light harvesting nanomaterials

Light Harvesting Nanomaterials

Editor

Surya Prakash Singh Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,

Hyderabad, India

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CONTENTS

Foreword i

Preface ii

List of Contributors iii

CHAPTERS

1. Metal Nanoparticle Induced Light-Trapping for Solar Photovoltaic Applications Brijesh Tripathi and Manoj Kumar

3

2. Low-Cost and Durable Tetrapyrrolic Sensitizers for Sensitization of Nanocrystalline TiO2 Lingamallu Giribabu, Kolanu Sudhakar and Challuri Vijaykumar

21

3. Polymer-Based Nanocomposite Materials for Functional Applications in Devices Sutapa Ghosh

55

4. Visible-Light Photocatalytic Organic Synthesis: Localized Surface Plasmon Resonance-Driven Oxidation Processes Using Au-TiO2 Nanocoupling Systems Shin-Ichi Naya and Hiroaki Tada

78

5. Microstructures and Photovoltaic Properties of C60-Based Solar Cells with Copper Oxides, CuInS2, Phthalocyanines, Porphyrin, Diamond and Exciton-Diffusion Blocking Layer Takeo Oku, Akihiro Takeda, Akihiko Nagata, Ryosuke Motoyoshi, Kazuya Fujimoto, Tatsuya Noma, Atsushi Suzuki, Kenji Kikuchi, Tsuyoshi Akiyama, Balachandran Jeyadevan, Jhon Cuya, Yasuhiro Yamasaki and Eiji Ōsawa

100

Index 142

i

Foreword

I am glad to write the foreword for timely eBook titled Light Harvesting Nanomaterials edited by Dr. Surya Prakash Singh. I believe this eBook is a mine of information for those who are pursuing work/research in this array. As fossil fuels are becoming more scarce, research on the development of various approaches to utilize renewable energy sources such as solar energy, wind and hydrogen is gaining importance.

Natural photosynthesis executes efficient light-harvesting by continuous unidirectional electron transfer between chromophores is magnificent aspect of nature’s light harvesting of solar energy has motivated researchers to mimic such a process. The science of light harvesting materials is experiencing remarkable growth. Till now conventional solid-state junction devices commanded photovoltaics, which are often made of silicon. Owing to the cheap fabrication and flexibility various light harvesting materials such as nanocrystalline materials, conducting polymer films and organic photoelectronic devices have emerged as alternatives.

Nanocrystalline materials provide possibilities of improved performance. Photosensitization of wide-band-gap semiconductors such as TiO2 by adsorbed dyes have enabled practical for solar cell applications. This eBook dealing with light harvesting nanomaterials should be of use in this interest.

Prof. C.N.R. Rao

National Research Professor Linus Pauling Research Professor & Honorary President

Jawaharlal Nehru Centre for Advanced Scientific Research Bangalore

India

ii

Preface

In the search of, renewable and clean energy, the solar cell is considered a major candidate for obtaining energy from the sun. To capturing the energy of entire solar spectrum is the main challenge to improve the performance of photovoltaic devices. Incorporation of nanomaterials as a light harvester made a revolutionary change in the field of solar cell technology by enhancing the efficiency with lower costs. First and foremost advantage of nanomaterials is tunable band offset and visible response with size quantization, which allows harvesting of desired portion of solar spectrum. Till date different kinds of nonmaterials have been tested for their light harvesting properties, such as compound semiconductor nanoparticles, quantum dots, metal nanoparticles etc. Understanding their energy transfer mechanism and charge carrier capacity are the important parameters to make it more useful for solar cell.

This eBook summarized the present scenario and fundamentals of synthetic approaches and tailoring the light harvesting properties of a variety of nanostructured materials and their application in photovoltaic industries with a systematic and coherent picture of the field.

The chapters of this eBook written by distinguished scientists, who are experts of their fields. We sincerely hope that this eBook will provide insight into subject and some new directions to material scientists as well as the new researchers.

Surya Prakash Singh Inorganic and Physical Chemistry Division

CSIR-Indian Institute of Chemical Technology Hyderabad

India E-mail: [email protected]

iii

List of Contributors

Akihiko Nagata Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Akihiro Takeda Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Atsushi Suzuki Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Balachandran Jeyadevan Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Brijesh Tripathi School of Solar Energy, Pandit Deendayal Petroleum University, Gandhinagar – 382007, India

Challuri Vijaykumar Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad - 500 607, India

Eiji Ōsawa NanoCarbon Research Institute, Ltd., 3-15-1 Tokida, Ueda, Nagano, 386-8567, Japan

Hiroaki Tada Department of Applied Chemistry, School of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan

Jhon Cuya Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Kazuya Fujimoto Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Kenji Kikuchi Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Kolanu Sudhakar Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad - 500 607, India

Lingamallu Giribabu Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad - 500 607, India

Manoj Kumar School of Solar Energy, Pandit Deendayal Petroleum University, Gandhinagar – 382007, India

Ryosuke Motoyoshi Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Shin-Ichi Naya Environmental Research Laboratory, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan

Sutapa Ghosh Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad - 500 607, India

iv

Takeo Oku Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Tatsuya Noma Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Tsuyoshi Akiyama Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan

Yasuhiro Yamasaki Orient Chemical Industries Co., Ltd., Department of New Business, Neyagawa, Osaka, 572-8581, Japan

Light Harvesting Nanomaterials, 2015, 3-20 3

Surya Prakash Singh (Ed) All rights reserved-© 2015 Bentham Science Publishers

CHAPTER 1

Metal Nanoparticle Induced Light-Trapping for Solar Photovoltaic Applications Brijesh Tripathi* and Manoj Kumar*

School of Solar Energy, Pandit Deendayal Petroleum University, Gandhinagar – 382007, India

Abstract: A complete treatment of the design, simulation, fabrication and characterization of metal nanoparticles and metal nanoparticle embedded structures is presented in this chapter. This treatment reveals that the metal nanoparticles play an important role in improving light absorption in the solar photovoltaic cell. This improves the understanding towards a promising approach to get better light trapping in solar photovoltaic cells and to enhance the power output from such a device. The examples presented here broaden the understanding of light trapping schemes and their effects.

Keywords: Design, fabrication and characterization, light absorption, light trapping, metal nanoparticle, simulation, solar photovoltaic cell.

1. INTRODUCTION

In context with solar photovoltaic cell, efficient light trapping techniques may help in reducing usage of active material while improving the net absorption of incident radiation. Anti-reflection coatings (ARC) have been an important area of study to minimize reflection from the front surface of the solar cells. ARCs have been optimized theoretically and experimentally on bare or encapsulated cells. Zhao and Green [1] theoretically optimized a number of different ARCs. Much of work has been reported on dual layer anti- reflection coating, where different type of materials were deposited like TiO2/SiO2/Si3N4, by Richards et al. [2], Arturo et al. [3], Bikash et al. [4] etc. A comparatively new method for achieving light trapping in solar cells is the use of metallic nanostructures that support surface plasmons, which are excitations of the conduction electrons at the interface between a metal and a dielectric. With appropriate engineering of these metallodielectric structures, light can be concentrated and ‘folded’ in to a thin semiconductor layer, thereby increasing the absorption [5]. Both localized surface

*Corresponding Authors Brijesh Tripathi and Manoj Kumar: School of Solar Energy, Pandit Deendayal Petroleum University, Gandhinagar – 382007, India; Tel: +91 79 2327 5343/5447; Fax: +91 79 23275030; E-mails: [email protected], [email protected]

4 Light Harvesting Nanomaterials Tripathi and Kumar

plasmons (LSP) excited in metal nanoparticles and surface plasmon polaritons (SPPs) propagating at the metal/semiconductor interface are interesting for light trapping application. In recent years extensive research is reported on surface plasmons, which are collective oscillations of the electrons in conductors, for biological and luminescence applications [6]. Catchpole and Pillai [7, 8] investigated the suitability of localized surface plasmons on silver nanoparticles for enhancing the absorbance of silicon solar cells in the IR region. They modelled Ag particles as scattering luminescent emitters and made use of the broadened emission peak to facilitate the near band (in IR region) light absorption in a Si thin-film. Ferry et al. [9] have reported the design, fabrication, and measurement of ultrathin film a-Si:H solar cells with nanostructured plasmonic back contacts, which demonstrated enhanced short circuit current densities compared to cells having flat or randomly textured back contacts (see Figs. 1 & 2).

Fig. (1). Design of solar cell with plasmonic light trapping. (a) Schematic cross section of the patterned solar cell. Patterns are made on the rear glass substrate, and there is conformal deposition of all layers over the patterns through the top ITO contact. Incident blue and red arrows indicate that blue light is absorbed before reaching the back contact while red light interacts more with the back patterns. (b) Photograph of finished imprinted patterned solar cell substrate. Each colored square is a separate device, with different particle diameter and pitch. (c) SEM of Ag overcoated patterns showing 290 nm diameter particles with 500 nm pitch. (d) SEM image of a cross section of a fabricated cell, cut using focused ion beam milling. Note that the ultrathin a-Si:H layer constitutes only a small part of the cell (reproduced from Ferry et al. [9] with permission from OSA).

Metal Nanoparticle Induced Light-Trapping Light Harvesting Nanomaterials 5

Fig. (2). Electrical measurements on plasmonic solar cells. Data are shown for a-Si:H with two different intrinsic layer thicknesses. (a) a-Si:H thickness 340 nm and (b) a-Si:H thickness 160 nm. Curves are shown for square grid patterns of 250 nm diameter plasmonic scatterers at pitches of 500 nm and 700 nm, the flat reference cell, and (in (b)) the randomly textured Asahi cell (reproduced from Ferry et al. [9] with permission from OSA).

Tsai et al. [10] have demonstrated the simulation results of absorption enhancement in an amorphous-Si (a-Si) solar cell by depositing metal nanoparticles (NPs) on the device top and embedding metal NPs in a layer above the Al back reflector. They have also reported that the solar cell absorption can be increased by 52% with a structure of vertically aligned top and bottom Al nanocylinders (NCs).

2. DESIGNING METAL NANO-PARTICLE LAYER FOR LIGHT TRAPPING

The subwavelength metal nanoparticles can be used to scatter incident light into a distribution of angles, increasing the path length of the light within the absorbing layer. This concept can be used to design ARCs with embedded metal nanoparticles for maximizing absorption in solar cells. The introduction of scattering objects into a solar cell results in the modified standard exponential absorption profile. The intensity at a given depth in a cell with a planar surface is related to the incident intensity 𝐼!" by 𝐼 = 𝐼!"𝑒!!", where 𝐿 is the path length of the light in the medium, and 𝑎 is the absorption coefficient of the material. There are many possible ways to utilize the concept of scattering in a solar cell to improve net absorption. Let us limit ourselves to following two architectures of nanoparticles for a solar photovoltaic cell application:


1) a sparse array of sub-wavelength metal scatterers deposited on the top of conventional solar photovoltaic cell as shown in Fig. (3a).

Fig. (3a). Sparse array of metal scatterers deposited on top of conventional solar photovoltaic cell.

2) metal scatterers embedded in ARCs which are thin-films used for minimizing reflection from top surface of conventional solar photovoltaic cell as shown in Fig. (3b).

Fig. (3b). Metal scatterers embedded in ARC on top of the solar photovoltaic cell.

2.1. Theoretical Treatment for Calculating Absorption in the Metal Nano-Particle Layer

Let us assume that transmission 𝑇(𝜆) through the complete solar photovoltaic cell is negligible, so that the power is either absorbed (in semiconductor, metal, or supporting layers) or reflected (including back scattering). One can neglect the interference between the scattering objects and so the fraction of total power absorbed in the semiconductor 𝐴(𝜆) can be written as:

𝑨 𝝀 = 𝝃𝑸𝒔𝒄𝒂𝒕 𝝀 𝒇𝒔𝒖𝒃𝒔𝒕𝒓𝒂𝒕𝒆 𝝀 + 𝟏 − 𝝃𝑸𝒔𝒄𝒂𝒕 𝝀 (𝟏− 𝑹𝒔(𝝀)) (1)

where 𝜉 is the fraction of the surface covered by scatterers, 𝑄!"#$ is the normalized scattering cross section of the nanoparticle relative to its physical size, 𝑓!"#!$%&$' is the fraction of the total scattered light that is forward scattered into the substrate, and 𝑅!(𝜆) is reflection from the semiconductor interface only, in the absence of a nanoparticle. If light is incident normally then the path length of the scattered light is increased to 1/cos(θ), where θ is the angle between the scattered light and surface normal. The reflection term 𝑅!(𝜆) represents the reflection from the semiconductor interface only, in the absence of a nanoparticle.


One can calculate the normalized quantity 𝑄!"#$ from the scattering cross section 𝜎!"#$ using 𝑄!"#$ = 𝜎!"#$/𝜎!"#$, where σ!"#$ is the geometrical cross section of the object. Under quasistatic limit, generally the size of nanoparticles are much smaller than the wavelength of incident light and the field on the particle is uniform, the scattering cross section is given by

𝝈𝒔𝒄𝒂𝒕 = 𝟏𝟔𝝅

𝟐𝝅𝝀

𝟒𝜶 𝟐 (2)

where 𝛼 represents the polarizability of the particle [11]. For spherical nanoparticles embedded in a semiconductor having the permittivity represented by 𝜀!, the polarizability can be calculated from

𝜶 = 𝟒𝝅𝒓𝟑 𝜺𝒎!𝜺𝒔𝜺𝒎!𝟐𝜺𝒔

(3)

where r represents the radius of the nanoparticle and 𝜀! represents the permittivity of the metal. When 𝜀! ≈ −2𝜀! the polarizability is at a maximum and the particle undergoes a dipolar surface plasmon resonance. The quantity 𝑄!"#$ depends on the size of the particle and local environment (generally dielectric material), both of these properties can be used to tune the extent of light scattering.

The absorption cross section 𝜎!"# in the metal depends on the polarizability as per the relationship given below:

𝝈𝒂𝒃𝒔 =𝟐𝝅𝝀𝐈𝐦[𝜶] (4)

For solar photovoltaic cell applications an important design criterion is that 𝜎!"#$ ≫ 𝜎!"# to keep metallic losses low, because sunlight absorbed in the metal is not expected to contribute to useful carrier generation within the photovoltaic cell. In the case of smallest nanoparticles, 𝜎!"#$ + 𝜎!"# ≈ 𝜎!"# but as the particle size increases 𝜎!"#$ grows and the dipolar plasmon resonance red shifts and broadens [9]. As the size of nanoparticle tends towards wavelength of incident light (𝜆), both the quasistatic approximation breaks down and multipolar modes contribute to the particle’s scattering cross section. All of the equations given above remain valid for dielectric particles as well as metallic ones and the difference lies in 𝜀! (dielectric particles have both more modest permittivities than metals and Re[𝜀!] > 0, and thus do not generally exhibit resonant behavior).


This resonant dipolar scattering behavior can be observed with many shapes, e.g. spherical metal nanoparticles, metallic voids, cylinders, hemispheres, core-shell particles, and anisotropic particles. Each different shape of particle would generally have a different 𝑓!"#!$%&$', so particle shape must be chosen to couple preferentially into the substrate rather backscatter into free space for light trapping application [12].

Let us now refer to the case of an isolated nanoparticle on top of a thin-film of absorbing material, as illustrated in Fig. (4). As shown in the figure that the incident light gets scattered off the object into a distribution of optical modes within the semiconductor. A fraction 𝜌 of the power scattered off the nanoparticle would couple to the escape cone (𝜌!) and a fraction 𝜌! to each guided mode, so that 𝜌! + 𝜌!!

!!! = 1, where N is the number of modes in the semiconductor. The effective size of the scatterer that couples to each mode is defined as the incoupling cross section 𝜌!"#,!, where

𝝆𝒊𝒏𝒄,𝒋 = 𝝆𝒔𝒄𝒂𝒕𝒇𝒔𝒖𝒃𝒔𝒕𝒓𝒂𝒕𝒆𝝆𝒊 (5)

Fig. (4). Diffusion model for light propagation inside the solar cell. Incident light is scattered off a scattering object, with a fraction ρ0 coupling to the escape cone and ρj to each guided mode. The modes propagate a distance L, with appropriate losses in the absorbing layer and cladding materials, until the next scattering object. At the next scattering object the mode is re-scattered (adapted from Saeta et al. [14] and Ferry et al. [15] with permission from OSA).


One can normalize it by the geometrical size of the object, this gives the normalized incoupling cross section 𝑄!"#,! = 𝜎!"#,!/𝜌!"#$ [13]. Each of these modes would have a characteristic overlap with the semiconductor that describes that a considerable fraction of energy would be absorbed in the semiconductor, which can be represented by 𝛤!,!. For a given mode at a particular wavelength, the fraction of power absorbed in the semiconductor layer of the cell is calculated by

𝑨 𝝀 = 𝝃𝑸𝒊𝒏𝒄,𝒋𝜞𝒔,𝒋 (6)

The fraction of total power absorbed in the semiconductor as a function of wavelength is calculated by

𝑨 𝝀 = 𝑨𝒋(𝝀)𝑵𝒋!𝟎 + 𝟏− 𝝃𝑸𝒔𝒄𝒂𝒕 𝝀 𝟏 − 𝑹𝒔 𝝀 − 𝑻 𝝀 (7)

2.2. Theoretical Treatment for Calculating the Reflection from the Composite Layer

The anti-reflection coating with embedded metal nanoparticles can be designed using the theoretical treatment outlined below. At each wavelength, the reflection coefficient 𝑅(𝜆) is calculated using the Matrix Method. In this method, each layer is represented by a characteristic matrix, M:

R(λ) =

n0B−Cn0B+C

2

(8)

where, n0 – refractive index of air and B and C are the components of the following matrix, M:

BC

!

"##

$

%&&=

Am iBm

iCm Dm

!

"

##

$

%

&&

m=1

n

∏()*

+*

,-*

.*

1ns

!

"##

$

%&& (9)

where, for m = 1, 2, …, n, Am = Dm = cosδm , Bm = isinδm ηm , Cm = isinδmηm and ns – refractive index of substrate. The two unknowns in these expressions are defined below:

δm ≡ phase difference = 2π nmdm cosθm

λ (10)


ηm = nm cosθm for s-polarization (11)

ηm = nm / cosθm for p-polarization (12)

where θm is the angle of incidence at layer m and nm is the complex refractive index of mth layer.

To describe the optical properties of a composite with suspended metal particles, an effective dielectric function theory was proposed by Maxwell-Garnett [16], in which the effective dielectric function of the composite is given by:

ε = εm

1+2 f ( εsph − εm ) / ( εsph +2 εm )1− f ( εsph − εm ) / ( εsph +2 εm ) (13)

where f is the volume fraction of metal particles in the composite. The complex refractive index n~ of the composite layer can be extracted from the relation

22 )(~~ iknn +==ε . Thus one can calculate complex refractive index of a composite layer and deduce reflection using equation (8).

3. EXAMPLES TO DEMONSTRATE THE DEPOSITION AND CHARACTERIZATION OF METAL NANOPARTICLES/METAL NANOPARTICLE EMBEDDED DIELECTRIC LAYERS FOR LIGHT TRAPPING

We refer to Pillai et al. [17] wherein it is reported that metal nanoparticles were deposited by thermal evaporation of thin layers of silver followed by annealing. The scanning electron microscope (SEM) image of the sample surface is shown in Fig. (5). This is one of the simplest methods of depositing metal particles onto a substrate. Silver is the metal of choice because of its low absorption losses when compared to other metals. The silver nanoparticles were formed by depositing varying mass thicknesses of a uniform layer of silver at the rate of approximately 2 Å/s at 1 x 10−5 Torr. The samples were then annealed in nitrogen at 200 °C for 50 min. Due to surface tension, the particles coalesce together to form islands. The variation in the size and shape of the particles with increasing mass thickness of silver can be seen in the SEM images in Fig. (5). The corresponding electroluminescence (EL) enhancement plots, with a clear redshift in the peak


Fig. (5). SEM pictures showing silver metal particles corresponding to a mass thickness of (a) 14 nm, (b) 16 nm, (c) 18 nm, (d) 27 nm of silver (reproduced from Pillai et al. [17] with permission AIP).

with increasing particle sizes, are shown in Fig. (6). These results suggest that surface plasmons offer a promising way to improve the efficiency of thin-film solar cell structures, avoiding the problem of increased recombination which occurs when silicon is textured directly.


Fig. (6). EL enhancement plots for particle sizes corresponding to different mass thickness of silver showing an increased enhancement for larger particle sizes and a clear redshift in the enhancement peaks as the particle size increases (reproduced from Pillai et al. [17] with permission from AIP).

Further, Beck et al. [18] have reported fabrication and characterization of nanoparticle induced enhancement of light trapping in a solar photovoltaic cell. They fabricated cells with a double dielectric layer structure with a thickness of 10 nm of thermally grown SiO2 and 8 nm of Si3N4, deposited by low pressure chemical vapor deposition, on both front and rear surfaces of the cells. The oxide layer was deposited to provide surface passivation for the cells and during the fabrication process nitride layer resulted due to process procedure. Before nano-particle deposition all the cell processing steps were completed. On half of the rear surface of the finished solar cells a random distribution of Ag nanoparticles was fabricated. Thin layers of Ag of eighteen nanometer thickness Ag were deposited by thermal evaporation followed by a 50 min anneal at 230°C, in an atmosphere of N2. So, the samples with half of the rear surface of the cells being covered with nanoparticles, as illustrated in Fig. (7a) could be made. A coating of TiO2 by atmospheric pressure chemical vapor deposition at 200°C was applied to the cells on both front and rear surfaces. The thickness of TiO2 coating on both sides of all the cells could be maintained as 89 nm on the rear, and 82 nm on the front. A single wavelength ellipsometer was used for determination of the TiO2 layer thickness and refractive index. The refractive index was measured to be 2.1 at a wavelength of 673 nm. A layer of 82 nm thick TiO2 was coated on the front of the cells to reduce reflection below 10% for wavelengths of 700–1200 nm, and


to less than 1% around 850 nm. Plasmonic light-trapping in thin c-Si solar cells could be demonstrated by this study. An enhancement of 47.9% in photocurrent was achieved by independently optimizing self assembled, rear-located nanoparticle arrays for light trapping and including a mirror and a dielectric ARC. An enhancement of 13.0% in photocurrent was attained due to light-trapping.

Fig. (7). (a) Schematic of the experimental geometry, (b) micrograph of the random array of Ag nanoparticles, (c) distribution of particle sizes and (inset) the percentage of particles in the six most populous bins (reproduced from Beck et al. [18] with permission from Wiley).


Before coating with TiO2 the scanning electron micrographs of the random nanoparticle arrays were taken at five points on the cells. Although by varying the deposited mass thickness of Ag thin-layers the nanoparticle size can be changed [19] the self-assembly method results in randomly positioned nanoparticles with a large distribution in diameter. This is clear from SEM image shown in Fig. (7b). Measurement of cross sectional area of each particle in the five micrographs was done and the diameter of the particles was taken to be equivalent to that of a circle of the same area. Fig. (7c) illustrates the particle size distribution calculated from all five micrographs and, inset, the percentage of the total number of particles which had the equivalent diameter. More than 89% of the particles had equivalent diameters between 50–204 nm, and the average equivalent diameter was 131 nm. The surface coverage of particles was 36% and they had a shape of ‘flattened’ hemisphere with a height of ~ 50 nm, estimated from the surface coverage of the particles and the volume of Ag deposited. From the known illumination intensity the external quantum efficiency (EQE) was then calculated as the fraction of incident photons that are converted to electrical current. An area of approximately 4 mm2 on the front of the cell was illuminated for each measurement. Separate measurements were taken on the half of the cell with nanoparticles on the rear and on the half of the cell without nanoparticles, for reference. Further, measurements were also taken with and without a detached Ag mirror behind the cells, having a reflectivity of 95% for wavelengths 750–1200 nm. Experimental EQE measurements for a 22 mm thick, c-Si solar cell with a TiO2 ARC (ARC, dash-dot line), with a TiO2 ARC and a mirror (ARC + Mirror, solid line), with rear-located nanoparticles and a TiO2 ARC (NP + ARC, crosses) and with rear-located nanoparticles, a TiO2 ARC and a mirror (NP + ARC + Mirror, circles) is shown in Fig. (8a). For reference the data for the same cell before TiO2 and nanoparticle deposition is included (Ref, dashed line in Fig. 8a). Transmission losses become significant at a wavelength of 750 nm for a 22 mm thick, c-Si cell and the data for cells with nanoparticles and/or mirrors exhibit EQE enhancements, compared to the ARC only case, due to the reduction in transmission at the rear of the cell. The performance of cell with a mirror is quite comparable to the cells with nanoparticles on the rear up to a wavelength of 870 nm. This may be attributed to the fact that specular reflection from the mirror increases the path length of light in Si to approximately 44mm which absorbs 90% of the incident light up to a wavelength of 870 nm. Beyond 870 nm the nanoparticle arrays (crosses and circles in Fig. 8a) perform better than the cells with only a mirror present (bare line in Fig. 8a). The nanoparticles could help to trap a significant portion of the light inside the Si by total internal reflection by scattering light at high angles, leading to an increase in the path length of light in the cell, compared to cells with


only mirror reflector. When a mirror is included behind the nanoparticles (circles in Fig. 8a), the portion of light which is not initially scattered by the particles into the Si substrate is reflected back to provide multiple scattering opportunities, increasing the EQE further.

Fig. (8b) shows the experimental EQE enhancements relative to the EQE of the cell with an ARC. Near the band edge of Si at a wavelength of 1150 nm, the experimental EQE enhancement is 1.6 for a cell with a detached Ag mirror (bare line in Fig. 8b), 4.3 for a cell with the Ag nanoparticle array (crosses in Fig. 8b), and 5.6 when a detached mirror is included with the nanoparticle array (circles in Fig. 8b) in the limit of weakly absorbed light.

Fig. (8). (a) Experimental external quantum efficiency (EQE) measurements on a 22 mm c-Si solar cell with a TiO2 ARC (ARC, dash-dot line), with a TiO2 ARC and a mirror (ARC + Mirror, solid line), with rear-located nanoparticles and a TiO2 ARC (NP + ARC, crosses) and with rear-located nanoparticles, a TiO2 ARC and a mirror (NP + ARC + Mirror, circles). The EQE spectrum of the same cell before TiO2 and Ag deposition is shown from reference (Ref, dashed line). Fig. (8b) shows EQE enhancements relative to the cell with a TiO2 ARC only (reproduced from Beck et al. [18] with permission from Wiley).


Let us consider another example of metal embedded dielectric layer studied with the help of a theoretical model and photovoltaic cell simulation software PC1D by Sircar et al. [20]. In this work theoretical treatment is done for investigating effect of embedded silver (Ag) nano-particle in silicon nitride anti-reflection coatings. The reflectance of ARC with different volume fractions of embedded Ag nano-particles is calculated as a function of the wavelength using the optical interference matrix theory and the Maxwell-Garnett theory. It could be concluded that the Ag nano-particles embedded silicon nitride ARC helps in performance improvement of commercial silicon solar cells. Theoretical calculations for the reflection coefficient of the composite layer are done with varying order of Ag volume fraction in SiN. The volume fraction of Ag in SiN is varied from 0% to 10% and reflection coefficient is plotted against wavelength as shown in Fig. (9).

Fig. (9). Reflectance from the modeled metal nano-particle embedded SiN anti-reflection coating (adapted from Sircar et al. [20]).

The reflection data generated using the theoretical treatment is used in the PC1D simulation software to calculate the enhancement in maximum power from the solar cell. The modeling parameters for PC1D simulation are listed in Table 1.

It was observed that variation in Ag volume fraction has different effect on different wavelengths. As shown in Fig. (10), the incident radiation of wavelength around 606.25 nm has minimum reflection for a volume fraction of around 2% of


Ag nano-particles, whereas with increasing wavelengths, i.e. for 650 nm and 693.75 nm the minima shift towards higher Ag volume fractions. We will find the optimum value of Ag volume fraction on the basis of other results.

Table 1. Parameters of the baseline model (reproduced from Sircar et al. [20]).

Solar Cell Parameters Values

Area 156.25 cm2

Thickness 200 µm

Emitter contact resistance 4.5 x 10-5 Ω

Base contact resistance 7.2 x 10-5 Ω

Emitter doping, N-type 3.81 x 1020 cm-3

Base doping, P-type 1.5 x 1016 cm-3

Rear doping, P-type 1 x 1018 cm-3

Bulk recombination lifetime 25 µs

Surface recombination velocity Sn = 4.5 x 105 cm/s Sp = 1000 cm/s

Primary light source • Intensity • Spectrum

0.1 W/cm2

AM 1.5

Fig. (10). Reflectance from the modeled metal nano-particle embedded SiN anti-reflection coating for wavelengths 606.25 nm, 650 nm and 693.75 nm (adapted from Sircar et al. [20]).


There is an increase in the short circuit current (ISC) with increasing Ag volume fraction in the silicon nitride ARC up to 6% as shown in Fig. (11) and then ISC starts decreasing for further increase in Ag volume fraction. Also, there is an increase in the maximum power (PMAX) generated by the cell with increasing Ag volume fraction in the silicon nitride ARC up to 6% and then PMAX starts decreasing for further increase in Ag volume fraction as shown in Fig. (12). This study shows that for a particle size of 5 nm, the optimum Ag volume fraction lies near 6%. The increase observed in ISC is around 1% to the baseline model (here baseline model corresponds to the one having 0% Ag volume fraction in the ARC). The cell performance is described by PMAX generated by the cell due to the incident radiation over its area. Around 2% increase in PMAX is observed due to the Ag nano-particle embedded silicon nitride ARC for a volume fraction of around 6% as shown in Fig. (12).

Fig. (11). Variation of short circuit current of the multi-crystalline silicon solar cell with respect to Ag volume fraction (adapted from Sircar et al. [20]).

Considerable improvement was observed in the output electrical parameters. For modeled ARC, increase of 1% in ISC and 2% in the PMAX delivered by the cell is reported. So it could be concluded that the Ag nano-particles embedded silicon nitride ARC helps in performance improvement of commercial silicon solar cells.


Fig. (12). Variation of maximum generated power of the multi-crystalline silicon solar cell with respect to Ag volume fraction (adapted from Sircar et al. [20]).

4. CONCLUSION

A complete treatment of the design, simulation, fabrication and characterization of metal nanoparticles and metal nanoparticle embedded structures is presented here. It could be observed from this study that the metal nanoparticles play an important role in improving light absorption in the solar photovoltaic cell.

ACKNOWLEDGEMENTS

One of the authors (BT) acknowledges the scientific discussions held with Dr. Ratna Sircar, Head, Physics Department, Feroze Gandhi College, Raebareli (India). Authors are thankful to Dipal Patel, Research Scholar, School of Solar Energy, PDPU for his technical help.

CONFLICT OF INTEREST

The authors confirm that this chapter contents have no conflict of interest.


REFERENCES

[1] Zhao, J.; Green, M.A. Optimized antireflection coatings for high-efficiency silicon solar cells, IEEE Trans. Electr. Dev., 1991, 38, 1925-1934.

[2] Richards, B.S.; Rowlands, S.F.; Honsberg, C.B.; Cotter, J.E. TiO2 DLAR coatings for planar silicon solar cells Prog. Photovolt. Res. Appl., 2003, 11, 27-32.

[3] Acevedo, A.M.; Arredondo, E. L.; Santana, G. Proceedings of the 29th IEEE PVSC. New Orleans, 2002, pp. 293-295.

[4] Kumar, B.; Baskara, P.T.; Sreekiran, E.; Narayanan, S. Benefit of Dual Layer Silicon Nitride Anti-reflection Coating, Conference Record of 31st IEEE Photovoltaics Specialists Conference, 2005, pp. 1205-1208.

[5] Harry, A.; Atwater; Albert, P. Plasmonics for improved photovoltaic devices. Nat. Mater., 2010, 9, 205-213.

[6] Maier, S.A. Plasmonics: fundamentals and applications. 1st ed. Springer, 2007. [7] Catchpole, K.R.; Pillai, S. Surface plasmons for enhanced thin-film silicon solar cells and light

emitting diode. J. Lumin., 2006, 121, 315. [8] Pillai, S.; Catchpole, K.R.; Trupke, T.; Green, M.A. Surface plasmon enhanced silicon solar cells. J.

Appl. Phys., 2007, 101, 093105. [9] Ferry, V.E.; Verschuuren, M.A.; Li, H.B.T.; Verhagen, E.; Walters, R.J.; Schropp, R.E.I.; Atwater.

H.A.; Polman, A. Light trapping in ultrathin plasmonic solar cells. Optics Express, 2010, 18, A237-A245.

[10] Tsai, F.J.; Wang, J.Y.; Huang, J.J.; Kiang, Y.W.; Yang C. C. Absorption enhancement of an amorphous Si solar cell through surface plasmon-induced scattering with metal nanoparticles”, Optics Express, 2010, 18, A207-A220.

[11] Bohren, C.F.; Huffman, D.R. Absorption and Scattering of Light by Small Particles, John Wiley & Sons, New York, 1983.

[12] Catchpole, K.R.; Polman, A. Design principles for particle plasmon enhanced solar cells. Appl. Phys. Lett., 2008, 93, 191113.

[13] Ferry, V.E.; Sweatlock, L.A.; Pacifici, D.; Atwater, H.A. Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells, Nano Lett. 2008, 8, 4391-4397.

[14] Saeta, P.N.; Ferry, V.E.; Pacifici, D.; Munday, J.N.; Atwater, H.A. How much can guided modes enhance absorption in thin solar cells? Optics Express, 2009, 17, 20975-20990.

[15] Ferry, V.E.; Munday, J.N.; Atwater, H.A. Design Considerations for Plasmonic Photovoltaics. Adv. Mater., 2010, 1–15.

[16] Cohen, R.W.; Cody, G.D.; Coutts, M.D.; Abeles, B. Optical properties of granular silver and gold films. Phys. Rev. B., 1973, 8, 3689-3701.

[17] Pillai, S.; Catchpole, K.R.; Trupke, T.; Green, M.A. Surface plasmon enhanced silicon solar cells, J. Appl. Phys., 2007, 101, 093105.

[18] Beck, F.J.; Mokkapati, S.; Catchpole, K.R. Plasmonic light-trapping for Si solar cells using self-assembled, Ag nanoparticles. Prog. Photovolt. Res. Appl., 2010, 18, 500–504.

[19] Xu, G.; Tazawa, M.; Jin, P.; Nakao, S. Surface plasmon resonance of sputttered Ag films: substrate and mass thickness dependence. Appl. Phys. A, 2005, 80, 1535-1540.

[20] Ratna, S.; Dibya, P.S.; Brijesh, T. Study of Ag Nano-particle Embedded Silicon Nitride Anti-Reflection Coating (ARC) for Silicon Solar Cells. Int. J. Photon., 2010, 2, 7-14.



CHAPTER 2

Low-Cost and Durable Tetrapyrrolic Sensitizers for Sensitization of Nanocrystalline TiO2 Lingamallu Giribabu*, Kolanu Sudhakar and Challuri Vijaykumar

Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 607, India

Abstract: In this chapter, recent progress in the applications of tetrapyrrolic free-base as well as its metal complexes in dye-sensitized solar cells (DSSC) is summarized and analyzed. Though ruthenium(II) complexes are dominant in DSSC, but they have certain drawbacks to necessitate the design of alternative sensitizers. Based on absorption, thermal and redox properties, tetrapyrrolic compounds are found to be alternative sensitizers to ruthenium(II) complexes. Tetrapyrroles are broadly divided into three categories and they are porphyrins, phthalocyanines and corroles. Both porphyrins and phthalocyanines are good sensitizers to wideband-gap semiconductors. Two classes of porphyrins are defined by connecting position of anchoring groups either at meso- or at pyrrole-β. The effect of efficiency of DSSC device with substituents on porphyrin ring is reviewed. Phthalocyanines are another class of tetrapyrrolic compounds and they are less explored for DSSC applications due to aggregation and solubility of macrocycle in common organic solvents. New unsymmetrical phthalocyanines are designed based on ‘push-pull’ concept is analyzed. The possibility of application of corroles as sensitizers for DSSC is also reviewed.

Keywords: Corroles, porphyrins, phthalocyanines, sensitizers, tetrapyrroles.

1. INTRODUCTION

A solar cell is a device source that converts light energy into electric energy utilizing an unlimited clean energy source, sunlight, without polluting the environment. Solar cells based on silicon and other semiconducting materials have been known ever since 1950’s [1]. However, their applications are limited due to various reasons [2]. To overcome the associated problems, in 1991 Prof. Gratzel and Brian O’Regan proposed next generation solar cells, i.e., Dye-Sensitized Solar Cells (DSSCs) based on the concept of artificial photosynthesis

*Address correspondence to Lingamallu Giribabu: Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 607, India; Tel: +91-40-271917124; Fax: +91-40-27160921; E-mail: [email protected]

22 Light Harvesting Nanomaterials Giribabu et al.

[3]. DSSC is a low-cost, easy to fabricate on flexible substrate, with short energy payback time (< 1 year), low sensitivity to temperature changes and environmentally benign device compared to the conventional solid state p-n photovoltaic devices [4].

Historically, DSSC started in 1972 with a chlorophyll sensitized zinc oxide (ZnO) electrode [5]. For the first time, photons were converted into an electric current by charge injection from excited dye molecules into a wideband-gap semiconductor. In the following years a lot of fundamental research was done on ZnO-single crystals [6], but the efficiency of these devices remains very low. The main problem was that a monolayer of dye molecules on a flat surface can only absorb up to 1 % of the incident light. Introduction of nanoporous TiO2 electrodes with a roughness factor of ca. 1000 dramatically increased the light harvesting efficiency and in 1991 solar cells of 7 % efficiency were introduced [3]. This triggered a boom in research activities and today cells of 11.2 % are state-of-the-art [7].

Fig. (1). Schematic of DSSC device structure.

The typical device structure of DSSC is shown in Fig. (1). It consists of nanocrystalline semiconductor which adsorbs the sensitizer on its surface, a Pt-doped counter electrode and a redox electrolyte of the type I-/I3

- in volatile organic solvent. In these devices sensitizer is one of the key components in achieving high efficiency and durability. The extensively used charge-transfer sensitizers employed so far in such cells are bis(tetrabutylammonium)cis-di(thiocyanato)-bis(4,4’-dicarboxy-2,2’-bipyridine)ruthenium(II) (the N719 dye) and trithiocyanato 4,4’,4”-tricaboxy-2,2’:6’,2”- terpyridine ruthenium(II) (the black dye), produced solar energy-to-electricity conversion efficiencies (η) of up to 11% under AM 1.5

Low-Cost and Durable Tetrapyrrolic Sensitizers Light Harvesting Nanomaterials 23

irradiation and stable operation for millions of turnovers [7]. Nevertheless, a substantial amount of research work is still needed to fill in the gap between today’s benchmark conversion efficiency and the Shockley-Queiser limit of η 32% predicted for a single junction cell [8]. To achieve high efficiency in DSSC, the most important component of the device is the sensitizer. Even though, ruthenium(II) polypyridyl complexes are more dominant in DSSC, they are very expensive due to the rarity of the metal, less durable due to the presence of two or three –NCS groups in its molecular structure and the absorption maximum of ruthenium complexes is limited at around 550 nm. As ruthenium(II) sensitizers are exclusive, metal-free sensitizers such as synthetic organic dyes and natural dyes are important for DSSC applications. Since these sensitizers do not contain noble metals, there are no concerns about resource limitation. However, organic dye based DSSCs are not useful for rooftop applications as these molecules are not very durable. For a light-harvesting system, the essential requirements are that the sensitizing dye absorbs light over a wide wavelength range, preferably one that encompasses the visible spectrum to near-IR, binds strongly to the semiconductor surface (needs to have an anchoring group), has a suitably high redox potential for regeneration following excitation and is stable over many years of exposure to sunlight [9]. Based on chemical, thermal, photophysical and redox properties, tetrapyrrolic compounds (porphyrins, phthalocyanines & corroles) are found to be suitable low-cost alternative sensitizers for DSSC applications [10-12].

2. PORPHYRIN SENSITIZERS

Nature accomplishes enhanced light absorption by stacking chlorophyll-containing thylakoid membranes of the chloroplast to form the grana structures that act as light-harvesting antenna [13]. These absorb the incident light and then channel the excitation energy to reaction centers, where light-induced charge separation takes place [14]. Given their primary role in photosynthesis, the use of porphyrins as light harvesters on semiconductors is particularly attractive. Owing to the delocalized macrocyclic structure and very strong absorption in the 400-450 nm region (Soret band) as well as two/four less intense absorption bands in the 500-700 nm region (Q-bands), porphyrins have long been studied as promising components of molecular electronic and photonic devices [15-17]. Numerous artificial photonic assemblies, based on multi-porphyrin architectures, have been designed to mimic photosynthetic solar energy transduction by converting excitation energy to chemical potential in the form of long-lived charge separation [18, 19].


One of the essential requirements of the porphyrin sensitizers is the anchoring group position. The position/s that are available on porphyrin for connecting the anchoring groups is either meso- or pyrrole-β position.

2.1. Anchoring Groups at Meso-Position

Over the last decade, the efficiency of meso- substituted porphyrins has significantly changed. A great variety of meso- substituted porphyrins have been used for the photosensitization of wideband-gap semiconductors like NiO, ZnO and TiO2 etc., the most common being the free-base and zinc derivatives of the meso-benzoic acid substituted porphyrin Por-1 [20]. Porphyrins exhibit long-lived (>1 ns) π* singlet excited states and only weak singlet/triplet mixing. The lowest unoccupied molecular orbitals (LUMO) of these porphyrins reside above the TiO2 conduction band and highest occupied molecular orbitals (HOMO) lies below redox couple in the electrolyte solution required for charge separation at SC/dye/electrolyte surface. Both Gratzel and co-workers as well as Fox and co-workers have reported efficiency charge injection from excited state of Por-Zn-1 into the conduction band of TiO2 (IPCESoret =42%, IPCEQ band = 8–10%) and (IPCESoret = 9.5%, Isc ≈ 2.5 µAcm−2), respectively [21, 22]. Boschloo and Goossens photosensitised TiO2 with Por-Zn-1, giving a low η of 1.1% (IPCESoret = 40%, IPCEQ band ≈ 10–16%, Voc = 0.36 V, Isc = 0.85 mAcm−2) [23]. Later, Cherian and Wamser reported a DSSC test cell device based on P-1 with an efficiency under AM 1.5 conditions (η = 3.5%, fill factor of 62%, IPCESoret = 55%, IPCEQ band = 25–45%, Voc = 485 mV, Isc = 6 mAcm−2) and deoxycholic acid (DCA) have been used as an additive in dye solution to minimize the aggregation of macrocycle [24]. Wamser have also reported a solid state based Grätzel cell using an aminophenyl tricarboxy phenyl porphyrin dye Por-2 with an aniline gel-based electrolyte system giving a η value of 0.8% [25]. The electron injection and charge recombination dynamics of N3, Por-1 and Por-Zn-1 on TiO2 was studied in detail by Durrant and co-workers [26]. Their investigations show that these three dyes have almost indistinguishable electron injection and recombination kinetics. The high efficiency reported for N3 dye probably originates from differences in the rate of electron transfer to the dye cation from the iodide redox couple used in these devices. It is also possible that the lower efficiency of porphyrin sensitisers results from the increased probability of exciton annihilation from close porphyrin proximity. Porphyrins have an inherent tendency to aggregate, and at high dye coverage, dipole/dipole interactions are expected to allow rapid migration of the excited state between neighbouring dyes, increasing the probability of exciton annihilation [27].


Ma and co-workers have compared the sensitization properties of porphyrin with only one carboxyl group Por-3 to that of the porphyrin with four carboxyl groups Por-1 [28]. The IPCE and overall conversion efficiency for Por-1 are higher than that of Por-3 probably because of the more stabilized adsorption on surface of TiO2 nanoparticles by the four carboxylic acid groups. A comparison on the binding ability between the carboxylic acid and sulfonic acid group has been made by the same group. They compared the binding ability between Por-1, sulfonic acid group Por-4 or hydrogen Por-4H on surface of nanocrystalline TiO2 and was found to drop following the order of –COOH > SO3H > H while IPCE and overall conversion efficiency decreased following the same order [29].

Bignozzi and co-workers have constructed a series of free base porphyrins with different anchoring groups for DSSCs [30]. They studied the effect of different anchoring groups and substitution positions on DSSC performance. Dyes Por-1 and Por-5 with carboxylic and phosphonic functions, respectively, did not exhibit significant differences in DSSC performance, which was probably because both dyes were electronically decoupled from TiO2 since the meso-aryl groups orientate perpendicularly to the porphyrin macrocycle. However, dyes Por-6 and Por-7 showed different IPCE values depending on the substitution position of the phosphonate anchoring groups. They pointed out that this could be due to the differences in the orientation and distance of the chromophore on the TiO2 surface imposed by the directionality of the anchoring groups. Dye Por-7 may lie closer to the surface of TiO2 than dye Por-6 with para-substituted porphyrin.

NH

NNH

N

CH2OOH

Por-3

NH

NNH

NR

R

R

R

R = SO3H; Por-4R = H; Por-4H


The number of anchoring groups in a sensitizer is also very essential in achieving higher efficiency of the device. Giribabu and co-workers have reported one (Por-8) or two (Por-9) rhodanine acetic acid groups at para position/s of meso- phenyl of either free-base or zinc porphyrin (Por-Zn-8 & Por-Zn-9) [31, 32]. The IPCE of DSSC device fabricated using Por-Zn-9 is 45% at Soret band and 20% Q-band; both of them are higher than that of the Por-8, Por-9 & Por-Zn-8. This may be due to the more stable binding of Por-Zn-9 on the surface of nanocrystalline TiO2, which induces more efficient electron injection to the conduction band of TiO2. Lee et al. have reported photoelectrochemical properties of porphyrins having either one (Por-Zn-10) or two (Por-Zn-11) triarylamine moieties at the meso- position/s [33]. The UV-Vis absorption spectra reveal that the substitutions result in large red-shifts in both the Soret band (∼60 nm) and the Q bands (∼125 nm), as well as enhancement of optical absorption. The enhancement is even more pronounced in the long-wavelength region of 575–725 nm, where the absorption of porphyrin Por-Zn-11 is eight times that of porphyrin Por-Zn-12. Both porphyrins Por-Zn-10 & Por-Zn-11 when compared to Por-Zn-12 shows enhanced optical properties that are attributed to the aggregation of the porphyrin molecules and geometry of the anchoring group.

Johnston and Waclawik have studied alkyl porphyrins i.e., tetrakis (3,5-di-tert-butylphenyl) porphyrin and its zinc derivative and their efficiency with N3 and Por-1 [34]. Differences in efficiencies of the porphyrin light absorbers are ascribed to poor electronic coupling of the latter alkyl porphyrin based sensitizers to the titania conduction band. Fukuzumi and co-workers have deposited porphyrin-modified TiO2 nanoparticles were deposited on nanostructured

NH

NNH

NPO3H2

PO3H2

H2O3P

PO3H2

Por-5

NH

NNH

N

PO3H2

PO3H2

NH

NNH

N

PO3H2

H2O3P

Por-6 Por-7


OTE/SnO2 electrode together with nanoclusters of fullerene (C 60) in acetonitrile–toluene (3/1, v/v) using an electrophoretic deposition technique to afford the porphyrin-modified TiO2 composite electrode denoted as OTE/SnO2/(porphyrin-modified TiO2 nanoparticle + C60)n [35]. The porphyrin-modified TiO2 composite electrodes have efficient light absorbing properties in the visible region, exhibiting the photoactive response under visible light excitation using I-/I3

- redox couple. The IPCE and efficiency of the system were enhanced that indicates formation of supramolecular complexes between porphyrins and fullerene on TiO2 nanoparticles plays an important role in improvement of the light energy conversion properties.

Gratzel and co-workers have integrated the porphyrin chromophore as π bridge into a D-π-A dye, Por-Zn-13 [36]. The sensitizer having a diarylamino donor group attached to the porphyrin ring acts as electron donor, ethylbenzoic acid moiety serves as an acceptor and the porphyrin chromophore itself constitutes π bridge in this particular D-π-A structure. The absorption spectra shows that both Soret and Q-bands are red shifted. This sensitizer has shown η of 8.8% (IPCESoret = 85%, IPCEQ band ≈ 80%, Voc = 0.735 V, Isc = 16.7 mAcm−2) on 11.5 µm thick

N

S

CH2COOH

S

O

N

NN

N

N

S

HOOCH2C

S

O

MN

S

CH2COOH

S

O

N

NN

NM

M = H; Por-8M = Zn; Por-Zn-8

M = H; Por-9M = Zn; Por-Zn-9

N

NN

NZn

COOH

COOH

N

NN

NZn

COOH

COOH

N

O

O

N

NN

NZn

COOH

COOH

N

O

O

N

O

O

Por-Zn-11Por-Zn-10 Por-Zn-12


nanoscrystalline TiO2 surface using liquid redox electrolyte. Although it shows an IPCE of 85%, but this sensitizer lacks absorption in the region 480-630 nm, which leads to the reduction of the JSC and efficiency values of the device. Hence, co-sensitization with an organic, which shows complementary spectral responses in the visible spectral range, was shown to have an efficiency of 11% (Voc = 0.77 V, Isc = 18.6 mAcm−2).

Wang et al. has replaced donor diarylamino group in Por-Zn-13 with a series of organic donor groups phenylethynyl, naphthalenylethynyl, anthracenylethynyl, phenanthrenylethynyl or pyrenylethynyl in D-π-A system [37]. The phenanthrenylethynyl or pyrenylethynyl sensitizer Por-Zn-14 has shown an efficiency of 10.06% (Voc = 0.711 V, Isc = 19.62 mAcm−2), which is highest efficiency among porphyrin based sensitizers. This is due the broad spectral response of the sensitizer between 400-800 nm.

2.2. Anchoring Groups at Pyrrole-β Position

The electronic π cloud of macorcyclic porphyrin is present above and below the plane of the macrocycle. Therefore anchoring groups connected at pyrrole-β position may give better efficiency than that at meso- positions. Through modifying the meso-tetraphenyl porphyrins by substitution at the pyrole-β position with functional groups could extend the π systems and enhance the red-absorbing Q bands due to the splitting of the four frontier molecular orbitals [38].

Gratzel and co-workers have compared DSSC device performance by using variety of anchoring groups at pyrrole-β position with different central metal ions [39]. The photovoltaic data shows that, for porphyrins with carboxylic binding groups, the Zn containing (Por-Zn-15) diamagnetic metalloporphyrins have very

N

NN

NZnN COOH

N

NN

NZn COOH

Por-Zn-13 Por-Zn-14


high IPCE values compared to those observed for the Cu containing (Por-Cu-15) paramagnetic metalloporphyrins. In addition, porphyrins with a phosphonate anchoring group show lower efficiencies than those with a carboxylate anchoring group (Por-Cu-16 & Por-Zn-16). Porphyrin having 2,5-dimethyl phenyl groups (Por-Zn-17) have shown the conversion efficiency of 4.80% (Voc = 0.66 V, Isc = 9.7 mAcm−2), which shows new avenues for improving the efficiency further of nanocrystalline solar cells for practical utility, by engineering suitable porphyrins with a smaller band-gap capable of absorbing in the visible and near-IR regions of the solar spectrum [40]. The same group have adopted extended-π conjugation concept and reported Por-Zn-18. This porphyrin has enhanced molar absorption coefficient both at Soret and Q bands. The device based on Por-Zn-18 using a hole transport material, spiro-MeOTAD has shown an efficiency of 3.0%.

Officer and co-workers further modified and reported at pyrrole-β position using cyano acetic acid as anchoring group, Por-Zn-19 [41]. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations show that key molecular orbitals (MOs) of porphyrin Por-Zn-19 is stabilized and extends out onto the substituent by π-conjugation, causing enhancement and red shifts of visible transitions thereby increasing the possibility of electron transfer from the substituent. The porphyrins were investigated for conversion of sunlight into electricity by fabricating a dye sensitized TiO2 solar cell using I-/I3

- redox electrolyte. The test cells yield close to 85% IPCE value with an overall conversion efficiency of 5.60% (Voc = 0.61 V, Isc = 13.0 mAcm−2). The same group have introduced another extended π-conjugation at pyrrole-β position and reported Por-Zn-20 [42]. This sensitizer is having dicarboxylic acid group as an

N

NN

NM

COOH

N

NN

NM

PO3H2

N

NN

N

COOH

Zn

M = Cu(II); Por-Cu-15M = Zn(II); Por-Zn-15

M = Cu(II); Por-Cu-16M = Zn(II); Por-Zn-16 Por-Zn-17


anchoring group. The molar absorption coefficient of both Soret and Q bands are further increased. When using liquid redox electrolyte this sensitizer has shown an overall conversion efficiency of 7.10%. The sensitizers with high ε values are very compatible with solid state redox electrolytes. This sensitizer has shown an efficiency of 3.60%, when spiro-MeOTAD used as hole transport material.

Giribabu et al. have substituted rhodanine acetic acid at pyrrole-β position of either free-base tetraphenyl porphyrin (Por-21) or zinc(II) tetraphenyl porphyrin (Por-Zn-21) [31]. They have studied photoelectrochemical studies based on these sensitizers using durable redox electrolytes and the zinc(II) porphyrin have shown better efficiency than the corresponding free-base porphyrin.

Ko and co-workers have used thiophene units as extended π-conjugation at pyrrole-β position Por-Zn-22, as a result both Soret and Q bands are bathochromic shift, intensification of absorption and an increased lifetime of the

N

NN

NZn

CN

COOH

N

NN

NZn

COOH

COOH

N

NN

NZn

COOH

COOH

Por-Zn-18Por-Zn-19 Por-Zn-20

N

S

CH2COOH

N

NN

NM

S

O

M = 2H; Por-21M= Zn(II); Por-Zn-21


excited state were observed [43]. However, the sensitizer undergoes aggregation on the surface of nanocrystalline TiO2 due to the planarity of the molecule. The problem of aggregation was resolved by using chenodeoxycholic acid as a co-adsorbate. The sensitizer has shown an efficiency of 4.0% (Voc = 0.61 V, Isc = 13.0 mAcm−2).

Osuka and co-workers have investigated electronic and photovoltaic properties of porphyrin based sensitizers having one Por-Zn-23 & Por-Zn-24 or two Por-Zn-25 anchoring carboxylic acid groups at one or two pyrrole-β position/s [44]. The electronic structures of the porphyrin macrocyclic core are strongly coupled with olefinic side chains so that the absorption spectrum largely exhibits broad and red-shifted Soret and Q-bands, especially up to 475 nm at the Soret band in a porphyrin doubly functionalized with malonic diacid groups (Por-Zn-25). The sensitizer exhibits the maximum overall conversion efficiency of 3.03% and the

N

NN

NZnC2H5

C2H5

C2H5

C2H5

S S

CN

COOH

Por-Zn-22

N

NN

NZn

COOH

N

NN

NZn

COOH

COOH

N

NN

NZn

COOHCOOH

COOHCOOH

Por-Zn-23 Por-Zn-24 Por-Zn-25


maximum IPCE value of 60.1% in the Soret band region, superior to others. From such photovoltaic performances, it can be suggested that multiple pathways through olefinic side chains at two β-positions enhance the overall electron injection efficiency and the moderate distance between the porphyrin sensitizer and the TiO2 semiconductor layer is important in retarding the charge recombination process.

β,β′-quinoxalino porphyrins containing different numbers of carboxylic acid anchoring groups have been evaluated as photosensitizers for DSSC by Imahori and co-workers [45]. Both of the sensitizers showed broadened, red-shifted, and amplified light absorption with the aid of π-extensions. DFT calculations reveal that the HOMO-LUMO gap of these sensitizers is very low due to the substitutions onto the β,β′-edge of the porphyrins, which mainly affect the energy levels for unoccupied orbitals of the porphyrins. Photovoltaic measurements of Por-Zn-26 and Por-Zn-27 sensitized TiO2 cells with P25 revealed power conversion efficiencies of 5.2% and 4.0%, respectively due to the number and the position of binding groups in the porphyrins have a large impact on the photovoltaic and photoelectrochemical performances.

The anchoring groups at pyrrole-β position were present either on extended π-conjugation or on the substituents so far in porphyrin based sensitizers for DSSC. The efficiency of DSSC device based porphyrins having anchoring groups directly connected to pyrrole-β position may give better efficiency since it is very near to electronic π cloud, which is above and below the macrocycle. Giribabu and co-workers have connected the anchoring carboxylic acid group directly to the pyrrole-β of free-base (Por-28) as well as corresponding zinc porphyrins (Por-Zn-28) [46]. The absorption spectra of both free-base and corresponding

NN

N

N

N

NZn

COOHN

N

N

N

N

NZn

COOH

COOH

Por-Zn-26 Por-Zn-27


zinc porphyrins are red shifted due to the electron withdrawing effect of carboxylic group. The LUMO of these sensitizers are above the TiO2 conduction band and HOMO below I-/I3

- redox couple. The DSSC test cell studies were yet to be reported.

2.3. Array Porphyrins

The maximum efficiency 10.06% was observed in DSSC devices based on porphyrins sensitizers is with Por-Zn-14, having anchoring group at meso- position. But, ruthenium(II) polypyridyl complexes have already shown an efficiency of 12% [47]. In order to further improve the efficiency of porphyrin based sensitizers, one has to improve its absorption between Soret and Q-bands region (420 to 510 nm). This is because the absorption of monomeric porphyrins in this region is minimum. Porphyrin arrays linked with conjugated acetylene bridges exhibit strong electronic coupling between porphyrin rings, resulting in splitting of the Soret band and broadening of the Q-bands [48]. Electronic absorption spectra of meso–meso-linked porphyrin arrays and their corresponding doubly and triply fused porphyrin arrays also show wide absorption covering the visible and near IR region. By dint of such spectral features, these porphyrin arrays are prospectively efficient sensitizers for application in DSSC.

The first dyad based porphyrin sensitizers for DSSC applications were reported by Koehorst et al. [49]. In this dyad, zinc porphyrin acts as antenna and shows sensitization when anchored on to TiO2 or ZrO2 substrate. Durantini have reported a dimeric porphyrin Por-29, which consists of 5,15-bis(4-carboxyphenyl) 10,20-bis(4-nitrophenyl) porphyrin (PH2) and Zn(II) 5-(4-aminophenyl)-10,15,20-tris(4-

N

NN

N

COOH

M

M = 2H; Por-28M = Zn(II); Por-Zn-28


methoxylphenyl) porphyrin (PZn) by an amide bond [50]. The PZn moiety bears electron-donating methoxy groups and a zinc ion, while the other porphyrin structure, PH2, is substituted by electron-withdrawing nitro groups and also anchoring carboxyl group. The PZn moiety in the dyad acts as an antenna. Dyad Por-29 sensitizes the SnO2 electrode and the photocurrent action spectrum closely matches the absorption spectrum. The photocurrent efficiency of dyad Por-29 is considerably higher than those of porphyrin monomers PZn and PH2. This dyad design, with PH2 in direct contact with the substrate through the free carboxylic acid group, is a promising architecture of organic material for spectral sensitization of semiconductor solar cells.

Campbell et al. have reported a series of linear and branched porphyrin arrays for the sensitization of nanocrystalline TiO2 [38]. In both the linear and branched arrays, the monormeric porphyrin units are linked in a graded series using appropriate spacers or bridging units, the higher energy chromophore transfers excitation energy to the lowest energy chromophore, which injects charge into the acceptor surface. There is no π-π interactions among the porphyrin macrocycles in these arrays. Both trimeric (Por-Zn-30 & Por-Zn-31) and pentameric porphyrins (Por-Zn-32 & Por-Zn-33) show low JSC values than the corresponding monomeric porphyrin. This is probably due to the small porosity of the TiO2 nanocrystalline layer which may be preventing adsorption of these very bulky dyes.

The two porphyrins macrocyles are connected by a rigid small spacer then there will be π-π interactions between porphyrin units as result the absorption spectra broaden. Yeh and co-workers have reported porphyrin dimers with varied

N

NN

N

OCH3

H3CO

OCH3

NH

CO NH

NNH

N

NO2

NO2

COOHZn

Por-29


connectivity between two porphyrin moieties (Por-Zn-34 – Por-Zn-37), their nature significantly influences their spectral, electrochemical and photovoltaic properties [51]. All porphyrin dimers exhibit a much broader absorption than that monomeric porphyrin. Dimer Por-Zn-35 exhibits a split Soret band ascribed to excitonic coupling. Among these porphyrin dimers, Por-Zn-35 exhibited the greatest photocurrent density because of its flat IPCE spectrum with external quantum efficiencies ∼70% covering the entire visible spectral region with an overall conversion efficiency of 5.23%. This is due to the effective excitonic coupling between two porphyrin macrocycles in this dimer is large when compared to other dimers.

N

NN

NZn

n-Bu n-Bu

n-Bun-Bu

Xyl

XylN

NN

N

Xyl

ZnXyl

COOH

HOOC COOHN

NN

N

COOH

Zn

N

NN

NZn

n-Bu n-Bu

n-Bun-Bu

COOH

COOHN

NN

N

COOH

ZnHOOC

COOH

HOOC COOHN

NN

N

COOH

Zn

Por-Zn-30 Por-Zn-31


N

NN

NZn

COOH

HOOC COOHN

NN

N

COOH

Zn

Xyl

XylN

NN

N

Xyl

ZnXyl

Xyl

XylN

NN

N

Xyl

ZnXyl

Xyl

XylN

NN

N

Xyl

ZnXyl

N

NN

NZn

COOH

HOOC COOHN

NN

N

COOH

Zn

COOH

COOHN

NN

N

COOH

ZnHOOC

COOH

N

NN

N

COOH

ZnHOOC

COOH

COOHN

NN

N

COOH

ZnHOOC

Por-Zn-32

Por-Zn-33

N

NN

NZn

Por-Zn-34

Ar

Ar

Ar

N

NN

NZn

Ar

Ar

COOHN

NN

NZnAr

Ar

Ar

N

NN

NZn

Ar

Ar

COOH

N

NN

NZnAr

Ar

Ar

N

NN

NZn

Ar

Ar

COOHN

NN

NZnAr

Ar

Ar

N

NN

NZn

Ar

Ar

COOH

Por-Zn-35

Por-Zn-36 Por-Zn-37Ar =


Osuka and co-workers have reported directly linked meso-meso porphyrin dimer Por-Zn-38, bearing a poly (ethylene glycol) end group at one meso- position and 2,4-pentadienoic acids at other end (two pyrrole-β positions) [52]. The Soret band and Q-bands of Por-Zn-36 are red shifted and more over Soret band split due to excitonic coupling between the porphyrin monomeric units. The IPCE spectrum extends up to near-IR region with an overall conversion efficiency of 4.2% (Voc = 0.60 V, Isc = 10.89 mAcm−2). This work strongly suggests that directly linked dimeric porphyrins would be an auspicious molecule as light-harvesting sensitizers in DSSC.

Mozer et al. have designed a Zn-Zn porphyrin dimer comprising two efficient monoporphyrin dyes linked in either a linear anti (Por-Zn-39) or a 90° syn (Por-Zn-40) fashion, representing simple building blocks of linear or branched 3-D multichromophore arrays [53]. DFT calculations show that each porphyrin dimer acts as two non interacting electronic entities. The Soret band of both the dimers

N

NN

NZn

Ar

Ar

N

NN

NZn

Ar

Ar

Por-Zn-38

COOH

COOH

OOOOO

OOOOO

N

NN

NZn

N

NN

NZn

NCCOOH

N

NN

NZn

N

NN

NZn

NC COOH

Por-Zn-39 Por-Zn-40


asymmetrically broadened and the molar extinction coefficients (ε) of their Q-bands are nearly double those of the monomeric units. By incorporating both the porphyrin dimers, it was observed an IPCE of 70%. Surprisingly, no major difference in dye uptake, injection efficiency, or device performance has been observed between the linear or angled dimer, suggesting both of these building blocks could, in principle, be used to construct larger 3-D multichromophore light harvesting arrays with efficient solar energy conversion.

3. PHTHALOCYANINE SENSITIZERS

Phthalocyanines are analogues to porphyrins and are manmade compounds. Like porphyrins, they are 18-π electron aromatic systems and posses an intense Soret-band at 350 nm and Q-bands at 650-700 nm regions, therefore they are an excellent alternative for solar-cell applications [54-57]. Even though providing good absorption in the red/near-IR region of the solar spectrum, phthalocyanines can be tuned to be transparent over a large region of the visible spectrum, thereby enabling the possibility of using them as “photovoltaic windows”: a red/near-IR absorbing photovoltaic cell, in place of a window, will allow visible light to enter a building whilst harvesting the solar power from the red/near IR part of the spectrum. In addition to directly generating power, this also reduces the solar heating of buildings, thereby reducing the power consumption. The thermal, electronic and redox properties of phthalocyanines are suitable for the sensitization of wideband-gap semiconductor oxides such as TiO2, SnO2 etc.

The first phthalocyanine that was used for the sensitization of nanocrystalline TiO2 was zinc tetra carboxy phthalocyanine (Pc-1) [58]. The anchoring carboxyl groups are present at peripheral positions. The absorption spectrum of Pc-1 in solution was showed a typical intense Q-band absorption transition (λmax = 690 nm, ε = 80,000 dm3 mol-1 cm-1). At λ= 690 nm incident light power P and the short-circuit current density ISc are 18 mW cm-2 and 0.4 mA cm-2, respectively. The IPCE is 4%. Deng et al. have reported another symmetrical gallium tetrasulfonato phthalocyanine (Pc-2) for DSSC applications [59]. The photocurrent conversion of this device was very poor. The poor conversion is mainly due to the existence of Pc-2 as a dimer on a nanostructured TiO2 electrode and more over sulfonic acid group not strongly adsorbed on nanoscrystalline TiO2 surface.


Gratzel and co-workers have utilized axial positions of ruthenium(II) phthalocyanines and reported a Pc-3 sensitizer. This sensitizer having bis-(3,4-dicarboxypyridine) at axial positions of (1,4,8,11,15,18,22,25-octamethyl phthalocyanine) ruthenium(II) [60]. This dye has the absorption in the range of 550-750 nm with absorption maxima at 650 nm having ε of 49,000 dm3 mol-1 cm-1. The IPCE was found as high as 60% but efficiency was very low. The major drawback of this dye was the tedious synthesis, very poor yield as well as desorption due to the coordinate mode of binding of pyridine ligand to ruthenium(II). The same group also reported a few zinc, aluminium, and ruthenium phthalocyanines [61]. These sensitizers have either carboxyl or sulfonic acid groups for anchoring onto nanocrystalline TiO2. The sensitizers having carboxyl anchoring groups shows superior binding to TiO2. Under similar test cell conditions zinc(II) tetracaboxy phthalocyanine (Pc-4) has showed better efficiency than aluminium(III) tetracarboxy phthalocyanine (Pc-5). At 700 nm, Pc-4 showed an IPCE of 43% where as Pc-5 showed an IPCE of 13%. Similarly, IPCE of Pc-6 and Pc-7 at 700 nm were 30 & 10%, respectively. The maximum efficiency that was observed in this series was 1% using Pc-4 sensitizer. The poor η of phthalocyanines might be due to the aggregation and poor electron injection from excited state of phthalocyanine into the conduction band of semiconductor. To avoid the aggregation of phthalocyanines, Gratzel and co-workers have use chenodeoxycholic acid as co-adsorbent to minimize the aggregation of phthalocyanines. This however, affects the efficiency of test cell device. Sundstrom and co-workers have used Zinc(II) phthalocyanines substituted with amino acids for the sensitization of nanocrystalline TiO2 [62]. Phthalocyanine having substitution with tyrosine (Pc-8) shows better photovoltaic efficiency of 0.54% than glycine (Pc-9) derivative, whose efficiency was only 0.13%.

N

N

N

N NN N

NZn

COOHHOOC

HOOC COOH

PC-1

N

N

N

N NN N

NGa

SO3--O3S

-O3S SO3-

PC-2


N

N

N

N NN N

NZn

Pc-4

COOHHOOC

HOOC COOH

N

N

N

N NN N

NAl

Pc-5

COOHHOOC

HOOC COOH

OH

N

N

N

N NN N

NZn

Pc-6

SO3HHO3S

HO3S SO3H

N

N

N

N NN N

NAl

Pc-7

SO3HHO3S

HO3S SO3H

OH

N

N

N

N NN N

NRu

N

N

COOHHOOC

COOHHOOC

Pc-3


Torres and co-workers have used axial positions of Titanium(IV) phthalocyanines for DSSC applications [63]. Unlike Grätzel have used coordinate bond at axial positions (Pc-3), Torres have connected anchoring 4-carboxy catechol at axial position by covalent bond (Pc-10). By using covalent bond, desorption from the surface of nanocrystalline TiO2 could be minimized. The solution absorption spectrum showed the expected intense Q-band absorption transition (λmax = 702 nm, ε = 1, 35,000 dm3 mol-1 cm-1) and the dye showed strong adsorption to

N

N

N

N NN N

N

ONH

COOH

HO

O

HNCOOH

HO

O

HN

HOOC

OH

O

NHHOOC

OH

Zn

N

N

N

N NN N

NZn

ONH

COOH

O

HNCOOH

O

NH

HOOC

O

NHHOOC

Pc-8 Pc-9

N

N

N

N NN N

NTi

O O

COOH

Pc-10


nanocrystalline titanium dioxide electrode. The IPCE were found 19 % and η of 0.2 %. The poor efficiency is due to the poor injection of electron transfer from S1 state of phthalocyanine to the conduction band of TiO2.

Yanagisawa et al. have used two unsymmetrical phthalocyanines with different metal ions at the phthalocyanine central cavity [64]. It has six alkoxy groups and two carboxyl anchoring groups. For zinc (II) sensitized film (Pc-11), the short-circuit photocurrent (JSC) is 0.17 mA.cm-2, open-circuit photo voltage 0.32 V and fill factor (FF) 61%, giving overall conversion efficiency (η) of 0.03%. For Ruthenium(II) sensitized film (Pc-12), the short-circuit photocurrent (JSC) is 1.71 mA.cm-2, open-circuit photo voltage 0.41 V, fill factor 57%, giving an overall conversion efficiency (η) of 0.40%. The IPCE for a Pc-11 sensitized TiO2 electrode is 1.6% at 690 nm, while an IPCE of 23% at 630 nm was observed for Pc-12 sensitized TiO2 electrode. The low efficiency of these phthalocyanines was probable due to the split in macrocyclic conjugation thereby results in poor electron injection from excited state to the conduction band of TiO2. Aranyos et al. have studied a few alkyl phthalocyanines for the sensitization of nanocrystalline TiO2 without any anchoring group/s [65]. They found that the phthalocyanines having alkoxy or bulky aryl groups in its peripheral sites have shown sensitization of nanocrystalline TiO2 with an overall conversion efficiency of 0.47%. The low efficiency is because of the fact that the sensitizers do not have any anchoring group/s in its molecular structure. Amao et al. have studied the

N

N

N

N NN N

NZn

OC5H11

C5H11O

OC5H11

O

C5H11O

OC5H11

OC5H11O

COOH

COOH

N

N

N

N NN N

NRu

C5H11O

C5H11OOC5H11

O

H11Oc5OC5H11

OC5H11O

COOH

COOH

N

N

Pc-11 Pc-12


aryloxy or aryl thiol aluminum(III) phthalocyanines for DSSC applications [66]. They observed very poor performance in these aluminum(III) phthalocyanines probably due to the lack of anchoring group/s.

Fig. (2). Design of new unysmmetrical Zinc phthalocyanines.

The efficiency of DSSC devices based on phthalocyanine sensitizers are very low when compared to ruthenium(II) polypyridyl complexes and is not economically viable for commercial applications. The low efficiency of cells incorporating phthalocyanines appear to be the lack of solubility of phthalocyanine macrocycle in common organic solvents, aggregation due to the planarity of the phthalocyanine macrocycle and lack of directionality in the excited state. One of

e-

e-

e -e-

e-

e -e-

NNN

N

N

N

NNM

RR

R R'

NNN

N

N

NH

NNH

Planar Phthalocyanine Aggregation

NNN

N

N

NH

NNH

NNN

N

N

NH

NNH

NNN

N

N

NH

NNH

NNN

N

N

NH

NNH

Localized*π+electron*cloud*

Delocalized*π+electron*cloud*Unsymmetrical*phthalocyanine*

R*=*alkyl*or*alkoxy,*R’*=*+carboxyl*group*


the essential requirements for the light-harvesting system of a molecular/seminconductor junction is that the phthalocyanine sensitizer possess directionality of its electronic orbitials in the excited state. This directionality should be arranged to provide an efficient electron transfer from the excited dye to the TiO2 conduction band by good electronic coupling between the lowest unoccupied molecular orbital (LUMO) of the dye and the Ti 3d orbital.

In order to further improve the efficiency of phthalocyanine based DSSC devices, Nazeeruddin, Giribabu and co-workers have designed an unsymmetrical phthalocyanines based on ‘push-pull’ concept. These phthalocyanines are having either three bulky tert-butyl groups (Pc-13) or six butyloxy groups (Pc-14), which serves to enhance the solubility in common organic solvents, minimizes the aggregation of phthalocyanine macrocycle, act as electron releasing (‘push’) group and to tune the LUMO level of the phthalocyanine that provides directionality in the excited state [67, 68]. These phthalocyanines also have two carboxyl groups, which act as electron withdrawing (‘pull’) and serve to graft onto nanocrystalline TiO2. The photovoltaic performance showed that these sensitizers showed an IPCE of 75 & 25%, with Pc-13 & Pc-14, respectively using a volatile redox electrolyte (0.6 M 1-butyl-3-methylimidazolium iodide, 0.05 M iodine, 0.05 M LiI and 0.5 M tert-butylpyridine in a 50:50 (v/v) mixture acetonitrile and valeronitrile). The I-V characteristics show an overall conversion efficiency of 3.05% (JSC = 6.50 mA cm-2, VOC = 635 mV and ff = 0.740) using Pc-13 and 1.13% (JSC = 2.81 mA cm-2, VOC = 0.525 V and ff = 0.764) using Pc-14 as sensitizer. These are the first DSSC test cell devices that cross 3% efficiency mark using phthalocyanines as sensitizers, which is a major breakthrough in this class

NNN

N

N

N

NN

COOHCOOH

NN

N

N

N

NN

N

COOHCOOH

C4H9O

OC4H9 C4H9O

OC4H9

OC4H9

C4H9O

Pc-13 Pc-14

Zn Zn


of compounds. Using non-volatile redox electrolyte (0.9 M 1,2-dimethyl-3-n-propylimidazolium iodide, 0.1 M I2, 0.5 M n-methylbenzimidazolium iodide and 0.1 M lithium saccaharide in γ-butyrolactone) Pc-13, has shown an overall conversion efficiency of 1.94% [69]. The DSSC devices showed excellent stability when subjected to long-term high-temperature stress. The DSSC device maintains more than 93% of the initial photovoltaic performance after aging at 60 oC in the dark. Pc-13 was also tested using hole transporting material (2,2’,7,7’-tetrakis (N,N'-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD), with an overall conversion efficiency of 0.87% (Voc = 0.72 V, Isc = 2.10 mAcm−2) [64].

Giribabu et al. have further redesigned and reported another unsymmetrical phthalocyanine, Pc-15 based on extended π-conjugation along with ‘push-pull’ concept [70]. This sensitizer was tested using liquid redox electrolyte with an efficiency of 2.35% (Voc = 0.56 V, Isc = 5.63 mAcm−2). Thermal stability studies show that the sensitizer is stable up to 150°C and suitable for roof top applications.

Torres and co-workers have further improved the efficiency of phthalocyanine based sensitizers and reported Pc-16 [71]. This unsymmetrical phthalocyanine has only one carboxylic anchoring group unlike two carboxylic anchoring groups in Pc-13. This sensitizer has shown an IPCE of 80% at 690 nm with an efficiency of 3.52% (Voc = 0.62 V, Isc = 7.60 mAcm−2). The high efficiency of Pc-16 over Pc-13 is probably due to the fact that excited state life time is less in former so that the recombination of excited electron with oxidized phthalocyanine is less. The low efficiency of phthalocyanine based sensitizers is probably due to the lack of

NNN

N

N

N

NNZn

CNCOOH

Pc-15


absorption in 450 to 600 nm region. To further improve the efficiency of DSSC devices, one has to improve the absorption in 450 to 600 nm region. For this reason, the same group have attempted co-sensitization by mixing an organic sensitizer whose absorption in 550 nm region with phthalocyanine. Such type of cocktail sensitizers have shown an efficiency of 7.74% (Voc = 0.67 V, Isc = 16.20 mAcm−2). The same group have reported a series of unsymmetrical phthalocyanines bearing an anchoring carboxylic function linked to the phthalocyanine ring through different spacers for the sensitization of nanocystalline TiO2. The modification of the spacer group allows not only a variable distance between the dye and the nanocrystalline TiO2, but also a distinct orientation of the phthalocyanine on the semiconductor surface.

The anchoring group is very essential in order to achieve good efficiency in phthalocyanine based DSSC devices. Torres and co-workers have further redesigned Pc-16 by the introduction of extended π-conjugation in the phthalocyanine macrocycle and reported Pc-17 [72]. The new sensitizer have two carboxylic anchoring groups and has shown an overall conversion efficiency of 3.96% (Voc = 0.60 V, Isc = 9.15 mAcm−2). The high efficiency of Pc-17 over Pc-16 is due to the slow back electron transfer dynamics of the former sensitizer.

Taya and co-workers have further redesigned phthalocyanine molecule by introducing bulky -tert butyl Pc-18 or phenoxy groups Pc-19 & Pc-20 in its peripheral sites [73]. The absorption spectra of Pc-17 both in solution and on TiO2 have showed a broader absorption around 600 nm, suggesting the formation of aggregation on the surface. In contrast, Pc-19 & Pc-20, showed a sharp Q band

NNN

N

N

N

NNZn

Pc-16 COOH

NNN

N

N

N

NNZn

COOHCOOH

Pc-17


both in solution and on TiO2, indicating a significant decrease in aggregation. The IPCE of Pc-18, Pc-19 & Pc-20 shows 30, 52 &78 %, respectively at 700 nm. The overall conversion efficiency of Pc-20 showed 4.6%, which is highest efficiency among phthalocyanine based sensitizers. This is due to the less aggregation of phthalocyanine on TiO2 surface than that of other phthalocyanine molecules.

Hardin and co-workers have co-sensitized Pc-16 with an organic sensitizer, N,N’-di (2,6-diisopropylphenyl)-1,6,7,12-tetra (4-tert-butylphenyoxy)-perylene-3,4,9, 10 tetracarboxylic diimide (PTCDI) [74]. The organic sensitizer does not have any anchoring group. It has an absorption 550 nm and emission maxima at 650 nm. The emission of PTCDI overlaps with the absorption phthalocyanine and hence an efficient energy transfer takes place from PTCDI to Pc-16. It was estimated an excitation energy transfer from PTCDI to Pc-16 was 47%. This has improved an overall efficiency of the device by 26%. Boguta and co-workers have adopted the co-sensitization of different macrocycles i.e., phthalocyanines and naphthalocyanines [75]. Both phthalocyanines and naphthalocyanines are physically

NNN

N

N

N

NNZn

Pc-18COOH

NNN

N

N

N

NNZn

Pc-19

COOH

O

O

OPh

Ph

Ph

Ph

Ph

Ph

NNN

N

N

N

NNZn

Pc-20COOH

O

O

OPh

Ph

Ph

Ph

Ph

PhO

O

O

Ph

Ph

Ph

Ph

Ph

Ph


mixed in an organic solvent and adsorbed on to nanocrystalline TiO2. The photovoltaic properties are better than the isolated macrocyclic sensitizer.

The co-sensitization of a phthalocyanine using an organic molecule having an absorption in 450 – 600 nm regions through chemical bond may give better efficiency. Giribabu et al. have reported a cocktail sensitizer based on zinc phthalocyanine and bulky organic molecules, triphenyl amine (Pc-21) [76]. The presences of bulky triphenyl amine groups further reduce the aggregation of the macrocyle. The absorption spectrum broadens due to the presences of triphenyl amine groups. It has shown an IPCE of 37% with an overall conversion efficiency of 1.09% (Voc = 0.42 V, Isc = 3.90 mAcm−2). The low efficiency of Pc-21 is probably due to the low energy transfer efficiency from triphenyl amine moiety to the phthalocyanine macrocycle.

4. CORROLE SENSITIZERS

Unlike the porphyrins and phthalocyanine, corroles are very less explored as sensitizers in DSSC. Corrole is an analogue of porphyrin, and is a contracted macrocycle where one meso position has been eliminated resulting in a direct pyrrole-pyrrole bond and possessing the 18-π electron aromaticity of porphyrins [77]. Corroles also having many optoelectronic applications like porphyrins and phthalocyanines [78, 79]. The structure of corrole represents an intermediate

NNN

N

N

N

NN

N

N

N COOH

Zn

COOH

Pc-21


between porphyrin and the corrin ring in the B12 cofactor. The first investigation on corrole was carried out by Johnson and Kay in the late 1960s when the macrocycle was produced as a byproduct during the synthesis of B12 [80]. Thus, while corroles have been known for more than 40 years, research in the field was slow to progress. However, investigations on corroles have recently increased with the synthetic work of the groups of Paolesse and Gross, each of whom reported a one-pot synthesis of triarylcorroles [81, 82].

Gross and co-workers have used free-base corrole (Cor-H2) and its Ga(III) (Cor-Ga) as well as Sn(IV) (Cor-Sn) for sensitization of nanocrystalline TiO2 [83]. These sensitizers have sulfonic acid as anchoring groups to TiO2. These sensitizers have shown an IPCE of 30, 32 & 7% with Cor-H2, Cor-Ga & Cor-Sn, respectively. The IPCE spectra extend from 500 to 700 nm, in all the sensitizers. The overall conversion efficiency of Cor-H2 was 0.8% (Voc = 0.44 V, Isc = 2.83 mAcm−2), with Cor-Ga 1.6% (Voc = 0.52 V, Isc = 4.55 mAcm−2) & with Cor-Sn 0.1% (Voc = 0.35 V, Isc = 0.58 mAcm−2). This is the only report available in the literature to best of our knowledge. However, corroles are promising sensitizers for DSSC.

CONCLUSION

In conclusion, DSSC is a promising alternative technology to the existing solid-state p-n photovoltaic devices. The sensitizer is one of the vital components in achieving high efficiency of these devices and the most commonly used

N

NN

NM

SO3H

SO3H

F5

F5

F5

M = 3H; Cor-H2M = Ga(III); Cor-GaM = Sn(IV); Cor-Sn(Cl)


sensitizers in these devices are ruthenium(II) polypyridyl complexes. However, ruthenium(II) complexes have certain drawbacks and hence the urgency to design alternative sensitizers. This is primarily due to fact that the ruthenium metal is expensive and rare and lack of absorption in near-IR region. Based on the thermal, electronic and redox properties tetrapyrrolic compounds are found to be the best alternative sensitizers with porphyrins, phthalocyanines and corroles belonging to the tetrapyrrolic class of compounds. Porphyrins having anchoring group/s at meso- position with zinc as central metal ion, have shown an efficiency of 10.06%, which are very close to that of ruthenium(II) complexes. There is hence a scope to further increase the efficiency of DSSC devices based on porphyrin sensitizers. Phthalocyanines are not only low-cost but also quiet efficient however is still poor when compared to porphyrins. Nevertheless, unsymmetrical phthalocyanines based on ‘push-pull’ concept have reached an efficiency of 4.6%. The highest efficiency that was observed using corroles as sensitizers are 1.6%. Modifications in all these tetrapyrrolic compounds may possibly lead to further improve the DSSC device performance by changing the design of the molecular structure.

ACKNOWLEDGEMENTS

DST-UK (‘APEX’) programme is greatly acknowledged for financial support of this work. KS and Ch.V are thankful to CSIR for fellowship.



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CHAPTER 3

Polymer-Based Nanocomposite Materials for Functional Applications in Devices

Sutapa Ghosh*

I & PC Division, Indian Institute of Chemical Technology, Tarnaka, Hyderabad, India

Abstract: In recent years Nanostructured polymer and polymer-based nanocomposites are key materials for new device applications. Fabrication of nanostructured polymeric materials (e.g.: monodispersed latex, block copolymers, nanocomposites) and inorganic materials have evolved rapidly. It is already known that the inherent properties of the polymer matrix are enhanced through nanocomposite formation by the synergistic interactions between the polymer matrix and the nanoparticles. The aspect of the device applications of polymer-nanocomposites can be broadened by modifying the surface properties of nanoparticles, doping and filling the matrix with nanoparticles of both p- and n-type and introducing functionalities in polymer matrix. These eBook chapter is dealing with the above observations with experimental evidences and references.

This eBook chapter also deals with synthesis of various nanocomposites by tailoring the properties of polymer materials and nanomaterials which has been explained with various examples. Fabrication of functional polymeric materials and their nanocomposites concerning their optical, electrical, mechanical and thermal properties in thin film as well as bulk morphologies using several polymerization techniques is also explained in this chapter alongwith various characterization techniques like XRD, DSC, FTIR, SEM, TEM etc. Applications of these materials for various device applications like photo diode, solar cell, light emitting diodes, magnetic storage materials etc. also explained briefly with examples in this chapter alongwith future prospects of those materials for device applications.

Keywords: Polymernanocomposite, composite, nanoparticle, devices, photo diode, solar cell, light emitting diodes, magnetic storage materials.

In recent years, metals and alloys have been replaced by polymers and polymer composites for various applications starting from the of household items to the aerospace polymer. The excessive research development in polymer science and technology has developed new polymers having enhanced electrical, mechanical

*Address correspondence to Sutapa Ghosh: I & PC Division, Indian Institute of Chemical Technology, Tarnaka, Hyderabad, India; Tel: 914027191385; E-mails: [email protected], [email protected]

56 Light Harvesting Nanomaterials Sutapa Ghosh

and thermal stability. Generally polymers are cheap, lightweight, fracture tolerant, pliable, mould into different shapes and size and can easily be processed and tailored (Gurunathan et al., 1999). Like living systems, chemical and physical changes can be observed in these materials by changing external factors like electrical field, pH, temperature, magnetic field, electric field, di-electric constant, biological molecules and light (Bar-Cohen, 2004) [1]. Shape-memory alloys, piezoelectric materials, etc. are named as intelligent materials (Zrínyi, 2000) and also known as active polymers. Polymers responding to external influences due to the change in its shape or size are in the centre of interest since several decades.

The inherent properties of the polymer matrix are enhanced by the synergistic interactions between the polymer matrix and the nanoparticles. This is due to the combination of various properties of constituent materials like metals, semiconductors and low and high molecular weight substances. It is a challenge for researchers to make nanocomposites at molecular level. Thus intercalation of the blocks effects the confinement and quantum size. This is why these properties of nanocomposites are not found in bulk materials. It is important to understand different interactions between the building blocks to invent better nanocomposites. Various types of nanocomposits could be possible depending on the nature of building blocks such as organic-organic, organic-inorganic and inorganic-inorganic.

Electronic energy transfer in various assemblies of organic molecules and polymers is understood by theoretical analysis and molecular modelling calculations with experimental analysis which has diverted most of the attention in optoelectronic applications. Various photoinduced processes mainly occur due to photosensitized reaction followed by electronic energy transmission and conduction in such systems. This happens through various assemblies of organic molecules to the energy trap followed by charge inception and charge segregation, or chemical reactions involved in those trapped molecules [2-5]. It has been found that the electronic energy transfer (EET) from donors to acceptors should be highly efficient for photovoltaic and optoelectronic applications.

Various research works in this field are focused to get EET by changing the conformation of pendent like chromophores in the flexible chain polymer in solution [6-14]. In addition to this, various research efforts have been reported on the effect of external influences such as solvent, pH value, temperature and polymer molecular structure etc. on the energy transfer from donor chromophores to acceptor chromophores via polymer chain [8-10]. Improvement has been

Polymer-Based Nanocomposite Materials Light Harvesting Nanomaterials 57

achieved to understand the photophysics of EET in organic molecules with different chromophores. Optimization of polymer chain conformation by suitable selection of donor-acceptor pairs results in high EET efficiencies (60-100 % in solution and 43 % in solid matrices) [6-14]. But, the achievable energy-transfer efficiency can be controlled by excimer formation within the donor chromophores within certain limit [4, 5]. These pendant-chromophore systems do not show efficient photovoltaic effects due to the limited distance for charge separation alongwith the resulting back electron transfer. Thus these pendant chromophore assemblies are mainly useful in photochemical reactions [15-18].

For example, photocatalytic activity studies of polymer micelles that have efficient electronic energy transfer [15-17] have been reported by Guillet and co-workers. Forster mechanism of EET between chromophores in solutions and solid polymer matrices [4, 18] has been also confirmed by various researchers. The Forster equation for energy transfer [3] by dipole-dipole interaction is as follows. The energy-transfer rate is inversely proportional to the 6th power of the inter chromophore distance, where donor emission extensively overlaps with acceptor absorption. It is found that there is a the close agreement between experimental data and Forster's theory and thus intermolecular distance is determined by using EET as a spectroscopic ruler. For determining Fluorescence spectroscopy could be used to determine molecular-level miscibility of polymer blends. Research work of Morawetz and coworkers is based on a series of such studies of polymer blends [19]. In this report, fluorescent chromophores as energy donors and acceptors were attached at polymer chain ends. The miscibility of the polymer blends was determined by using emission spectroscopy of donor and acceptor [19]. Electronic energy transfer in conjugated polymers has also been studied by researchers [20]. Energy migration between segments various conjugation lengths of the same species has been reported [20, 21]. Electronic energy migration is found to be facile in conjugated polymers [20, 21] as emissions are dominated by the lowest-energy state. It is observed that incorporation of flexible-coil components into conjugated rigid-rod polymers, enhances the fluorescence quantum efficiency by several order of magnitude [22]. Such rod-coil polymer nanocomposites, are found to have novel photophysical properties and they are promising materials for optoelectronic application. These properties can be regulated by supramolecular structure and morphology of the materials [22].

After the invention of the conductivity mechanism in conducting polymers [23-25], significant attention has gained in the use of low molecular weight organic molecules and polymers for optoelectronic, electronic and photonic devices.


These polymer based devices have been extensively studied by various research groups all over the world for various applications such as LED, photodiodes, solar cells, gas sensors, field effect transistors etc. Some of them are tested for commercialization also [26].

Some of the advantages of the polymer-based devices are listed below:

1. These are flexible and light weight.

2. With low cost for device fabrication.

3. High resistance to fracture.

4. Possibility of making devices with new geometry using new principles which are not possible by traditional methods.

5. These materials can be made environment friendly by changing formulations.

6. It is possible to vary the properties of those materials (low molecular weight organic and polymer materials used for device components) by varying the composition.

Nanocomposite based devices, have significant features such as improved long-term stability compared to fully organic ones. Inorganic non-conductive Polymer nanocomposite based devices are found to have compared to fully organic based devices and thus higher stability finds enormous applications. This may be due to the fact that polymer matrix prevents the inorganic substituent from inter diffusion. This usually limits their operational time.

It is possible to obtain p-n nanojunctions using polymer nanocomposites. Here junctions between highly doped semiconductor particles and polymer composites allow the formation of interpenetrating networks of nanoparticles with modification of many valuable properties useful for device operation. In addition the use of highly doped nanoparticles results in the high electric fields within a layer, which often exceed 108V/cm. LED photodiode characteristics could be easily manipulated by varying the size of the nanoparticles simply by tailoring the band gap. For example emission wavelength of binary or ternary chalcogenides, generally used as inorganic LED materials, can be tailored to a much broader range by this method in comparison to the process of modification by change in


the stoichiometry of those materials. Since the size of nanoparticles can be tailored to less than half that of the visible light wavelength, the composite behave like a single crystal by following Raleigh mechanism. Another typical feature of polymer-nanocomposites is refractive index which can be modified by varying the concentration of nanoparticles in polymer matrix and introducing extremely high interface area between the nanoparticles and the polymer matrix. These properties are very useful for device application of those materials.

These materials can be synthesized by sol-gel and guest host inclusion processes. These materials can be made as continuous networks or matrix separated microcrystals. In case of guest host inclusion reaction makes use of the intercalation process—the insertion of guest substances into inorganic or organic solids various dimensions such as with molecules-zero dimensional, chains-one dimensional, layered systems-two-dimensional or frameworks-three-dimensional structures. All these techniques have been used to elucidate the structure, bonding and guest dynamics in intercalated materials.

n AND p TYPE NANOPARTICLES BASED POLYMER NANOCOMPOSITES SYNTHESIS

These can be prepared by conventional mechanical mixing method and in situ method. The mechanical mixing involves the synthesis of polymer and nanoparticle which are mechanically mixed to form the composites. But in this method nanoparticles are not dispersed properly due to agglomeration of nanoparticles and high viscosity of polymer matrix. Hence in situ synthesis of nanoparticle in the polymer matrix is efficient for the synthesis of the polymer nanocomposite.

a) Synthesis of p-type nanoparticle based polymer nanocomposite by in situ method. In the first step the polymer matrix (such as PVA) is dissolved in water by heating. Later it is cooled to room temperature. In the second step the p-type semiconductor nanoparticle precursor (CuCl2) is added to this polymer matrix solution followed by reduction (with NaI). Casting gives p-type polymer nanocomposite [27]. The chemical bath deposition method is used for the synthesis of n-type nanoparticle based polymer nanocomposite. It involves the addition of p-type semiconductor nanoparticle precursor (such as (CH3COO)2Cd.2H2O) to polymer matrix (PVA) solution followed by heating to dissolve. To this solution thiourea is added to form n-type


nanoparticle polymer nanocomposite followed by casting on the glass substrate [28].

b) p - n particles conversion Reaction: Chemical conversion of one sulfide species into another is done by this method when performed from one side of the solution-swelled composite film. This conversion allows the formation of mixed layer in the middle of the film and only one sulfide compound at the edges of the composite film. Another synthetic method includes casting or spin casting of colloid solutions of sulfide nanoparticles of both conductivity types along with dissolved conductive polymer.

PREPARATION OF DOPED NANOPARTICLES

Synthesis of doped nanocrystals (NCs) has become a major field of recent researchers. This is due to the fact that doped ions provide good traps for the excited electrons, which can be used in optoelectronic devices, solar cells and light-emitting diodes. High intrinsic conducting nanoparticles are synthesized by introducing dopants as donor or acceptor impurity. This type of doping can change the carrier concentration within certain range. The conductivity of the composites synthesised with doped nanocrystals (CdS doped with Indium) can be enhanced by more than 3 times compared to undoped ones.

SELF ASSEMBLY BASED SYNTHESIS

Developmenet of the self assembled molecular electronic devices consist of stratification [28]. This involves spontaneous separation of polymer layers using different concentrations of nanoparticles during spin coating using common bicomponent solution. The stratification is carried out with two different solvents. Both dissolved polymer and nano particle colloid solution can be stratified to these solvent mixtures. These solvents possess different solubility towards each component and different boiling points.

Solvent with higher vapour pressure can be evaporated faster thereby precipitating one of the component during casting process. This stratification leads to the formation of diffused p-n junctions or interpenetrating donor-acceptor networks with a gradual connectivity change.


SYNTHESIS OF CORE-SHELL NANOPARTICLES

Core shell nanoparticles have been synthesised by several methods, such as chemical precipitation, sol-gel, microemulsion and inverse micelle formation. For example CdSe/CdS nanoparticles can be synthesized by the inverse miceller technique. This method involves the H2S and H2Se gaseous mixture injection into a solution of cadmium chlorate [29]. Based on their ratio and the sequence of gas injection, different structures (domain like, core shell) can be formed. Completely different properties will be obtained on the basis of the confinement potential which further dependence on whether the lower band gap material forms a core or a shell of the particle. It is a matter of concern to know where the electrons and holes tend to rest, once the core shell particles are in contact. The latter is essential for both LED and photovoltaic applications. The oxide layer (core shell) present on the nanoparticles in composites changes the composite properties such as conductivity. The time period for the formation of this oxide layer is several weeks and after a month a remarkable quantity of nanoparticles is oxidized, leaving only a smaller inner core of metal. Due to this, electrophysical, optical and magnetic properties of nanocomposites will be changed.

NANOPARTICLE SURFACE MODIFICATION

Semiconductor nanoparticles have many potential applications but the disadvantages of these nanoparticles are aggregation and uncontrollable size. Surface properties of the nanoparticles are of great importance for applications of those materials. Since these surface defects determine the electron-hole recombination process which plays an important role in optoelectronic devices performance. Trapping of electron hole pair has to be either suppressed, as in case of photodiode and solar cells or enhanced, as in the case of LED applications. This has to be monitored depending on the end use.

Numerous methods are there to tailor the surface of sulfide nanoparticles. These are capping method or organic ligand attachment procedure. The latter procedure involves the covalent bond formation of organic molecules (such as thiophenolate etc.) with surface of nanoparticle to form complex. In general reaction of cadmium and chalcogenide ions in solution, synthesizes the nanoparticles. This can be terminated when thiophenolate is mixed to the solution. By changing the concentration of these reagents nanoparticles of required size covered or capped with thiophenolate ligand groups can be obtained. Distribution of nanoparticle in the polymer matrix can be influenced by Capping of nanoparticles.


PREPARATION OF POLYMER-NANOCOMPOSITES BY GAS PHASE DEPOSITION METHOD

Number of methods depending on gas-phase deposition of monomers on a substrate followed by polymerization has been developed [30]. Gas-phase deposition of organometallic monomers such as p-cyclophanes with organogermanium or organotin substituents is already known. On pyrolysis of these compounds yields respective p-xylene monomers with organometallic substituents without the dissociation of the organometallic bonds this after deposition and polymerization, forms poly (p-xylylenes) (PX) with organometallic component. The thermal treatment of the synthesised system in an inert atmosphere leads to rupture of the organometallic bonds and which further leads to the formation of PX composites with Ge and Sn particles.

Another method is the co-condensation of vapours of metals such as Pd, Ag, Zn, Cd, Ga, In, Ge, Sn, Sb and Bi in a vacuum with vinyl monomers onto a cooled substrate. Further the colloids are polymerized via heating or by the radical initiators [31]. Polymer metal composites can also be synthesized by polymerization in plasma (glow discharge) initiated by high energy ions [32].

In cryochemical method [33] polymer-nanocomposite is synthesized with metal and semiconductor thin films in different morphologies that are not possible by other methods. It involves the co-condensation of metal nanoparticle and monomers vapours at low temperature. Further, the solid state polymerization of the co-condensate can be performed at low temperature under irradiation. Polymer nanocomposites of metals, (Mg, Pd, etc.) and semiconductors (PbS and CdS) have been prepared using this method [34].

CHARACTERIZATIONS OF POLYMER NANOCOMPOSITES

Analytical Characterizations

a) Powder X-ray Diffraction: It is used to identify the distance between successive atomic planes and positions of atoms or ions within a crystal. This technique also can be used to determine the crystallinity in polymers [1], relative proportions of different materials in composites by comparing diffraction line intensity. In polymer nanocomposites it determines the extent of exfoliation of nanoparticle such as clay materials or graphite materials in the polymer matrix. While these clay materials are dispersed in


the polymer, they are forced to delaminate, by the interaction between polymer matrix and the silicate layers present in the clay materials there by increases the distance between the silicate layers. This process of delamination is called as intercalation and it results in greater distance between the silicate layers. The distance of separation is identified by the change in position of the peak from that of the respective pure clay on an XRD diagram. Bragg’s Law is commonly used to determine the distance between the silicate layers present in the clay and it is given by the following equation.

nλ = 2dsinθ

where n is an integer indicating the order of reflection.

λ is the wavelength of the incident x-rays.

d is the interplanar spacing of the crystal.

θ is the angle of incidence of the x-ray beam.

b) Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA/DSC)

Thermogravimetric analysis (TGA) is the most widely used thermal method. This method is used to determine the composition, degradation temperature, thickness of the polymer layer surrounding the nanoparticles and confirm the composition as well as thermal stability of the composites. TGA continuously measures the amount and rate of change in the weight of a sample with change in temperature. The sample is placed in a pan held in a microbalance. The pan and sample are heated in a controlled manner (at different heating rates) and weight is measured continuously throughout the heating cycle. While we are heating the sample it undergoes weight loss at different temperatures. These weight losses are due to a specific reaction or decomposition of the sample. The weight loss experienced during the decomposition experiment corresponds to the amount of polymer that was attached to the particles in the sample. It appears that the introduction of inorganic material to the polymer increases the thermal stability of pure polymer.


c) DSC is used to observe various thermal events with change in the specific heat capacity of the polymers [1]. It provides the information about thermal changes that do not involve a change in sample mass and measures the crystallization temperature, glass transition temperature, melting temperature and heat capacity.

d) Transmission Electron Microscopy (TEM): The transmission electron microscope (TEM) uses a high energy electron beam transmitted through a very thin sample to image and analyze the size of the particles present in the nanocomposite systems. TEM enables the visualization of internal structure of crystal samples and provides two dimensional images magnified as high as 100,000 times by using transmitted electrons. Because electrons can only travel a short distance through matter, samples must be very thin to enable acceptable image resolution. TEM images of polymer samples [1] are challenging because of the thin sample requirement and because the high intensity of the electron beam can burn away polymer films before images can be produced [35].

e) Scanning Electron Microscopy (SEM): Scanning electron microscopy (SEM) is one of the most versatile instruments used to measure the particle size and distribution and to examine fracture surfaces. The SEM consists of an electron gun producing a source of electrons at an accelerating voltage range of 1-40 keV. Electron lenses reduce the diameter of the electron beam and place a small focused beam on the specimen with a spot size of less than 10 nm. The electron beam interacts with the near-surface region of the specimen and penetrates to a depth of about 3µm depending on the accelerating voltage and generates signals to form an image. The smaller the beam size, the better the resolution of the image. The more conductive the material the better it will behave under higher voltages. Higher voltages (15-30kv) are generally used for achieving high resolution at high magnifications, although this can damage the specimen very quickly if it is not highly conductive. Thus, when imaging polymers and ceramics it is appropriate to use voltages below 10 kV. SEM is run under a vacuum to minimize beam interactions with gas molecules which would block the electrons before reach of the specimen in the sample chamber and retard resolution. Non-conductive specimens, such as most polymers, often suffer from variations in surface potential which introduce astigmatism, instabilities, and false X-ray signals. Charging, a condition during which charge accumulates on the surface of a nonconducting specimen causing


excessive brightness, often occurs making it difficult to obtain quality images. We can avoid these issues by using sputter coating on non-conductive samples with a fine gold layer [36].

(f) Fourier Transform Infrared Spectroscopy: Fourier transform infrared spectroscopy (FT-IR) is used to analyze the interaction between the polymer matrix and nanoparticles [1]. This technique is based upon the fact that a chemical substance shows the absorption in the infrared region gives rise to absorption bands called as an IR absorption spectrum, over a wide wavelength range. Usually the signal detected is presented as plots of intensity versus wavenumber (in cm-1). These bands correspond to characteristic absorption of functional groups present in the sample. Using this absorption data, one can identify molecular components and structures and interaction between polymer and nanoparticles. The absorption spectrum is most often compared against a spectrum from a known sample for identification. Absorption bands in the wavenumbers range of 4000-1500 are due to functional groups such as -OH, C==O, N—H, and CH3. The wavenumbers range from 1500-400 cm-1is referred to as the fingerprint region and generally specific to each material [37].

Mechanical Characterizations

(a) Tensile Testing

Tensile testing is the most fundamental type of mechanical test performed on material to measure elastic modulus, ultimate stress, and ultimate strain. In tensile testing, specimen must have two shoulders and a gauge (section) in between them. The shoulders are large and are placed in the grips of movable and stationary fixtures in a screw driven device, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area. The test process involves the applying tension to the sample until it breaks and measures applied load versus elongation of the sample. The testing process requires specific grips, load cell and extensometer for each material and sample type. The grips, used in tensile testing, must properly fit the specimens, and they must have sufficient force capacity so that samples are not damaged during testing. The load cell is a finely calibrated transducer that provides a precise measurement of the force applied. The extensometer is used to measure the stress-strain measurements and tensile tests. Output from the device is recorded in a text file including load and elongation data. Mechanical properties are determined from a stress vs strain plot of the load and elongation data. Tensile testing is a destructive characterization


technique. The American Society for Testing and Materials (ASTM) provides the following relevant standard test methods:

• D638 - Tensile Properties of Plastics

• D3039 - Tensile Properties of Polymer Matrix Composite Materials

(b) Dynamic Mechanical Analysis (DMA)

DMA is a thermal analysis technique that determines elastic modulus, loss modulus, and damping coefficient as a function of temperature, frequency, or time for materials as they are deformed under periodic stress. The method is often used to determine the storage and loss modulus, tan δ, complex and dynamic viscosity of the sample. Further it also measures the storage and loss compliance, glass transition temperatures, creep, and stress relaxation, as well as related performance attributes like rate and degree of cure, sound absorption and impact resistance, and morphology, as well. Samples in DMA, depending on the equipment, can be quite small, in the range of 40 mm X 5 mm X 1 mm. The sample is clamped into movable and stationary fixtures and then enclosed in a DMA chamber. The DMA involves the measuring of complex modulus at low constant frequency varying the sample temperature. Experimental inputs into the equipment include frequency and amplitude of oscillations, static initial applied load, and temperature range. Results are plotted as a function of temperature for elastic modulus, loss modulus, and damping coefficient of the sample. DMA, like tensile testing, is a destructive technique. The American Society for Testing and Materials (ASTM) provides the following relevant standard test methods:

• D4065 - Dynamic Mechanical Properties: Determination and Report of Procedures

• D4092-Terminology Related to Dynamic Mechanical Measurements on Plastics

(c) Nanoindentation

Nanoindentation is a power full technique used to determine the hardness and elastic modulus of materials, including layers and coatings <100 nm thick from experimental readings of indenter load and depth of penetration. Nanoindentation is a specialized indentation test in which forces involved are usually in the mili or micronewton range and the depth in the order of nanometers. Because the sample


surface area and depth requirements are so small, thin film samples are appropriate for this type of testing. Very small material volumes having the order of several tens of nanometers can be accessed with the tip of the nanoindenter and material properties can be evaluated for such a small piece of the material. One of the key factors in analyzing indentation data is the contact area between the indenter and specimen. With nanoindentation, however, the area is on the order of microns and is too small to measure accurately [38-40].

Instead, the depth of penetration into the specimen surface is measured and combined with the known geometry of the indenter to calculate the contact area.

While doing the nanoindentation test, force and displacement are recorded as the indenter tip is pressed into the test material’s surface with a given loading and unloading profile. The results are plotted as load versus displacement. Several studies of nanoindentation on polymers, such as PS and PMMA, have shown larger modulus values than reported by tensile testing or DMA on the same materials and are tend to increase with indentation depth. Unlike tensile testing and DMA, nanoindentation is a non-destructive technique. The American Society for Testing and Materials (ASTM) Task Group E28.06.11 is developing a standard test method for indentation testing.

APPLICATION OF POLYMER NANOCOMPOSITE BASED DEVICES

Electroluminescence in Polymer Nanocomposites and Light Emitting Diodes

Electroluminescence- by electrical excitation has been observed in a wide range of conjugated molecules. It is observed due to radiative recombination of electrons and holes in a material, usually a semiconductor material (Fig. 1). In polymers, it was first reported for poly (p-phenylene vinylene), PPV, as illustrated in Fig. (1) in the 1990s [41]. This phenomenon can be extensively utilized in optoelectronic devices such as light emitting diodes (LEDs). Here the selection of proper material for the device is an important issue. The material for electroluminescence devices should have high injection current densities of both the electron and the holes into the active region of the device, with high probability of carrier recombination. These materials should also have good electrical contacts with the electrodes without any chemical reaction which may reduce the device performance and lifetime.


Fig. (1). Scheme of radiative recombination in a p-n junction. Left area base; right area emitter.

Electroluminescent devices performance is measured by external quantum efficiency (EQE) given by

η= γ ηe ηopt

where γ is the injection coefficient

ηe is the internal quantum efficiency

ηopt is the light output coefficient 20-30% of EQE reaches for the best inorganic semiconductor LEDs. In case of

Fig. (2). The schematic representation of device structure of an ITO/PPV/Al LED.


The process responsible for EL required the injection of electrons from one electrode and holes from the other, the capture of one by another, and the radiative decay of the excited state produced by this recombination process. The ITO layer works as a transparent electrode in the device. This allows the light generated within the diode to leave the device. The top electrode is obtained by thermal evaporation of a metal. LED operation is achieved when the diode is biased to achieve injection of positive and negative charge carriers from opposite electrodes. Capture of oppositely charged carriers by the broader region of the polymer layer can then result in photon emission.

The advantage of polymer nanocomposite materials is that one can control the important properties of the material such as the desired colour emission and their intensity by band gap engineering through chemical synthesis and fictionalization of the material. Numerous efforts are being attempted in the fabrication of electronically pumped polymer lasers. Research group at Cambridge Display Technologies Co. [42] has reported the enhanced operational stability of polymer LEDs. Those LEDs are having a very high propensity to degrade in an ambient atmosphere. It is due to oxygen diffusion into the conjugated polymer matrix and the emitted radiation initiated the photochemical reactions. The operational time of these LEDs has increased to 20,000 h by the use of sealings, protective coatings and special additives which are adequate for most device applications.

(a) Light Emitting Diode Based on Polymer Nanocomposites

The advantages of polymer nanocomposite LEDs are the tailoring of the band gap. This causes the quantum confinement results in, which leads to the possibility of tuning the emission variation based on the nanoparticle size. Colour of the emitted light of these LEDs is voltage dependent. Variation in the ratio of electron hole pairs and the nanoparticles changes the injection conditions from the cathode and anode. This further affects the carriers penetration inside the structure leading to the variation of the colour of the emitted light. Variation in the electron injection and electron penetration depth obtained by the variation in voltage are important for the shift in recombination zones resulting in the emission colour change. It is possible to produce the desirable colour coordinates changing the particle sizes and the thickness of the layers. By using suitable Polymer Nanocomposite material.

The energy level diagram for an ITO/PPV/Al-LED under forward bias is shown schematically in Fig. (2). It has been found that there is a small barrier for hole


injection from the ITO-electrode into the valence band states (or highest occupied molecular orbital, HOMO) [43]. For aluminium as cathode, a significantly larger barrier for electron injection into the PPV conduction band states (or lowest unoccupied molecular orbital, LUMO) is observed.

Fig. (3). Schematic energy level diagram for an ITO/PPV/Al-LED under forward bias, showing the ionization potential (I.P.) and electron affinity (E.A.) of PPV, the work functions of ITO and Al (ϕITO and ϕAl ), and the barriers to injection of electrons and holes (ΔEe and ΔEh).

PPV has an energy gap between π and π* states of about 2.5 eV that produces yellow-green luminescence in a band below this energy, with the same spectrum as that produced by photo excitation (Fig. 3) [44, 45].

(b) Photodiodes and Solar Cells Based on Polymer-Nanocomposites

Recently polymer photodiodes have gained considerable interest of materials scientists and technologists due to the advantage of making large area arrays of addressable photodiodes. These are cheap substitute for video i-cones and MIS-capacitor CCD used in digital video cameras or night vision devices. Flexible organic solar cells are quite inexpensive and environmentally friendly substitutes to their inorganic counter parts. The best result obtained is for the molecular crystals of p-n junction devices formed by perylene and copper phthalocyanine in which the energy efficiency reached to 2%. The main utility of polymer based cells is the feasibility of making large area flexible structures by using low cost processing methods such as doctor blading or solution casting or spin coating methods. The extent of photoluminescence quenching in the nanocomposites


Fig. (4). Photoluminescence spectra of PPV. It is shown that the optically excited luminescence from PPV as measured for a thin film on a glass substrate (free space emission) and from a micro cavity structure (dimensions as shown). Similar micro cavity devices which can be electrically excited are made by forming a layer of ITO onto the distributed Bragg reflector (DBR) mirror before deposition of the polymer layer.

should always be examined before the fabrication of photovoltaic cells. In the photovoltaic devices based on CdS nanoparticles with different sizes in a poly (vinyl alcohol) (PVA) matrix, (which is non conductive). Since PVA is a non -luminescent matrix, quenching in the PVA matrix is more complete, and thus only the luminescence of CdS nanocrystals must be quenched. In case of CdS nanoparticles in the non-conductive and non-luminescent PVA matrix the electron hole separation takes place between neighbouring nanocrystals, by the high local fields [46]. Therefore matrix plays a passive role in the photocurrent generation process. A number of structures studied so far have been depicted in Fig. (4). The nanocomposites with highly p- and n-doped nanoparticles are more favourable for photovoltaic applications than the nanocomposites with particles of one conductivity type due to the following reasons:

i) All the advantages of fractal p-n junction can be utilized.

ii) It is possible to prevent inter diffusion by using various heterojunctions such as II-VI sulfides based solar cells.

iii) Excitons generated in the conductive polymer matrix can be decomposed by introducing p-i-n device structure with extremely high local fields.


iv) The topology of conduction networks formed by p- and n-type particles provide a gradual increase in the connectivity in the built in field direction. This minimizes the amount of network dead ends in the nanoparticle chains.

v) Larger area and flexibility can be achieved in these p-n nanocomposite photovoltaics cells.

A system of p-type Cu2S nanoparticles mixed with n-type CdS, doped with indium was investigated which was in order to increase carriers concentration by in situ synthesis in PVA matrix. Cu2S and CdS placed in contact form a hetero junction which delivers 12% of the power conversion efficiency when made by liquid epitaxy. By preparing the nanocomposites of Cu2S and CdS in a polymer matrix, diffusion was prevented, thus enhancing the long term stability of the photovoltaic cells significantly.

Fig. (5). Structure of p-n nanojunction based photovoltaic cells.

The best photovoltaic properties were observed by the CdSe/Polymer nanocomposites having 90 wt.% of CdSe. When illuminated by the solar spectrum energy conversion efficiency was found to be around 0.1%, which is not as high


as the values exhibited by the other inorganic composite materials based solar cells. However, the efficiency of the system can be significantly enhanced by adding dye molecules to the polymer matrix with optimization of the topology of the semiconductor nanoparticles networks.

The chains of CdS nanoparticles can also act as anode dispersed into the polymer matrix, depending on the possibility of matching the valence band levels of the polymer matrix and the semiconductor nanoparticles. In the case of the PANI/CdS system, a hole can be injected into the semiconductor nanoparticle which can charge the particle positively. Positively charged particle can show larger hopping probability compared to a neutral one. Thus it can be transferred to the anode.

(c) Polymer Nanocomposites as Magnetic Storage Materials

Polymer nanocomposites have been also extensively studied as high capacity magnetic storage media and elements of nanoscale sprintonics.

Immobilized particles in the polymer such as iron ions can react with NaOH followed by in a reducing atmosphere. This leads to the formation of mixed iron oxide ferromagnetic nanoparticles. Iron oxide system being a single domain one, by varying the concentration of particles in polymer we can easily find out the effect of interparticle distance on magnetic properties. Another interesting parameter is the magnetic anisotropic energy. This property can be monitored by growing particles in uniaxially stretched polymer matrices.

Polymer mediated self assembly of magnetic colloids on flat substrates to form nanocomposite thin films was experimented by Sun and the IBM group. Ligand exchange of poly (2-vinylpyrrolidone), or poly (ethyleneimine) onto Co, or FePt nanoparticles was reported to form assemblies as magnetic storage media [47].

(d) Characterization of Magnetic Materials

By magnetization measurement information about the magnetic moments can be obtained. When the measurements are conducted under the constant magnetic field the information concerning the low frequency dynamics is obtained if pulsed magnetic field is used.

Mossbauer spectroscopy is a very sensitive method. This gives the information about magnetic momentum dynamics and also i the magnetic moment relaxation dynamics information within the time scale of 10-8 s.


Magnetic force microscopy (MFM) provides the information about local fields by using an AFM tip made of ferromagnetic material in the tapping mode. MFM maps the local magnetic fields of the nanoparticles in a polymer matrix.

CONCLUSION

For the applications of polymer nanocomposites numerous cases with large effective surface area have been exploited ranging from antimicrobial coatings, to optical and magnetic materials, where surface plasmon resonances and the effective suppression of eddy currents are have proved to be advantageous respectively. The huge drop of the resistivity near the percolation threshold can be used for sensing and switching applications. Various opportunities arising from the nanoscale metallic fillers have also been exploited. Usually it is difficult to control filling factor and filling factor profile, particle size, alloy composition and other parameters. Physical vapour deposition has proved to be successful due to its good process control.

The development of polymer-nanocomposites based devices can be summarized based on the following facts. Compared to the microelectronic circuitry, which operates on the basis of ordered functional elements such as logical gates, polymer nanocomposites, can utilize the properties of disordered systems. The properties of disordered systems, such as percolation threshold, structural change and the dependence of the topology of the interconnected chains on the filler concentration, synthesis of the infinite clusters of conductive network with the programmable topology, are used for LEDs, photodiodes, solar cells and in the fabrication of gas sensors. The properties of nanoparticles, quantum dots, can be utilized by varying the nanoparticle size during the synthesis thereby monitoring electrophysical, optical and magnetic properties. The aspect of the device applications of polymer-nanocomposites can be broadened by modifying the nanoparticle surface, doping and filling the matrix with nanoparticles of both p- and n-type.

FUTURE PERSPECTIVES OF NANOCOMPOSITE-BASED DEVICES

Design of polymer nanocomposites by ordered arrays of nanoparticles in the polymer matrix [48] has proved to be one of the most upcoming fields. It is observed that ordered two-dimensional arrays of nanoparticles are obtained by simultaneous evaporation of sulfides along with the chemical vapour deposition of polymers. Achieving narrow particle size distribution in material can make the


material useful opportunities for the construction of electronic circuits. For nanocomposite circuit boards, the first possibility could be to convert non-conductive poly (paraxylylene), to semiconducting poly (paraphenylenevi-nylene) by heating with an electron beam. The second option can be light-induced doping of polyaniline-based nanocomposites, thereby changing the conductivity of the polyaniline from an insulating phase to a semiconductor or metallic one depending on the extent of doping. The above two methods can be used to design nanoscale circuits which includes conductors, semiconductors and isolating areas. In such circuits tunnel resonant diodes and single electron transistors are usually used as structural elements, similar to the devices described elsewhere [49]. In a report single electron transistor was made operational by introducing one CdSe nanoparticle using e- beam lithography [49]. It can be clearly understood from above discussions that colloid chemistry could be used as a means for producing a single-nanoparticle device, such as a tunnel resonant diode. It is also possible to prepare two- or three-dimensional ordered systems of nanoparticle by templating them to DNA molecules [50], bacterial self-assembled layers [51] or micelle-like structures [49]. By using these templates, one can assemble quite complex arrays of functional nanoparticles in electronic and optolelectronic applications. Polymer nanocomposites have been also used for neutral networks applications where nanoparticles are used as knots in a network, and connections between these knots can be formed by local electrochemical doping, which transforms the polymer from an insulating state into a conductive state. Due to reversible nature of electrochemical doping, one can either establish or break connections between knots, allowing the learning function and adaptivity of the networks.

ACKNOWLEDGEMENTS

SG Acknowledges Director CSIR-IICT for her support. She also acknowledges DST (SERB) and CSIR (Specs and Multifun) India for funding support.


The author confirms that this chapter contents have no conflict of interest.

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78 Light Harvesting Nanomaterials, 2015, 78-99


CHAPTER 4

Visible-Light Photocatalytic Organic Synthesis: Localized Surface Plasmon Resonance-Driven Oxidation Processes Using Au-TiO2 Nanocoupling Systems Shin-Ichi Naya1 and Hiroaki Tada*,1,2

1Environmental Research Laboratory, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan; 2Department of Applied Chemistry, School of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan

Abstract: This chapter reviews a newly developed visible-light consisting of gold nanoparticles (NPs) and titanium(IV) oxide (Au/TiO2), the so-called “plasmonic photocatalyst”. Unlike the hitherto studied semiconductor photocatalysts that are activated by the band gap excitation, the redox ability of the “plasmonic photocatalyst” is induced by the excitation of the localized surface plasmon resonance (LSPR) of Au NPs. Au/TiO2 with strong and broad visible absorption well matching the solar spectrum is a very promising photocatalyst for the solar chemical transformations. Subsequently to the introduction, Section 2 describes the fundamentals of the “plasmonic photocatalyst” in the following of what the LSPR is, the photo-induced interfacial electron transfer, and coupling between LSPR and interband transition of Au NPs. Section 3 summarizes the Au/TiO2-photocatalytic oxidation processes as organic synthesis, degradation of organic pollutants and water splitting reported so far. Section 4 deals with the important factors affecting the visible-light activity (support effect, Au particle size effect, and reaction field effect) for the design of highly active “plasmonic photocatalysts”.

Keywords: Conduction band, green technologies, photocatalysis, TiO2 oxidation processes, valence band, UV-light-activity.

1. INTRODUCTION

The development of “green technologies” for producing chemicals is a key to realizing the sustainable society. The ideal synthetic process should satisfy the following requirements: (i) no emission of CO2 and by-products, (ii) utilization of

*Address correspondence to Hiroaki Tada: Department of Applied Chemistry, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan; Tel: +81-0-6721-2332; Fax: +81-6-6727-2024; E-mail: [email protected]

Visible-Light Photocatalytic Organic Synthesis Light Harvesting Nanomaterials 79

indispensable energy source such as sunlight, (iii) high atom efficiency, and (iv) safe process without use of harmful reagents. A possible approach to achieve this is the application of TiO2 photocatalysis to organic synthesis. Since the discovery of the Honda-Fujishima effect in 1972 [1], a great deal of endeavor has continuously been devoted to the researches. In the TiO2 photocatalytic reactions, reductions and oxidations are concomitantly induced by the conduction band (CB) electrons and valence band (VB) holes, respectively, on the surface (Scheme 1). From a viewpoint of organic synthesis, the reactions can be conveniently divided into “reductions” and “oxidations” according to the targeted products. The CB electrons with a mild reducing ability can be utilized for the “reductions”. Further, the loading of Au nanoparticles (NPs) on TiO2 (Au/TiO2) can greatly increase the UV-light-activity and selectivity for some kinds of reductions due to the charge separation enhancement resulting from the interfacial electron transfer (IET) from TiO2 to Au [2, 3]. Also, TiO2 shows photocatalytic activities for many important “oxidations” including epoxidation of alkenes, oxidations of alkanes and alcohols to ketones, and hydroxylation of aromatic compounds using O2 and H2O as the oxygen source [4]. However, the VB holes have so strong oxidation power as to cause complete degradation of most organic compounds to CO2 and H2O. Consequently, it seems to be difficult to achieve high selectivity in the TiO2-photocatalyzed “oxidations”, whereas TiO2 is promising as an environmental purification catalyst. In addition, the total reaction efficiency is severely limited by the fact that TiO2 only absorbs UV-light occupying 3-4% of the sunlight.

Scheme 1. TiO2 photocatalytic reaction.

On the other hand, Au NP strongly absorbs visible-light due to the localized surface plasmon resonance (LSPR) derived from the collective oscillation of

80 Light Harvesting Nanomaterials Naya and Tada

conductive electrons. The LSPR-enhanced strong local electric field near the particle surface entails the surface-enhanced Raman scattering and infrared absorption effects [5, 6]. Recently, the LSPR-excitation of Au/TiO2 has been shown to cause the IET from Au to TiO2 [7]. As a result, a mild oxidation power is induced by the lowering in the Fermi energy (EF) of Au NPs (Scheme 2) [8]. Also, Au/TiO2 has a broad absorption in the whole visible range with a peak located near 550 nm, which well fits the solar spectrum (Fig. 1). These features in addition to those of TiO2 have opened up new applications of Au/TiO2 to the “partial oxidations” of organic compounds as organic synthesis. In this system, Au NP plays a role in an active nano-photoanode with the EF controlled by light,

Scheme 2. Au/TiO2 LSPR-PC reaction.

Fig. (1). UV-vis absorption spectra of Au/TiO2 and TiO2, and Solar spectrum.


while TiO2 acts as a support maintaining Au NPs with clean surfaces in a highly dispersion state, a tuner for the absorption spectrum of Au NPs, and an electron acceptor. It is worthy noting that the Au/TiO2 LSPR-driven photocatalytic (LSPR-PC) reactions utilize the mild oxidation power of LSPR-excited Au NPs and the visible-light as an energy source, which is crucial for achieving efficient “green oxidation processes”.

This chapter describes the oxidation processes as organic synthesis using Au/TiO2 as the LSPR-PC under visible-light irradiation. First section outlines the fundamentals of the Au/TiO2 LSPR-PC. Second section reviews the LSPR-PC oxidations reported so far. Last section discusses the parameters affecting the activity for the LSPR-PC in detail.

2. FUNDAMENTALS OF LSPR-DRIVEN OXIDATION PROCESSES

2.1. Localized Surface Plasmon Resonance of Au/TiO2

Plasmon is the quantized collective oscillation of conductive electrons. In bulk materials, plasmon is a longitudinal wave, which can not be excited by a light with transversal oscillation mode. On the other hand, the surface plasmon containing a transverse wave component can couple with the irradiated light. There are two types of the surface plasmon, i.e., propagating surface plasmon and localized surface plasmon. The propagating mode occurs in smooth surface of metals, and the couple of the surface plasmon with electromagnetic wave is called the surface plasmon polariton. The localized surface plasmon mode appears at a nanometer-sized metal particle or a nano-structured metal surface. Under the resonance conditions, the particle polarization induced by the incident light oscillates in the same frequency of the light (Scheme 3). Thus the localized

Scheme 3. Localized surface plasmon resonance.


surface plasmon resonance (LSPR) causes the strong local electric field near the particle surface. Mie theory exactly describes the optical response of a spherical particle [9-11]. In the region where the particle size d is much smaller than the wavelength (λ) of the incident light, the dipole term mainly contributes to the particle polarization since the retardation effect can be neglected. Thus, the quasi-static approximation provides the absorption cross-section (Cabs) of the particle by eq. 1.

(1)

where is the complex dielectric function of the particle, ε m is the dielectric constant of the surrounding medium, d is particle size (= 2r), and λ is the wavelength of the incident light.

Eq. 1 indicates that the maximum value of Cabs is obtained by the minimized denomination under eq. 2.

(2)

Eq. 2 is called the Fröhlich condition, and the LSPR peak appears at that wavelength. In air (ε m = 1), the Fröhlich condition requires ε1(λ) = -2, which is satisfied for Au NPs at λ = 490 nm. Eq. 2 indicates Au NP-LSPR peak position explicitly depends on the dielectric constant of the supporting medium. Also, the Au NP size (d) effect implicitly influences the LSPR peak through the dependence of the complex dielectric function of Au NP [12]. Further, the LSPR shape and position are influenced by the surface-adsorbents and the particle shape and positions [13].

2.2. Interfacial Electron Transfer Between Au NP and Metal Oxides (UV Light vs Visible-Light Irradiation)

Visible-light irradiation to Au/TiO2 excites the Au NP-LSPR mode to trigger the electron transfer from Au to TiO2 [7]. As a result of the lowering in the Fermi energy (EF) of Au NPs [8], a mild oxidation power is induced on the Au NPs. A chemically comprehensive way to understand the direction of the IET is viewing it through the reversible reactions. An excellent example is a conversion between thiol and disulfide [14]: Au/TiO2 was added to a solution of 2-mercaptopyridine


(PySH). After the suspension was stirred in dark for 1 h, visible-light (λ > 420 nm) was irradiated under aerobic conditions. The electronic absorption spectral change of the filtered solution with irradiated time (tp) is shown in Fig. (2). With increasing tp, the absorptions of PySH at 371 and 288 nm weaken, and a new absorption of 2,2’-dipyridyl disulfide (PySSPy) grows at 230 nm with isosbestic points located at 198, 216, and 267 nm. Under anaerobic conditions and under aerobic conditions using TiO2 without Au NPs loading, no reaction proceeded. Clearly, O2 acts as an acceptor of electrons injected into the CB(TiO2) from Au NPs. Fig. (3) shows the concentrations of PySH ([PySH]) and PySSPy ([PySSPy]) as a function of tp. Upon prolonging tp, the [PySH] decreases, while the [PySSPy] increases. The turnover number (the molecule number of PySH consumed / the number of Au surface atoms) reached 19 at tp = 13 h. The value of ([PySH] + 2 × [PySSPy]) is almost constant, indicating that the LSPR-PC oxidation of PySH to PySSPy proceeds stoichiometrically (eq. 3).

Au/TiO2, Vis (λ > 420 nm) 2PySH ＋ 1/2 O2 → PySSPy ＋ H2O (3)

Fig. (2). Change in electronic absorption spectra of a PySH solution with Vis-irradiation (λ > 420 nm) in the presence of Au/TiO2.

This reaction apparently follows the zero-order kinetics toward [PySH], which results from the supply of the sufficient amount of PySH to the Au surface due to the extremely high affinity of Au NPs for sulfur-containing compounds [15].


Fig. (3). Plots of concentrations of PySH and PySSPy and total molecular amounts derived from PySH as a function of tp.

No over-oxidized product such as sulfoxide suggests the smooth desorption of the product from the catalyst surface. Thus, high efficiency and selectivity of this reaction can be explained within the framework of the idea of “reasonable delivery photocatalytic reaction systems (RDPRS)” [2].

On the other hand, the UV-light irradiation to Au/TiO2 excites the electrons in the VB to the CB of TiO2, which are rapidly transferred from the CB(TiO2) to Au. As a result of the rise in EF of Au NPs, a mild reducing ability is induced on Au NPs. Interestingly, the direction of the reaction is reversed by UV-light irradiation (λ > 300 nm) to Au/TiO2, i.e., the catalytic reduction of PySSPy to PySH proceeds (eq. 4) [16].

Au/TiO2, UV (λ > 300 nm) PySSPy ＋ H2O → 2PySH ＋ 1/2 O2 (4)

Consequently, the reversible formation and cleavage of S-S bond can be achieved photocatalytically by the selection of irradiation wavelength (Scheme 4). Reversible formation and cleavage of S-S covalent bond, i.e., the thiol-disulfide conversion, is one of the most important reactions in the biological processes such as transformations of the cysteine-cystine and dihydro lipoic acid-lipoic acid as well as the controls of the secondary and tertiary structures in protein [17]. The


conventional methods for the conversion between thiol and disulfide needs stoichiometric amounts of oxidant and reductant. Thus, the photocatalytic conversion may also be important from the viewpoint of the “green chemistry”.

Scheme 4. Light wavelength-switchable reversible photocatalytic reactions.

2.3. Coupling Between LSPR and Interband Transition of Au NPs

Au NP is known to have an absorption in the whole visible range due to the interband transitions from Au 5d band to 6sp and within 6sp band in addition to the LSPR absorption the maximum around 550 nm [10, 11]. Recent studies using time-resolved absorption spectroscopy have clarified that the excitation of the interband mode incurs broadening and bleaching of the LSPR absorption [18-20]. The reciprocal of the LSPR line width (Γ−1) is approximately proportional to the LSPR-lifetime (τ) in eq. 5.

τ ∝ Γ −1 (5)

Broadening of the LSPR is accompanied by the simultaneous excitation of the interband mode, which further shortens the LSPR-lifetime. F(R∞) denotes the Kubelka-Munk function corresponding to the absorption intensity. Thus, F(R∞) /ω is proportional to the imaginary part of the particle dipole polarizability. By the Pakizeh and Langhammer analysis [21, 22], the value of F(R∞)/ω of Au/A-TiO2 (d = 3.6 nm) was calculated as a function of ω. The spectrum can be characterized by the intrinsic Fano line-shape function expressed by eq. 6 [23].


(6)

where q is Fano asymmetry parameter, ω0 is effective resonance position, Γ is line width, and A is constant.

Right-hand side of eq. 6 consists of symmetric Lorentzian term, constant term, and residue term, which correspond to the LSPR mode, the interband mode, and the Fano interference mode, respectively [21]. The asymmetric profile stems from the intrinsic Fano interference between the discrete LSPR mode and the continuum absorption due to the interband mode. Fig. (4) shows the three functions obtained by the best-fitting of Au/TiO2 (d = 3.6 nm) spectrum with eq. 6. The sign of the Fano interference term changes from negative to positive with increasing ω through the LSPR peak (ω0). The positive value of the Fano interference term at the higher energy side of the LSPR peak shows the strong

Fig. (4). Optical response of Au/TiO2 (d = 3.6 nm). Absorption of Au/TiO2 (solid line), the fitting curve by eq. 6 (bold broken line), the LSPR term in eq. 6 (blue line), the interband term (red line), and the Fano interference term (green line).


coupling between the LSPR mode and the interband mode. As a result, the absorption maximum shifts towards higher energy (2.33 eV) from the LSPR absorption maximum (2.10 eV). Fig. (5) shows plots of Γ −1 as a function of q: A-TiO2 and R-TiO2 denote anatase-TiO2 and rutile-TiO2, respectively. In this case, the increase in q means the decrease in the coupling of the LSPR mode with the interband transition mode. The increase in Γ−1 with increasing q suggests the prolongation of the LSPR-lifetime by the decoupling between the two modes. The elongation of the LSPR-lifetime can result in the increase in the efficiency of the LSPR-driven IET from Au to TiO2 (section 4.1) [24].

Fig. (5). Plots of Γ -1 vs q.

3. OXIDATION PROCESSES

This section reviews the LSPR-PC oxidations reported so far. Tables 1 and 2 summarize LSPR-PC selective oxidations [14, 24-29], and degradations of organic pollutants [25, 26, 30-36] and water splitting [37], respectively. The pioneering work by Kowalska et al. reported the Au/TiO2-LSPR-PC selective oxidation of 2-propanol to acetone, clarifying that the reaction is initiated by the excitation of Au NP-LSPR from the analysis of the action spectrum [25, 26]. The first report on the LSPR-PC oxidation suitably referred to as “organic synthesis” is probably the oxidation of PySH to PySSPy as described in Section 2 [14]. The reaction afforded modest yield (58%) with high selectivity (96%). Also, the


Table 1. Au/TiO2 LSPR-PC selective oxidations.

Reactant Product Q.E. (λ ex)a Yield Selectivity Conditions Ref.

2-PrOH acetone 0.025%

(550 nm) (0.32

µmol / h) ---

H2O, Xe 150 W λ > 450 nm, 298 K

[25, 26]

PySH PySSPy --- 58%

(25 h) 96%

H2O, Xe 300 W λ > 430 nm, 298 K

[14]

cinnamyl alcohol

cinnamaldehyde

0.6% (585 nm)

50% (6 h)

95% H2O, Xe 300 W

λ > 520 nm, 298 K [27]

benzyl alcohol

Benz- aldehyde

--- 16% (6 h)

> 99% H2O, Xe 300 W

λ > 520 nm, 298 K [27]

benzene phenol --- 62% (7 h)

96% H2O, Xe 150 W λ > 420 nm

[28]

benzene phenol --- 61% (3 h)

89% H2O, Xe 300 W λ > 400 nm

[29]

oxidation of cinnamyl alcohol and benzyl alcohol derivatives with excellent selectivity was reported [27]. Aromatic aldehydes are easily over-oxidized to carboxylic acids especially in H2O. The high selectivity owes to the characteristic of the LSPR-PC reactions (or the LSPR-induced mild redox abilities). In heterogeneous catalysis, the selective oxidation of benzene to phenol is one of the most challenging processes. Interestingly, the LSPR-PC oxidation of benzene was significantly enhanced to afford phenol in 62% with 96% selectivity by addition of phenol [28, 29].

At present, purification of water and air is an important subject in chemical technology. The LSPR-PCs can also be applied to degradations of various organic pollutants (Table 2). In both gas- and liquid-phases, the reactions proceeded with high yields. On visible-light irradiation, tert-butyl methyl ether was partially decomposed to yield a mixture of iso-butyl alcohol and tert-butyl alcohol probably due to the mild oxidizing power [30]. Also, the degradation of phenol proceeds 85% with the generation of benzoquinone (8%) [31]. On the other hand, acetic acid and formic acid were completely mineralized [32-34]. Au is known to


Table 2. Au/TiO2 LSPR-PC degradations of organic pollutants and water splitting.

Reactant Product Q.E. (λ ex)a Yield Selectivity Conditions Ref.

tBuOMe decomp. --- 80%

(2.5 h) ---

H2O, Kr lamp λ > 495 nm

[30]

phenol (benzo-

quinone) ---

85% (3 h)

8% H2O, Xe 400 W

λ > 450 nm, 298 K [31]

CH3CO2H decomp. --- (0.8

µmol / h) ---

H2O, Xe 150 W λ > 435 nm, 298 K

[25, 26]

HCO2H decomp. --- 87% (2 h)

--- gas, 293 K

400 < λ < 500 nm, [32]

HCO2H decomp. --- 9%

(6 h) ---

H2O, Xe 150 W λ > 520 nm, 298 K

[33, 34]

Soman decomp. --- 100% (2 h)

--- gas, Ne 200 W λ > 420 nm

[35, 36]

VX decomp. --- 100% (2 h)

--- gas, Ne 200 W λ > 420 nm

[36]

sulfer mustard

decomp. --- 100% (2 h)

--- gas, Ne 200 W λ > 420 nm

[36]

H2O H2 O2

7.5 % (560 nm) 5.0 % (560 nm)

(2.7 mL/h) (1.9 mL/h)

H2O, Xe 220 W λ > 400 nm, 311 K

[37]

aQuantum efficiency (excitation wavelength).

have large affinity for sulfur and halogens. By taking advantage of this nature, chemical warfare agents containing sulfur or halogens (Soman, VX, and sulfur mustard) was successfully detoxified [35, 36].

If H2O can be split into H2 and O2 under illumination of sunlight, the combination with fuel cell provides a completely sustainable energy system. Very interestingly, Au/TiO2 exhibits visible-light-induced catalytic activity for water splitting. By using EDTA and Ag+ as sacrificial agents, H2 and O2 evolved from H2O with very high quantum efficiencies, respectively [37].


4. IMPROVEMENT OF THE ACTIVITY OF THE LSPR-ACTIVATED PHOTOCATALYSTS

4.1. Support Effect

This section deals with effective approaches to increase the activity of LSPR-PCs. Firstly, the way for the enhancement of the LSPR itself is discussed. As suggested in section 2.3, the decoupling between the LSPR and the interband transition modes of Au NPs is a key to increasing the photocatalytic activity. The LSPR peak position of Au NPs significantly redshifts with increasing the permittivity (ε) of the supporting medium [10-12], while the interband transition intensifies with decreasing wavelength almost independently of ε. Thus, the reduction of the coupling by loading Au NPs on a support with high permittivity allows us to expect enhancing catalytic activity.

Rutile-TiO2 (R-TiO2) has a very large permittivity (ε = 114) that is more than twice of anatase-TiO2 (A-TiO2) (ε = 48) [38], whereas they have the identical chemical composition and comparable band energy positions. The support effect on the efficiency of Au/TiO2-LSPR-excited chemoselective oxidation of alcohols to carbonyl compounds has been studied [39]. To vary Au particle size (d/nm) with its loading amount maintained constant, Au/A-,R-TiO2 was synthesized by the deposition–precipitation (DP) method where both heating temperature and time were altered [40]. Surprisingly, the Au/R-TiO2 system exhibited much higher activity for cinnamyl alcohol oxidation as compared with the Au/A-TiO2 system, and the activity ratio (TON(Au/R-TiO2)/TON(Au/A-TiO2)) exceeded 3 at d ≈ 5.0 nm. The quantum efficiencies (Φ, molecules produced/incident photons) for the Au/R-TiO2 and Au/A-TiO2 systems were calculated to be 1.4 × 10-3 at λ = 585 ± 15 nm and 0.33 × 10-3 at λ = 555 ± 15 nm, respectively, by assuming a two-electron process for the oxidation.

As shown by the UV-vis absorption spectra in Fig. (6), the LSPR peak of Au/R-TiO2 is present at longer wavelength by ca. 40 nm as compared with that of Au/A-TiO2 at comparable d. The shape of the LSPR absorption for Au/A-TiO2 has a more asymmetric character than that for Au/R-TiO2, i.e., the absorption intensifies at the shorter wavelength side (400-500 nm) of the LSPR peak. The reciprocal of the LSPR line width (Γ−1) also increases with increasing size d (section 4.2). Previous density functional theory calculations and experimental results indicated that in the Au/TiO2-catalyzed CO oxidation, the Au-TiO2 interfaces and/or low-coordinated surface Au atoms can become the active sites [41-43]. To rule out the


Fig. (6). Diffuse reflectance UV- vis spectra of R-TiO2 and Au/R-TiO2 as well as A-TiO2 and Au/A-TiO2.

effect on the number of the catalytically active site, TON (the reacted molecule number/ the number of Au surface atoms) per unit active site (TON site-1) is plotted vs Γ−1 in Fig. (7). A clear positive correlation suggests that the elongated LSPR lifetime increases the efficiency of the LSPR-excited IET from Au NPs to TiO2 to enhance the activity of Au/TiO2. Consequently, we can summarize this support effect as follows (Scheme 5): Replacing of A-TiO2 by R-TiO2 with much higher permittivity as a support of Au NPs redshifts the LSPR absorption. The reduction in the coupling between the LSPR and the interband modes suppresses the Fano interference [24]. The resulting elongation of the LSPR lifetime favors


the interfacial electron transfer from Au NPs to TiO2. Consequently, the visible-light-reactivity of Au/TiO2 for the oxidations remarkably increases.

Fig. (7). Plots of TON site-1 vs Γ-1.

Scheme 5. Support effect.

In summary, under the prerequisite that the efficient IET is possible, the solid with high permittivity is advantageous as the support of Au NPs to decouple the interaction between the LSPR and interband transition modes, and increase the activity of the LSPR-PCs.


4.2. Au Particle Size Effect

This section discusses the Au particle size (d) effect on the activity of the LSPR-PCs. The size dependence of LSPR properties has well been studied. In the particle size region below the electron mean free path in Au (~30 nm), the Γ is expressed by eq. 7 due to the boundary effect [12].

Γ = Γ0 + A × vF / d (7)

where Γ0 is the LSPR-linewidth without interface damping, vF is Fermi velocity, and A is constant.

Eq. 7 indicates that increasing d sharpens the LSPR absorption to lengthen the LSPR-life time [12]. As a result, the efficiency of the LSPR-induced IET from Au to TiO2 would increase, whereas the number of active site sharply decreases [41,43].

Fig. (8) shows the TONs for two selected reactions as a function of d. In the Au/A-TiO2-LSPR-PC oxidation of PySH, the TON increases with increasing d [14]. This trend can be accounted for in terms of the increases in the absorption intensity and the LSPR-life time. Due to the large affinity of Au for sulfur, the sufficient amount of PySH is adsorbed on the Au NP-surface, which would veil

Fig. (8). Turnover number (TON) of the PySH consumed at tp = 24 h and the cinnamaldehyde generation at tp = 6 h as a function of d.


the decrease in the active sites on the catalyst surface. In contrast, in the Au/R-TiO2-LSPR-PC oxidation of cinnamyl alcohol, a volcano-type profile is observed with a maximum activity reached at d ≈ 5 nm [39]. In this case, the number of the active sites can also be a governing factor for the activity because of the relatively lower affinity of Au NP for alcohols. On increasing d, the number of the active sites decreases to lower the catalytic activity, whereas both the LSPR absorption intensity and LSPR-life time increase. The optimum d value would be determined by the balance between them. When the support is replaced by A-TiO2 in the same reaction, the factors encounter so that the TON is almost independent of d [27].

Fig. (9). Plots of Ph-C=C-CHO concentrations at tp = 6 h (a) and adsolubilization amounts of Ph-C=C-CH2OH (b) vs Csurf.

In this manner, the activity of the LSPR-PC oxidations strongly depends on the Au particle size. According to the type of reactions, the Au particle should be optimized by taking the LSPR absorption intensity and the LSPR-life time as well as the number of catalytically active sites.

4.3. Reaction Field Effect

In view of the idea of RDPRS [2], the sufficient supply of the alcohol to the oxidation sites on the Au NP surface and the prompt separation of the product from the reaction field is the clue to achieving high activity and selectivity. The additive effect of trimethylstearylammonium chloride (C18TAC) on the rate of the cinnamyl alcohol (Ph-C=C-CH2OH) oxidation was examined. Fig. (9a) shows plots of the concentration of cinnamaldehyde (Ph-C=C-CHO) produced at tp = 6 h vs C18TAC concentration (Csurf) [27]. The reaction is drastically enhanced at


Csurf = 0.10 mM near to the critical admicelle concentration (CAMC) [44], while no decomposition of C18TAC occurred. Ph-C=C-CHO was generated in 95.3% selectivity (4.7 % cinnamic acid) with 24% conversion at tp= 6 h. The Φ value at λ = 555 ± 15 nm attained to 2.0 × 10-3 at Csurf = 0.10 mM. The Ph-C=C-CHO concentration sharply increases in the region of Csurf from 0.02 to 0.10 mM, decreasing at Csurf > 0.10 mM. The amount of Ph-C=C-CH2OH dissolved into the admicelle (adsolubilization amount per amount of TiO2, Γsol) from the bulk solution was determined as a function of Csurf (Fig. 9b). The Γsol also reaches a maximum at Csurf = 0.10 mM, and at Csurf > 0.10 mM, the competitive Ph-C=C-CH2OH incorporation into the micelles in the bulk solution decreases the value. At Csurf = 0.10 mM, the ratio of the Ph-C=C-CH2OH concentration in the admicelle to that in the bulk solution is estimated to reach as much as 6000 by assuming that the admicelle thickness is approximately 5 nm. The positive correlation between the reaction rate and Γsol indicates that the effect of the admicelle concentrating Ph-C=C-CH2OH in the vicinity of the Au surfaces greatly accelerates its oxidation. Another intriguing point is that the amount of Ph-C=C-CHO produced at tp = 6 h (24 µmol at Csurf = 0.1 mM) is larger than that of Ph-C=C-CH2OH adsolubilized before reaction (0.12 µmol at Csurf = 0.1 mM) by a factor of ca. 200. Further, this heterosupramolecular system was applied for several alcohols (Table 3). The remarkable acceleration of the chemoselective oxidations in selectivity > 99% for all the alcohols tested shows the wide effectiveness of this methodology.

Table 3. Results of alcohol oxidation with and without surfactant.

Alcohol a Carbonyl Compound Formationb Ratio d

With C18TAC c Without

4-HO-C6H4CH2OH 62.2 2.1 29.6

4-MeO-C6H4CH2OH 43.0 12.1 3.6

C6H5CH2OH 27.4 8.3 3.3

4-Cl-C6H4CH2OH 18.9 3.3 5.7

C6H5CH(OH)Me 31.2 9.3 3.4 aC0 = 500 µM. bµM. cCsurf = 0.1 mM. dEnhancement ratio by addition of C18TAC.

On the basis of these results, the essential mechanism on this heterosupramolecular visible-light photocatalytic reaction can be summarized as follows (Scheme 6): The C18TAC admicelle is formed on the Au/A-TiO2 surfaces at CAMC, and the alcohol is incorporated into the hydrophobic nanospace of the admicelle from the water phase to be concentrated near the Au NP surfaces.


Visible-light excitation of the Au NP-LSPR of Au/TiO2 triggers the electron transfer from Au to TiO2. Au NPs with lowered Fermi energy oxidize the alcohol on their surfaces, while O2 is reduced by the electrons accumulated in TiO2. The resulting hydrophilic intermediates like R-C=O+H are spontaneously transported into the water phase to yield carbonyl compound. As a result of the consumption of the alcohol in the reaction field, it is further supplied from the water phase through the passive transport.

Scheme 6. Heterosupramoleculer system.

CONCLUDING REMARKS

Au/TiO2 exhibits high photocatalytic activities under both UV-light and visible-light irradiation. This chapter discusses its visible-light-activity particularly with the emphasis placed on the application to organic synthesis. The visible-light-activity is induced by the excitation of the LSPR of Au NPs, while the UV-light-activity originates from the interband transition of TiO2. This new LSPR-PC is still in its infancy, but its wide possible application to organic synthesis such as the oxidations of alcohols to aldehydes, thiol to disulfide, and benzene to phenol is currently being revealed, in addition to degradation of organic pollutants and water splitting. Timely, we have shown that the activity of the Au/TiO2 LSPR-PCs strongly depends on the kind of support, Au particle size, and the reaction field. The present information about the design for the efficient LSPR-PCs should widely contribute to the field of LSPR-applied technology named as “Plasmonics”.


ACKNOWLEDGEMENTS

None declared.



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CHAPTER 5

Microstructures and Photovoltaic Properties of C60-Based Solar Cells with Copper Oxides, CuInS2, Phthalocyanines, Porphyrin, Diamond and Exciton-Diffusion Blocking Layer Takeo Oku*,1, Akihiro Takeda1, Akihiko Nagata1, Ryosuke Motoyoshi1, Kazuya Fujimoto1, Tatsuya Noma1, Atsushi Suzuki1, Kenji Kikuchi1, Tsuyoshi Akiyama1, Balachandran Jeyadevan1, Jhon Cuya1, Yasuhiro Yamasaki2 and Eiji Ōsawa3 1Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, 522-8533, Japan; 2Orient Chemical Industries Co., Ltd., Department of New Business, Neyagawa, Osaka, 572-8581, Japan; 3NanoCarbon Research Institute, Ltd., 3-15-1 Tokida, Ueda, Nagano, 386-8567, Japan

Abstract: C60-based bulk heterojunction solar cells were fabricated, and the electronic and optical properties were investigated. C60 were used as n-type semiconductors, and copper oxides, porphyrin, phthalocyanines, CuInS2 and nano-diamond were used as p-type semiconductors. An effect of exciton-diffusion blocking layer of perylene derivative on the solar cells between active layer and metal layer was also investigated. Optimized structures with the exciton-diffusion blocking layer improved conversion efficiencies. Electronic structures of the molecules were investigated by molecular orbital calculation, and energy levels of the solar cells were discussed. Nanostructures of the solar cells were investigated by transmission electron microscopy, electron diffraction and X-ray diffraction, which indicated formation of mixed nanocrystals.

Keywords: Copper oxides, CuInS2, diamond, exciton-diffusion blocking, fullerene, microstructure, phthalocyanine, porphyrin, solar cell.

1. INTRODUCTION

Solar cells are clean energy devices that provide electricity, and produce no carbon dioxide semi-permanently and hazardous waste gases causing global warming. Silicon (Si) solar cells have a high conversion efficiency and long

*Address correspondence to Takeo Oku: Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, 522-8533, Japan; Tel: +81-749-28-8368; Fax: +81-749-28-8590; E-mail: [email protected]

C60-Based Solar Cells Light Harvesting Nanomaterials 101

lifetime. However, silicon solar cells have some problems such as supply limit of silicon raw materials, there expensive production cost due to the complicated process and long energy payback time. Therefore, development of new solar cells instead of silicon solar cells is mandatory.

Carbon-based nanostructures such as fullerenes, nanocapsules, onions, nanohorns, nanotubes and nanodiamonds have been widely reported and investigated [1-6]. These C structures show different physical properties, and the bandgap energies are in the range of 0 eV to 5.5 eV. Solar cells with amorphous carbon thin films have been studied [7, 8], and photovoltaic efficiencies were obtained by using a chemical vapor deposition (CVD) method.

Recently, C60-based polymer/fullerene solar cells have been investigated and reported [9-13]. These organic solar cells have a potential for use in lightweight, flexible, inexpensive and large-scale solar cells [14-16]. However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants. One of the improvements is donor-acceptor (DA) proximity in the devices by using blends of donor-like and acceptor-like molecules or polymers, which are called DA bulk-heterojunction solar cells [17-20]. The bulk heterojunction is an efficient method to generate free charge carriers, and the charge transfer (electrons and holes) is possible at the semiconductor interface. An acceptor with highest occupied molecular orbital can receive electrons from the conduction band of an opposite semiconductor (donor).

In the present review, fabrication and characterization of C60-based solar cells with copper oxides, CuInS2, phthalocyanine, diamond, porphyrin and exciton-diffusion blocking layer were reported. In the present work, C60 and fullerenol (C60(OH)10-12) were used for n-type semiconductors, and diamond particles, nanodiamond (ND), metal phthalocyanine derivative (MPc) and porphyrin were used for p-type semiconductors. Structures of the present C60-based solar cells are shown in Fig. (1). Phthalocyanines which have a photovoltaic property, heat-resistance, light-stability, chemical stability, and high optical absorption at visible range are used for an oxidation catalyst, catalyst of fuel cells and solar cells. An efficiency of ~5% was achieved for organic solar cells by employing small molecules such as copper phthalocyanine and fullerene [21-23]. Many studies on the metal phthlocyanine (MPc) monomer have been performed, and the properties are different by changing central metal and chemical substitution. The organic-inorganic hybrid device structures were produced, and nanostructure, electronic property and optical absorption were investigated.

102 Light Harvesting Nanomaterials Oku et al.

Fig. (1). Structure of the present C60-based solar cells with bulkheterojunction (BHJ) and heterojunction (HJ) structures.

2. CuOx/C60 SOLAR CELLS

2.1. Background of CuOx

Semiconductor oxides are a promising alternative to silicon-based solar cells because they possess high optical absorption and are composed of low cost materials. Copper oxides (CuO and Cu2O) are known to be p-type semiconductor oxides, and crystal structures of CuO and Cu2O are shown in Fig. (2). They are suitable materials for high efficiency solar cells because their direct bandgaps of ~1.5 and ~2.0 eV, respectively, are close to the ideal energy gap for solar cells, and well matched with the solar spectrum. A maximum efficiency of ~2% has been obtained for Cu2O solar cells using high-temperature annealing and vacuum evaporation techniques [24]. Solar cells consisting of Cu2O and ZnO fabricated by electro- and photochemical deposition methods have also been reported [25-27]. Although CuO has been used as a hole transfer layer and barrier layer for dye-sensitized solar cells [28, 29], few solar cells have been reported using CuO as a p-type semiconductor active layer. The use of Cu2O and CuO is advantageous because of its simple production method for solar cells [30, 31].

The purpose of the present study was to fabricate Cu2O/C60 and CuO/C60 thin film solar cells by electrodeposition and spin-coating methods, and to investigate the effect of Cu2O, CuO and C60 layers on their electronic properties. Fullerene (C60) is a good electronic acceptor, and has been used as n-type semiconductor active

Al (Electrode)

PEDOT:PSSITO or FTO

Glass substrate

-+p-type n-type

ZnTTP C60

CuO C60

Cu2O C60

CuInS2 C60

Diamond C60

CuPc(ND) C60(OH)10-12

CoPc(ND) C60

GaPc dimer C60

Bulk HeterojunctionHeterojunction

PTCDA


layers for organic thin film solar cells [12, 20]. Spin-coating is a low-cost method, and electrodeposition is a method for homogeneous thin film formation, which are essential for the mass production of any solar cells [17, 18]. Cu2O and CuO thin films prepared by sol-gel spin-coating and electrodeposition have also been previously reported [25-27, 32]. Cu2O/C60 and CuO/C60 solar cells were investigated by structural analysis and measurements of optical absorption and photovoltaic activity in the present study.

Fig. (2). Crystal structures of CuO and Cu2O.

2.2. Experimental Procedures

Four different CuOx/C60 solar cells were fabricated in the present study [31]. Cu2O nanoparticles were synthesized by reducing copper-amine complex with 1-heptanol in the presence of tetramethyl ammonium hydroxide. The details of the synthesis scheme were as follows: Copper acetate and oleylamine were introduced into 1-heptanol and heated under a nitrogen atmosphere at 120°C for one hour to remove the moisture in the system. Then, tetramethyl ammonium hydroxide (TMAOH) dissolved in 1-hepatanol (~150°C) was introduced and the suspension was heated at 150°C for 2 hours. Next, the suspension was cooled to room temperature and Cu2O nanoparticles were recovered by centrifuging. Finally, the Cu2O nanoparticles were washed with methanol to remove excess oleylamine, and the Cu2O nanoparticles (~100 mg) were dispersed in toluene (10 mL). A thin layer of polyethylenedioxythiophene doped with polystyrene-sulfonic acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated at 2000 rpm on precleaned indium tin oxide (ITO) glass plates (Geomatec Co., Ltd., ~10 Ω/□). After annealing at 100°C for 10 minutes in N2 atmosphere, semiconductor layers were

Cu2OCuO

c

ab

a

a a

Cu

O

Cu

O


prepared on a PEDOT layer by spin coating using a mixed solution of a toluene with Cu2O nanoparticle and a C60 solution (16 mg/mL) with C60 powders (Material Technologies Research, 99.98%) in odichlorobenzene. The thickness of the bulk heterojunction structure was approximately ~150 nm. The Cu2O:C60 layers were annealed at 100°C for 30 minutes in N2 atmosphere.

CuO layers were spin-coated on pre-cleaned F-doped SnO2 (FTO) glass plates (Asahi Glass, ~9.3 Ω/□). Copper acetate monohydrate, Cu(CH3COO)2·H2O (0.5 mol/L, Sigma Aldrich, 99.99%) was dissolved into 2-propanol/monoethanol-amine. Cu layers were prepared by spin-coating of a CuO precursor solution at 1000 rpm and annealing at 450°C for 30 min under air atmosphere on an FTO substrate.

Cu2O and CuO layers were also prepared on pre-cleaned indium tin oxide (ITO) glass plates by an electrodeposition method using platinum counter electrode. Copper (II) sulfate (CuSO4, 0.4 mol/L, Wako 97.5%) and l-lactic acid (3 mol/L, Wako) were dissolved into distilled water. The electrolyte pH was adjusted to 12.5 by adding NaOH. The electrolyte temperature was kept at 65°C during electrodeposition. The electrodeposition of Cu2O and CuO layers was carried out at voltages of -0.35 and +0.70, respectively, and the quantity of electric charges was 2.2 cm–2.

C60 layers with thickness of ~100 nm were prepared on the Cu2O and CuO layers by vacuum evaporation from C60 powder (Material Technologies Research, 99.98%). For all the present copper oxides semiconductor solar cells, aluminum (Al) metal contacts of thickness ~100 nm were deposited as top electrodes, and annealed at 140°C for 20 min in N2. These four types of the present solar cells were denoted as ITO/Cu2O/C60/Al, ITO/CuO/C60/Al, ITO/PEDOT:PSS/Cu2O: C60/Al and FTO/CuO/C60/Al.

Current density-voltage (J-V) characteristics were measured (Hokuto Denko Corp., HSV-100), both in the dark and under illumination at 100 mWcm-2 using an air mass (AM) 1.5 solar simulator (San-Ei Electric, XES-301S). The solar cells were illuminated through the substrate side, with an illuminated area of 0.16 cm2. Optical absorption of the solar cells was investigated by means of UV-visible absorption spectroscopy (Hitachi Ltd., U-4100). The microstructures of the copper oxides thin films were investigated by X-ray diffractometry (XRD, Philips X’Pert-MPD System) with CuKα radiation at 40 kV operating voltage and 40 mA operating current. Transmission electron microscopy (TEM, Hitachi H-8100, 200 kV operating voltage) were also carried out for nanostructure analysis [33].


Thermodynamical calculations for the copper oxide reactions were carried out by HSC Chemistry (Outokumpu Research Oy. Poli, Finland).

2.3. RESULTS AND DISCUSSION

J-V characteristics of the present solar cells under illumination showed photocurrents with short-circuit currents and open-circuit voltages. Measured parameters of the present CuOx/C60 solar cells are summarized in Table 1. Cu2O/C60 structures fabricated by electrodeposition and spin-coating methods provided better power conversion efficiency (η) of ~4×10-3% compared to CuO/C60 solar cells. The Cu2O/C60 solar cell prepared by the electrodeposition gave an η of 4.2×10-3%, a fill factor (FF) of 0.25, short-circuit current density (JSC) of 67 µAcm-2 and open-circuit voltage (VOC) of 0.20 V.

Table 1. Measured parameters of CuOx-based solar cells.

Sample VOC (V) JSC (µAcm−2) FF η (%)

CuO/C60 (Spin) 0.025 36 0.25 2.3×10-4

Cu2O:C60 (Spin) 0.17 110 0.23 4.3×10-3

CuO/C60 (ED) 0.24 1.8 0.25 9.0×10-5

Cu2O/C60 (ED) 0.20 67 0.25 4.2×10-3

Fig. (3) show optical absorption of the Cu2O/C60 and CuO/C60 solar cells. The solar cells prepared by the electrodeposition indicated higher absorption in the range of 300-700 nm compared to those by spin-coating, which would be due to the film thickness. Absorption peak at ~360 nm and ~500 nm were due to copper oxides, and peaks at ~340, ~440 and ~610 nm corresponded to C60.

Crystalline components in the Cu2O and CuO thin films were investigated by XRD, as shown in Fig. (4). Diffraction peaks corresponding to Cu2O and CuO were observed for the Cu2O and CuO thin films, and they consisted of cuprite phase with cubic system (space group of Pn3m and lattice parameters of a = 0.4250 nm) and cupric phase with monoclinic system (space group of C2/c and lattice parameter of a = 0.4653 nm, b = 0.3410 nm, c = 0.5018 nm, β= 99.48°) [34]. The particle sizes were estimated using Scherrer’s equation: D = 0.9λ/Bcosθ, where λ, B, and θ represent the X-ray wavelength, full width at half maximum, and Bragg angle, as listed in Table 1. The crystallite sizes of Cu2O and CuO


Fig. (3). Optical absorption of CuOx/C60 solar cells.

produced by the electrodeposition are ~40 nm, which are larger compared to those prepared by spin-coating. The increase of crystallinity which was due to the thicker film thickness led to an increase in open-circuit voltage. Crystallite sizes of Cu2O and C60 of the Cu2O:C60 bulk heterojunction thin film were determined to be 7.2 nm and 25.7 nm, respectively.

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ITO/CuO/C60/Al

Electrodeposition

ITO/Cu2O/C60/Al

Electrodeposition

(d)(c)


Fig. (4). XRD patterns of Cu2O and CuO thin films prepared by (a) spin-coating and (b) electrodeposition.

Fig. (5a & 5b) are a TEM image and a selected area electron diffraction pattern of a Cu2O:C60 bulk heterojunction layer, respectively. The TEM image indicated aggregated Cu2O nanocrystals with sizes of 10~20 nm. Line broadening of the Debye-Scherrer rings in Fig. (5b) indicates nanocrystal structures of Cu2O. Optimization of the nanocomposite structure with Cu2O and C60 would increase the efficiencies of the solar cells.

A TEM image of the CuO thin film prepared by spin-coating is shown in Fig. (5c), which indicated CuO nanocrystals of 10-20 nm sizes, which agreed well with the XRD results. Debye-Scherrer rings in the electron diffraction pattern in Fig. (5d) indicated polycrystalline CuO structures with no favored nanocrystal orientation within the films.

Fig. (5e & 5f) are a TEM image and a selected area electron diffraction pattern of a Cu2O layer prepared by the electrodeposition, respectively. The TEM image indicated Cu2O nanocrystal structures with sizes of 40~50 nm, which agreed well with the XRD results, as listed in Table 1. The comparatively larger crystallite sizes of the Cu2O resulted in unclear Debye-Scherrer rings in Fig. (5f), which indicated the higher crystallinity of the Cu2O nanoparticles.

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ity (a

.u.)

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nsity

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.)

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CuOSpin-coating

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Cu2OElectrodeposition

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ITO

ITO

ITOCuO202

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CuOElectrodeposition

(b)(a)


Fig. (5). TEM images and electron diffraction patterns of (a, b) Cu2O:C60 bulk heterojunction layer, (c, d) CuO nanoparticles (spin-coating), and (e, f) Cu2O thin film (electrodeposition), respectively.

Cu2O200

Cu2O220Cu2O310

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Ellingham diagram of copper oxides for an O2 gas molecule (per mol) was investigated by thermodynamical calculation, as shown in Fig. (6). The copper element would be oxidized to CuO and Cu2O completely by annealing at elevated temperatures, and Cu2O would be more stable compared to CuO from the Ellingham diagram.

Fig. (6). Ellingham diagram of copper oxides for an O2 molecule.

Energy level diagram of ITO/Cu2O/C60/Al and ITO/CuO/C60/Al solar cells is shown in Fig. (7), in which previously reported values of energy levels were used [31, 35, 36]. Separated holes could transfer from the valence band of the Cu2O to the ITO, and separated electrons could transfer from the conduction band of the Cu2O to the Al electrode, respectively. It has been reported that VOC is nearly proportional to the semiconductor bandgap [37], control of the energy levels is important for increasing cell efficiency. Compared with a Si semiconductor with an indirect transition band structure, Cu2O and CuO have direct transition bandgaps and greater optical absorption. Thus, ultrathin films of copper oxide layers could potentially provide efficient charge injection.

In the present study, Cu2O and CuO were selected as a p-type semiconductor oxide for solar cells. These copper oxides are advantageous as a low-cost reagent

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4Cu + O2(g) = 2Cu2O

2Cu + O2(g) = 2CuO


with a simple fabrication process. Their lower conversion efficiencies compared to the Si may have been due to presence of heterogeneous grain size copper oxides in the active layer. Defects produced by inadequate crystallinity may have caused carrier recombination, and the formation of higher quality copper oxide thin films may overcome this problem.

Fig. (7). Energy level diagram of the (a) ITO/Cu2O/C60/Al and (b) ITO/CuO/C60/Al solar cells.

3. CuInS2: C60 BULK HETEROJUNCTION SOLAR CELLS

3.1. Background of CuInS2

I-III-VI group compounds called chalcopyrite are expected as next generation solar cell materials. Chalcopyrite compounds have advantages of high optical absorption and high resistivity to cosmic rays compared to conventional silicon solar cells. In addition, they have a band structure of direct transition, which shows high quantum efficiency. Therefore, development of chalcopyrite compounds solar cells have been performed [38, 39]. Although CdS is used for most of the chalcopyrite solar cells to obtain high efficiency, development of the Cd-free solar cells has continued because of the poisonous influence of Cd on the environment.

The purpose of the present work is to investigate the Cd-free inorganic-organic hybrid solar cells with CuInS2 (CIS) for p-type semiconductor and with C60 for an organic n-type semiconductor. CIS is one of the representative chalcopyrite

-4.5eV

-6.2eV

-2.8eV

-4.9eV

ITO Cu2O C60 Al

-4.7eV -4.3eV

e-

e-

h＋

hν

(b)(a)

-4.5eV

-6.2eV

-3.7eV

-5.2eV

ITO CuO C60 Al

-4.7eV -4.3eV

e-

e-

h＋

hν


compounds, and C60 is suitable for n-type semiconductors due to strong electron affinity.


CIS solution for p-type semiconductors were produced by dissolving CuI (Sigma Aldrich Corp., 99.99%) and InCl3 (Sigma Aldrich Corp., 99.99%) in a mixture of triphenylphosphite (1 mL) (Sigma Aldrich Corp., 97%) and acetonitrile (2 mL) (Nacalai Tesque, Inc., 99.5%), dropping bis(trimethylsilyl)sulfide (Tokyo Chemical Industry Co., Ltd., >95%) [40, 41]. The solution for n-type semiconductors was prepared by dissolving C60 in o-dichlorobenzene. A thin layer of polyethylenedioxythiophene doped with polystyrene-sulfuric acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated on a pre-cleaned fluorine dope tin oxide (FTO) glass plates (Asahi Glass, ~9.3 Ω/□). Then, semiconductor layers were prepared on a PEDOT:PSS layer by spin coating, and annealed at 120 ºC for 10 min in N2 atmosphere. The FTO was used because of the high temperature annealing process.

The thickness of the blended device was ~150 nm. To increase efficiencies, PTCDA with a thickness of ~20 nm was also added over the active layers as shown in Fig. (1). After annealing at 100°C for 30 min in N2 atmosphere, PTCDA (Wako Pure Chemical Industries Ltd.) was evaporated between active layer and metal layer. Finally, aluminum (Al) metal contacts were evaporated as a top electrode, and annealed at 140°C for 20 min in N2 atmosphere.

3.3. Results and Discussion

Measured parameters of a CuInS2:C60 bulk heterojunction structure are summarized in Table 2. A solar cell with CIS:C60 bulk heterojunction structure provided power convergent efficiency of 8.0×10−4%, fill factor of 0.28 and open-circuit voltage of 0.18 V. The p-n interfaces, which are photoelectron conversion areas were increased by using blend structures of p-type and n-type semiconductors.

Table 2. Measured parameters of CuInS2/C60 solar cells.


CuInS2/C60 0.12 23 0.23 6.2×10-4

CuInS2:C60 0.18 16 0.28 8.0×10-4


Fig. (8) shows a measured optical absorption of the solar cells based on CIS. These solar cells show a wide optical absorption range from 400 to 800 nm, and the heterojunction solar cell show as higher optical absorption range from 350 nm to 550 nm than that of the bulk heterojunction. Since the FTO substrate was set as an incident side, the optical absorption of the CIS layer was high for the heterojunction structure. On the other hand, optical absorption of the bulk heterojunction would be lower compared to that of the heterojunction structure because C60 were mixed with the CIS layer.

Fig. (8). Optical absorption spectra of heterojunction and bulk heterojunction solar cells.

An X-ray diffraction pattern of CIS:C60 bulk heterojunction is shown in Fig. (9). Several diffraction peaks are observed, which correspond to 112, 204 of CIS and 111, 220, 311, 222, 422, 511 of C60. The average particle sizes of CuInS2 and C60 were calculated from Scherrer’s formula to be 5 nm and 13 nm, respectively. The 204 peak of CIS is too small to be used for the calculation of the CIS grain size, and only one peak of 112 was used for the calculation.

Fig. (10a) is a TEM image of CIS, and many CIS particles are observed. Fig. (10b) is an electron diffraction pattern of CIS. Debye-Scherrer rings are observed in the diffraction pattern, which shows crystallite structures of CIS particles.

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Energy (eV)3.5 3.0 2.5 2.04.0


Fig. (9). X-ray diffraction pattern of CIS:C60 thin film.

Fig. (10). TEM image of CIS nanoparticles and electron diffraction pattern of CIS.

An interfacial structure of CIS and C60 was observed by TEM as shown in Fig. (11). Filtered Fourier transform of the HREM image of CIS:C60 bulk heterojunction layer is shown in Fig. (11a). Fig. (11b) is an inverse Fourier transform of (a), and arrows show the interface of CIS and C60. Lattice fringes of

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211204312314217

000

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{101} of CIS and {111} of C60 were observed. The enlarged image of a part of C60 in (b) is shown in Fig. (11c). Arrangements of C60 molecules are observed in the image. CIS and C60 have size distribution, and the crystal sizes of them observed in the TEM image are larger compared to the averaged sizes.

Fig. (11). (a) Filtered Fourier transform of HREM image of CIS:C60 bulk heterojunction layer. (b) Inverse Fourier transform of (a). (c) Enlarged image of a part of C60 in (b).

Fig. (12). Energy level diagram of CIS:C60 solar cells.

C60 {111}

3nm CIS {101}

C60 200C60 111

000 CIS 101

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e- e-

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Optimization of the nanocomposite structure with CIS and C60 would increase the efficiencies of the bulk heterojunction solar cell. From the present TEM observation, CIS and C60 were not mixed in a molecular scale. If the mixture structure of CIS and C60 is improved to a nanoscale, it is believed that the area of the p-n junction interfaces is increased, and the efficiency would be improved. In addition, it is important to search the most suitable mixture ratio of the p-type and n-type semiconductors for bulk heterojunction solar cells.

An energy level diagram of CIS/C60 solar cells is summarized as shown in Fig. (12). Previously reported values were used for the energy levels of the figures by adjusting them to the present work [12, 42, 43]. When light is incident from the FTO side, excitation by the light absorption happens in the p-n interface, and electrons and holes are produced by charge separation. Carriers would transport from -4.5 eV to -4.3 eV by hopping conduction. Improvement of the present bulk heterojunction solar cells would be possible by the introduction of a buffer layer, change of annealing conditions, and the improvement of the microstructure is also necessary to obtain high efficiency.

The evaporation method provided high quality thin films, but a high vacuum and high temperature process are necessary. Although CISCuT method is a productive process, it requires a high temperature process [44]. On the other hand, the present spin coating method is simpler compared to the other formation methods. In addition, we can apply the spin coating method to plastic substrates without high vacuum and high temperature processes.

4. METAL PHTHALOCYANINE: DIAMOND/C60 SOLAR CELLS

4.1. Background of Metal Phthalocyanine and Diamond

The purpose of the present work is to fabricate and characterize C60/phthalocyanine-based bulk heterojunction and heterojunction solar cells. In the present work, C60 and fullerenol (C60(OH)10-12) were used for n-type semiconductors, and diamond particles, nanodiamond (ND), metal phthalocyanine derivative and µ-oxo-bridged gallium phthalocyanine (GaPc) dimer were used for p-type semiconductors. Phthalocyanines which have a photovoltaic property, heat-resistance, light-stability, chemical stability, and high optical absorption at visible range are used for a oxidation catalyst, catalyst of fuel cells and solar cells. Many studies on the metal phthlocyanine (MPc) monomer have been performed, and the properties are different by changing central metal and chemical


substitution. The organic-inorganic hybrid device structures were produced, and nanostructure, electronic property and optical absorption were investigated.


A schematic diagram of the present C60/phthalocyanine-based bulk heterojunction (BHJ) and heterojunction (HJ) solar cells is shown in Fig. (1). A thin layer of polyethylenedioxythiophen doped with polystyrene-sulfonic acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated on pre-cleaned indium tin oxide (ITO) glass plates (Geomatec Co., Ltd., ~10 Ω/□). The PEDOT:PSS has a role as an electron blocking layer for hole transport. Then, semiconductor layers were prepared on a PEDOT layer as shown in Fig. (1).

Three types of solution for p-type semiconductors were produced [45]. The first is a diamond:C60 BHJ solar cell, which was prepared by spin coating using a mixed solution of C60 (Material Technologies Research, 99.98%) and diamond powder (New metals & Chemicals Co. Ltd., >95%) in 1,2-dichlorobenzene. Although the diamond powder did not have high purity, the impurities would not be activated, and the carrier concentration would be low. Total weight of diamond:C60 was 18 mg, and weight ratio of diamond:C60 was 1:8.

The second was produced by nanodiamond (NanoCarbon Research Institute, Ltd., ND) and tetra carboxy phthalocyaninate cobalt (Orient Chemical Industries Co. Ltd., Tc-CoPc) in de-ionized water. The solution for n-type semiconductors was prepared by dissolving C60 in 1,2-dichlorobenzene. On the thin layer of PEDOT:PSS, p-type semiconductor layers were prepared by spin coating a mixed solution of Tc-CoPc and ND in de-ionized water. The nanodiamonds were dispersed in the Tc-CoPc thin film. The n-type semiconductor layers were deposited on the top of the p-type semiconductor layer by spin coating a C60 solution.

The third was also produced by tetra carboxy phthalocyaninate copper (Orient Chemical Industries Co. Ltd., Tc-CuPc), fullerenol (Honjo Chemical Co. Ltd., 99.5%, C60(OH)10-12) and ND in de-ionized water. On the thin layer of PEDOT:PSS, semiconductor layers were prepared by spin coating using a mixed solution of Tc-CuPc, C60(OH)10-12 and ND in de-ionized water. The nanodiamonds were obtained by using the bead milling method in water, and were dispersed in the active layer.



Measured parameters of diamond-based thin films are summarized in Table 3 [45, 46]. Power conversion efficiency, fill factor, short-circuit current density and open-circuit voltage are denoted as η, FF, Jsc, and Voc, respectively. A thin film with the diamond:C60 structure provided η of 4.3×10-5%, FF of 0.35, Jsc of 5.3 µA/cm2 and Voc of 0.023 V. In Table 4, thin films structure with ND provided a higher cell performance than that of thin films structure without ND.

Table 3. Measured parameters of diamond-based solar cells.


Diamond:C60 0.023 5.3 0.35 4.3 × 10-5

Tc-CoPc/C60 0.011 4.6 0.28 1.4×10-5

Tc-CoPc:ND/C60 0.012 7.0 0.24 2.0×10-5

Tc-CuPc:C60(OH)10-12 0.20 0.92 0.24 4.4×10-5

Tc-CuPc:ND:C60(OH)10-12 0.21 1.8 0.21 7.9×10-5

Fig. (13) shows optical absorption spectra of the nanodiamond-based thin films. In Fig. (2a), the diamond:C60 nanocomposite structure provided photo-absorption in the range of 350 to 500 nm, and shows high absorption at 339, 402 and 506 nm, which correspond to 3.7, 3.1 and 2.5 eV, respectively. Absorption peaks of the C60 were confirmed within the range from 300 to 400 nm, and an absorption peak of 506 nm corresponds to the diamond. In Fig. (2b & 2c), a solid line and dashed line show thin film structure with ND and thin film structure without ND. These thin films provided photo-absorption in the range of 300 to 800 nm, and thin film structure with ND indicates a higher optical absorption compared to that of thin film structure without ND. The optical absorption property of the thin film was improved by adding the nanodiamond to the active layer.

Fig. (14) shows X-ray diffraction patterns of diamond powder and the present thin films. In Fig. (3a), diffraction peaks of the diamond powder were confirmed as 111, 220 and 311 of the diamond structure. A grain size of diamond powder was determined to be 12 nm, which was calculated by Scherrer’s equation. An increase of photo-absorption above ~600 nm would be due to the nanostructure of diamond particles, which will be discussed later. In Fig. (14b & c), diffraction


Fig. (13). Optical absorption spectra of (a) diamond:C60 layer, (b) Tc-CoPc:ND/C60 and Tc-CoPc/C60 layers, (c) Tc-CuPc:ND:C60(OH)10-12 and Tc-CuPc:C60(OH)10-12 layers.

peaks corresponding to diamond are observed for the Tc-CoPc:ND/C60 and Tc-CuPc:ND:C60(OH)10-12 sample. The average particle sizes of the nanodiamond were calculated be 4.5 and 5.5 nm from Scherrer’s formula.

A TEM image, an enlarged image and an electron diffraction pattern of the diamond:C60 composite layer are shown in Fig. (15a, 15b & 15c), respectively.

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Fig. (14). X-ray diffraction patterns of (a) diamond powder, (b) Tc-CoPc:ND/C60 and Tc-CuPc/C60 layers, (c) Tc-CuPc:ND:C60(OH)10-12 and Tc-CuPc:C60(OH)10-12 layers.

Diamond powder has an fcc structure with a lattice parameter of a=0.357 nm. C60 has also an fcc structure with a lattice parameter of a=1.42 nm. In the electron diffraction pattern of Fig. (15c), expansion of C60 reflections was observed, which indicates a disordered structure of the composite layer. In the present TEM observation, diamond and C60 were not mixed well in nanoscopic scale, and the fabricated thin film would show low conversion efficiency.

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ND 111

(c)


Fig. (15). (a) TEM image, (b) enlarged image of a part of (a) and (c) electron diffraction pattern of diamond:C60 layer.

Fig. (16a) is a TEM image of C60 layer, and lattice image of C60 {111} is observed. Fig. (16b) is an electron diffraction pattern of C60 layer, and diffraction peaks of C60 are observed. Fig. (16c) is a HREM image of the Tc-CoPc:ND composite layer. In Fig. (16c), lattice image of diamond {111} is observed. Tc-CoPc shows dark contrast in the image. Fig. (16d) is an electron diffraction pattern of the Tc-CoPc:ND composite layer and diffraction peaks of diamond 111, 220, 311 are observed. Since no diffraction peak of Tc-CoPc was observed, Tc-CoPc would have an amorphous structure.

Fig. (17a) is a TEM image of the Tc-CuPc:ND:C60(OH)10-12 composite layer. The TEM image indicated ND with the size of 4-6 nm as indicated by arrows, which agree well with the XRD result. Fig. (17b) is an electron diffraction pattern of the active layer, and diffraction peaks of diamond 111, 220, 311 are observed. Since no diffraction peak of Tc-CuPc and C60(OH)10-12 were observed, Tc-CuPc and C60(OH)10-12 would have amorphous structures.

An energy level diagram of nanodiamond-based solar cells is summarized as shown in Fig. (18). Previously reported values were also used for the energy levels [45]. An energy gap of diamond estimated from Fig. (13a), which corresponds to absorbance of 506 nm, is used for the model. From a theoretical calculation [47], nanodiamonds are composed of three layers; a diamond core (sp3), a middle core (sp2+x) and a graphitized core (sp2). Therefore, a band gap of the nanodiamond is decreased by the existence of the sp2+x bonding [48].

The carrier transport mechanism is considered as follows; when light is incident from the ITO substrate, light absorption excitation occurs at the p-n

200nm 20nm

111220

311

000

ba c


Fig. (16). (a) TEM image and (b) electron diffraction pattern of C60 layer. (c) HREM image and (d) electron diffraction pattern of Tc-CoPc:ND layer.

Fig. (17). (a) TEM image and (b) electron diffraction pattern of Tc-CuPc:ND:C60(OH)10-12 layer.

10 nm

(111)

111220311

422

000

ba

000

111

220311

diamond 1110.21 nm

1nm

Tc-CoPc

Tc-CoPc

Tc-CoPc dc

ND

ND


000


111

220311

20nm

ND

ba


Fig. (18). Energy level diagrams of (a) diamond:C60, (b) Tc-CoPc:ND/C60, and (c) Tc-CuPc:ND:C60(OH)10-12 solar cells.

heterojunction interface, and electrons and holes appear by charge separation. Then, the electrons transport through C60 or C60(OH)10-12 toward the Al electrode, and the holes transport through PEDOT:PSS to the ITO substrate. Since it has been reported that Voc is nearly proportional to band gaps of semiconductors [37], control of energy levels is important to increase the efficiency. For the present sample, the low Voc would be due to the voltage drop by resistance increase, which would be caused by low carrier density of nanocomposite layer and contact resistance in metal/semiconductor interface. The low cell performance would also be due to the insufficient dispersion of diamond and C60 in the composite layer, and further control of the nanocrystals is needed.

An advantage for the nanocomposite structure is increase of p-n heterojunction interface. However, due to disarray of the donor/acceptor microstructure,

ITO PEDOT:PSS C60diamond Al

-4.3eV-5.0eV-4.7eV

Active layer

e-

-5.5eV

-3.0eV

-6.2eV

-4.5eV

h+

hν

ITO PEDOT:PSS C60Tc-CoPc+ND Al

-4.3eV-5.0eV

-4.7eV

Active layer

e-

-4.9eV

-3.4eV

-6.2eV

-4.5eV

h+

NDhν

(a)

(b) (c)

ITO PEDOT:PSS C60(OH)10-12Tc-CuPc+ND Al

-4.3eV-5.0eV

-4.7eV

Active layer

e-

-5.3eV

-3.5eV

-6.2eV

-4.5eV

h+

ND hν


electrons and holes could not transport smoothly by carrier recombination at the electronic acceptor/Al interface, and at the PEDOT:PSS/electronic donor interface, respectively. To solve these problems, introduction of a layer preventing carrier recombination and improvement of crystalline structure with few defects are needed.

In the present work, nanodiamond-based solar cells were fabricated and characterized. For the carbon-based solar cells in previous works, thin films are fabricated by a CVD method [7, 8]. In the present work, solar cells with C60, C60(OH)10-12 and metal phthalocyanine derivative as an organic semiconductor, and diamond particles and nanodiamond as an inorganic semiconductor were fabricated by a spin coating method, which is a low cost method. The performance of the present thin films would be dependent on the nanoscale structures of the organic-inorganic materials, and control of the structure should be investigated further.

5. GALLIUM PHTHALOCYANINE DIMER/C60 SOLAR CELLS

5.1. Background of Phthalocyanine Dimers

When the nearest neighbor two phthalocyanines with substituent such as amino group and hydroxy group are connected by hydrogen bridged substituent, high photoconduction was observed [49, 50]. However, few phthalocyanine dimers have been reported, and high photoconduction can be expected for the covalently-bridged phthalocyanine dimers. The purpose of the present work is to fabricate and characterize phthalocyanine dimer/fullerene heterojunction solar cells. In the present work, µ-oxo-bridged gallium phthalocyanine (GaPc) dimer is used for p-type semiconductors, and fullerene with excellent electron affinity is used for n-type one. The second purpose is to investigate molecular orbital of gallium phthalocyanine (GaPc) dimer and fullerene which is a good electronic accepter, and to examine this structure as solar cell material.

5.2. Experimental and Calculation Procedures

A thin layer of polyethylenedioxythiophen doped with polystyrenesulfonicacid (PEDOT:PSS) (Sigma Aldrich Corp.) was spin-coated on pre-cleaned indium tin oxide (ITO) glass plate (Geomatec Co., Ltd., ~10 Ω/□). The PEDOT:PSS has a role as an electron blocking layer for hole transport. After annealing at 100 ºC for 20 minin N2 atmosphere, µ-oxo-bridged GaPc dimer (Orient Chemical Industries, Co. Ltd.) and fullerene (C60, Material Technologies Research, 99.98%) layer were


prepared on a PEDOT:PSS layer by evaporation (1.8×10−3 Pa). Finally, alimunium metal contacts were evaporated as a top electrode and annealed at 140 ºC for 20 min in N2 atmosphere [51]. The active area of the solar cells was 4 mm×4 mm. A schematic diagram of the present solar cells is shown in Fig. (1). GaPc monomer with axial Cl ligand was also used for the comparison.

First principle calculations of the electronic properties have been used to predict the geometry of molecular structures and vibration spectra for phthalocyanine [52]. The optimized geometries and energies of all the present structures were calculated by ab-initio molecular orbital calculations using Gaussian 03. A detailed analysis of ground states of the electronic structures is based on self-consistent solutions of the Kohn–Sham (KS) molecular orbital model on density functional theory (DFT) with hybrid B3LYP function [53]. The KS equation is defined as a followed equation (1).

− 𝟏𝟐𝛁𝟐 + 𝛒(𝐫!)

𝐫!𝐫!𝐝𝐫! + ∂E𝒙𝒄 𝝆 𝒓

𝛛𝛒+ 𝛄 𝐫 𝛗𝐢 = 𝛜𝐢𝛗𝐢 (1)

Exchange-correlation potential of the third term in the (1) is defined as the functional derivative of EXC [ρ(r)] with respect to ρ(r). The isolated molecular structures were optimized by the DF calculation using restricted Hartree-Fock (RHF) and hybrid B3LYP function based on the Becke exchange function and the Lee-Yang-Parr correlation function, with the KS orbital expanded to LANL2DZ and STO-3G*, 3-31G* and 6-31G* basis sets. Electronic densities and energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and electronic densities were investigated.


The measured J-V characteristic parameters of ITO/PEDOT:PSS/GaPc dimer/C60/Al solar cell and ITO/PEDOT:PSS/GaPc/C60/Al are summarized in Table 1. Each structure shows a characteristic curve for open circuit voltage and short circuit current density. A solar cell using GaPc dimer provided η of 4.2×10−3%, FF of 0.27, Voc of 0.14 V and Jsc of 0.11 mA/cm2. All parameters were improved by using GaPc dimer compared to GaPc monomer, as listed in Table 4.

Fig. (19) show a measured optical absorption of GaPc dimer, C60 and GaPc dimer/C60 cells. The solar cells show a wide optical absorption ranging from 320 nm to 800 nm (which correspond to 3.8 and 1.5 eV, respectively). The absorption


spectrum of GaPc dimer was almost the same as that of the monomer, but a new peak was observed around 450 nm.

Table 4. Measured parameters of GaPc-based solar cells.

Sample VOC (V) JSC (mAcm−2) FF η (%)

GaPc-dimer/C60 0.14 0.11 0.27 4.2×10-3

GaPc-monomer/C60 1.4×10-4 0.15 0.25 5.3×10-6

Fig. (19). Optical absorption spectra of GaPc dimer/C60 solar cells.

X-ray diffraction pattern of GaPc dimer layer is shown in Fig. (20). The diffraction pattern shows a peak at 2θ=6.9°, which corresponds to lattice spacing of 1.27 nm. A particle size of GaPc dimer was calculated from Scherrer’s formula to be 16.6 nm.

Fig. (21) is a structure of µ-oxo-bridged gallium phthalocyanine dimer used in the present study. Two GaPc planes are parallel to one another, and the rotational

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

300 400 500 600 700 800Wavelength [nm]

Abs

orpt

ion

[a.u

.]

GaPc dimer/C60

C60

GaPc dimer


Fig. (20). XRD pattern of deposited GaPc dimer thin film.

Fig. (21). Molecular structure of GaPc dimer after structural optimization (a) and energy levels of GaPc monomer and GaPc dimer.

5 10 15 202θ 　[degree]

Inte

nsity

[a.u

.]

GaPc dimer

41.35 °0.17 nm

GaPcCl GaPc dimer

-‐0.20

-‐0.10

-‐0.15

-‐0.25

-‐0.05

ΔEg

ΔEg

Energy

/ hartree


degrees are 41.35 º. The plane distance between GaPc monomer is ~0.34 nm [51]. When the nearest neighbor two phthalocyanines are arranged with hydrogen bridged substituent, high photoconduction can be expected for the covalently-bridged phthalocyanine dimer.

Fig. (22) shows HOMO and LUMO energy levels of the GaPc dimer with C60 after structural optimization using DFT/6-31G*. In Fig. (22), electronic densities of LUMO, LUMO+1, and LUMO+2 are localized for the fullerene side, while the HOMO is localized for the GaPc-dimer side, which suggests electron transfer between the GaPc dimer and fullerene. The similar localization of frontier orbital was previously reported for other donor-fullerene supramolecular systems [54-56].

Fig. (22). Molecular orbital spatial orientation for the HOMO, LUMO+K (K=0, 1, 2) energy levels of the GaPc dimer+C60. (a) HOMO, (b) LUMO, (c) LUMO+1 and (d) LUMO+2.

(a) (b)

(c) (d)


A schematic diagram of energy levels of GaPc dimer, C60, and GaPc with C60 is shown in Fig. (23). The energy levels of the four highest occupied and four lowest unoccupied orbital levels are summarized in Table 5. It should be noted that the LUMO energy levels of the GaPc dimer+C60 are comparable to the LUMO energy levels of fullerene, and the HOMO energy levels of the GaPc dimer + C60 are close to the HOMO energy levels of GaPc dimer. However, the symmetry of the GaPc dimer seems to be lowered because of decreasing of degeneracy, which would be due to the interaction with C60.

Fig. (23). Comparison of the energy levels of GaPc dimer, C60 and GaPc dimer+C60.

Electronic density of the GaPc dimer was widely distributed around π orbital on the both aromatic rings as observed in Fig. (22a). Energy levels of GaPc dimer at HOMO and LUMO were calculated to be -5.165 eV and -3.051 eV, as listed in Table 5. The energy level of GaPc dimer would indicate advantage of GaPc dimer for charge-transfer of the excited carriers to conduction band of C60. The HOMO

-6

-2

-4

-8

0

-10

Orb

ital e

nerg

y le

vels

(eV)

GaPc dimer C60GaPc dimer + C60

EgEg

Eg


Table 5. Comparison of the four highest occupied and four lowest unoccupied orbital levels.

States GaPc dimer (eV) GaPc dimer + C60 (eV) C60 (eV)

LUMO+3 -2.978 -2.832 -2.092

LUMO+2 -2.978 -3.106 -3.231

LUMO+1 -3.051 -3.105 -3.231

LUMO -3.051 -3.103 -3.231

HOMO -5.165 -4.972 -6.294

HOMO-1 -5.214 -5.027 -6.294

HOMO-2 -6.449 -6.079 -6.294

HOMO-3 -6.449 -6.085 -6.294

LUMO+3 -2.978 -2.832 -2.092

of GaPc dimer and the LUMO of C60 is closely related with open circuit voltage (Voc) of the solar cells, which is described by Voc = (1/e) (|EGaPcHOMO| - |EC60LUMO|) - 0.3 (V), where e is the elementary change [57]. The value of 0.3 V is an empirical factor and this is enough for efficient charge separation [58]. The model of photovoltaic mechanism would be described by this equation, and the control of the energy levels is important for improving conversion efficiency. It is believed that the present GaPc dimer would be useful for the solar cells as previously reported fullerene-based supramolecules for high-performance photovoltaic devices, and excitation and charge transfer during light irradiation should be investigated further.

The energy level diagram and electronic structures of the solar cell were calculated and summarized as shown in Fig. (24). Previously reported values except GaPc dimer were used for the energy levels of the figures by adjusting to the present work [12, 20]. The HOMO and LUMO levels of GaPc in Fig. (24), and HOMO and LUMO of two phthalocyanine monomers were stirred and piled up, respectively. The interaction between two phthalocyanine monomers is not able to be confirmed in Fig. (24). When light is incident from the ITO side, excitation by the light absorption happens at the p-n interface, and electrons and holes are produced by charge separation. Electrons are transported to an Al electrode, and holes are transported to an ITO substrate. Energy barrier would exist near the semiconductor/metal interface. Carriers would transport from -4.5 eV to -4.3 eV by hopping conduction. Improvement of the present solar cells would be possible by the introduction of a buffer layer, change of annealing conditions, and the improvement of the microstructure is also necessary to obtain high efficiency.


Fig. (24). Energy level diagram of GaPc dimer/C60 solar cells.

Although the energy gap and energy level of GaPc dimer were hardly changed by dimerization in the present work, the power conversion efficiency was significantly improved. It is known that the π electron system is enhanced by dimerization, and high career mobility can be expected [59]. Since enhancing the π electron system was not confirmed by dimerization in the present study, the improvement of efficiency is believed that molecular orientation and crystallinity were improved by ordered array due to dimerization, which led to the decrease of career recombination. As a result, open-circuit voltage was greatly improved, which led to the high conversion efficiency.

Various crystallizations of µ-oxo-bridged gallium phthalocyanine dimer have been reported [49, 50]. When the crystallographic structure is different, initial surface potential, photosensitivity and residual surface potential are also different. Further crystallographic structure should be investigated in the future.

6. ZnTPP: C60 BULK HETEROJUNCTION SOLAR CELLS SOLAR CELLS

6.1. Background of Porphyrin

The purpose of the present work is to fabricate and characterize porphyrin:C60 bulk heterojunction solar cells. In the present work, 5,10,15,20-Tetraphenyl-

-5.0eV-4.5eV

-6.2eV

-3.0eV

-5.1eV

ITO PEDOT:PSS

GaPcdimer

C60 Al

hν-4.8eV

-4.3eV

e-

h+


21,23H-porphin zinc (ZnTPP) was used for p-type semiconductors [12, 60], and C60 with excellent electron affinity was used for n-type one. Porphyrin has high optical absorption in the visible spectrum and high hole mobility [61-63], and was expected to form cocrystallites with C60 [64, 65] that would be suitable for the bulk heterojunction structure [66, 67]. The second purpose is to investigate an effect of exciton-diffusion blocking layer (EBL). 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is perylene derivative with a simple structure, which was reported to be used for solar cells [68]. In the present work, PTCDA was used as the EBL for porphyrin/C60 bulk heterojunction solar cells. EBL prevents hole transfer between active layer and anode, and improvement of conversion efficiency was expected by an introduction of the EBL.


A thin layer of polyethylenedioxythiophen doped with polystyrene-sulfonic acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated on pre-cleaned indium tin oxide (ITO) glass plates (Geomatec Co., Ltd., ~10 Ω/□). The PEDOT:PSS has a role as an electron blocking layer for hole transport. Then, semiconductor layers were prepared on a PEDOT layer by spin coating using a mixed solution of C60 (Material Technologies Research, 99.98%), ZnTPP (Sigma Aldrich) in 1 ml o-dichlorobenzene. Total weight of ZnTPP:C60 was 18 mg, and weight ratio of ZnTPP:C60 was changed in the range of 1:9 ~ 5:5. The thickness of the blended device was ~150 nm. A schematic diagram of the ZnTPP:C60 bulk heterojunction solar cells is shown in Fig. (1). To increase efficiencies, PTCDA with a thickness of ~20 nm was also added over the active layers as shown in Fig. (1). After annealing at 100°C for 30 min in N2 atmosphere, PTCDA (Wako Pure Chemical Industries Ltd.) was evaporated between active layer and metal layer. Finally, aluminum (Al) metal contacts were evaporated as a top electrode.

The isolated molecular structures were optimized by ab-initio molecular orbital calculations using Gaussian 03. Conditions in the present calculation were as follows: calculation type (FOPT), calculation method (RHF) and basis set (6-31G). Electronic structures such as energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and electron densities were investigated.


Measured J-V characteristic of ZnTPP:C60 bulk heterojunction solar cells under illumination is shown in Fig. (25). The bulk heterojunction indicates a one layered


composite structures with p- and n-type semiconductors, which is denoted as ZnTPP:C60. Effects of PTCDA addition to the ZnTPP:C60 bulk heterojunction solar cells were also investigated, which is denoted as ZnTPP:C60/PTCDA. Each structure shows a characteristic curve for open circuit voltage and short circuit current, and measured parameters of the present solar cells are summarized in Table 6. Power conversion efficiency, fill factor, short-circuit current density and open-circuit voltage are denoted as η, FF, JSC, and VOC, respectively. As shown in Fig. (25) and Table 6, current density of ZnTPP:C60 increased by PTCDA addition, and the best efficiency was obtained for the ZnTPP:C60/PTCDA sample.

Fig. (25). Measured J-V characteristic of ZnTPP:C60 bulkheterojunction solar cells under illumination.

Table 6. Measured parameters of ZnTTP:C60 solar cells.


ZnTTP:C60 0.30 0.074 0.26 5.8 × 10−3

ZnTTP:C60/PTCDA 0.33 0.62 0.38 7.8 × 10−2

Fig. (26) shows the optical absorption of C60, ZnTPP, ZnTPP:C60 and ZnTPP:C60/PTCDA bulk heterojunction solar cells. The ZnTPP:C60/PTCDA structure provided higher photo-absorption in the range of 300 to 800 nm (which correspond to 4.0 and 1.5 eV, respectively), compared to the ZnTPP:C60 structure. Exciton migration of C60 can be efficiently suppressed by use of PTCDA, and

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0 0.1 0.2 0.3 0.4

ZnTPP:C60/PTCDA

ZnTPP:C60

Voltage (eV)

Cur

rent

den

sity

(mAc

m-2

)


exciton would be generated for both ZnTPP/C60 and C60/PTCDA interfaces, which results in the increase of conversion efficiency, as listed in Table 6.

Fig. (26). Absorbance spectrum of (a) C60, ZnTPP and (b) ZnTPP:C60 bulk heterojunction solar cells.

X-ray diffraction patterns of ZnTPP and ZnTPP:C60 bulk heterojunction layers are shown in Fig. (27a & 27b), respectively. In Fig. (27a), diffraction peaks corresponding to ZnTPP crystal are observed. After formation of ZnTPP:C60 bulk heterojunction layer, the diffraction peaks corresponding to ZnTPP disappeared, and C60 peaks are observed as shown in Fig. (27b). In addition, a new diffraction peak is observed as indicated by an arrow, which would be believed to be porphyrin/C60 cocrystallites [64, 65].

Fig. (28) is an electron diffraction pattern of ZnTPP:C60 bulk heterojunction layer, taken along the [-123] direction of C60. A twin structure with the (112) twin plane is observed in Fig. (28), as indicated by a dotted line. Diffraction spots which would correspond to cocrystallites of ZnTPP:C60 are also observed as indicated by arrows.

Electronic structures of the molecules were calculated, and energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shown in Fig. (30). HOMO levels in Fig. (29a), electrons are localized around the pyrrole rings of the ZnTPP. Energy levels of LUMO of C60 and PTCDA are also shown in Fig. (29b & 29c), respectively. The separated carriers would transfer from ZnTPP to C60, and from ZnTPP/C60 to PTCDA.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

300 400 500 600 700 8000.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce

Energy (eV)3.5 3.0 2.5 2.04.0

(a) (b)Energy (eV)3.5 3.0 2.5 2.04.0

Wavelength (nm)

Abso

rban

ce

ZnTPP:C60

ZnTPP:C60:PTCDA

ZnTPP

C60


Fig. (27). X-ray diffraction pattern of (a) ZnTPP and (b) ZnTPP:C60 bulk heterojunction layer.

Fig. (28). Electron diffraction pattern of ZnTPP:C60 bulk heterojunction layer.

5 10 15 20 5 10 15 20Diffraction angle 2θ (degree)

Inte

nsity

(a. u

.)(a) (b)

Diffraction angle 2θ (degree)

Inte

nsity

(a. u

.)

ZnTPPC60 111

ZnTPPC60 220

111

511

242

000

420331

511

111

420331


Fig. (29). Calculated LUMO and HOMO levels of (a) ZnTPP (b) C60 and (c) PTCDA.

An energy level diagram of the ZnTPP/C60/PTCDA solar cell is summarized in Fig. (30). Previously reported values [12, 20] were used for the energy levels of the figures by adjusting to the present work. The incident direction of light is from the ITO side. Energy barrier would exist near the semiconductor/metal interface. Electronic charge-transfer separation was caused by light irradiation from the ITO substrate side. Electrons are transported to an Al electrode, and holes are transported to an ITO substrate. The VOC of organic solar cells is reported to be determined by the energy gap between HOMO of donor molecule and LUMO of accepter molecule, and a relation between VOC and polymer oxidation potential is VOC = (1/e)(|EZnTPPHOMO| – |EC60LUMO|) – 0.3 (V), where e is the elementary charge [57]. The value of 0.3 V is an empirical factor, and this is enough for efficient charge separation [69]. The present experimental data of VOC indicated smaller compared to the calculated ones from the equation, which might be due to the voltage descent at the metal/semiconductor interface, and control of the energy levels is also important to increase the efficiency.

ZnN

CH

(a) (b) (c)

O

HOMO HOMO HOMO

LUMO LUMO LUMO

Zn

O


Fig. (30). Energy level diagram of ZnTPP:C60/PTCDA solar cell.

In the present work, efficiencies of the solar cells were increased by addition of PTCDA layers, which would work as the exciton-diffusion blocking layer for porphyrin:C60 bulk heterojunction solar cells. The PTCDA layers prevents hole transfer between the porphyrin:C60 active layer and aluminum, and the conversion efficiencies were improved.

Since the microstructure of ZnTPP and C60 bulk heterojunction layer is strongly dependent on the weight ratio of these, it is necessary to control the microstructure to form cocrystallites of ZnTPP:C60. In the present work, higher efficiencies were obtained for the ZnTPP:C60 sample with the weight ratio of 3:7, which would be suitable for the cocrystallite formation, as observed for weak reflections in X-ray and electron diffraction patterns. Recombination of electrons of C60 and holes of ZnTPP would occur in the bulk heterojunction layer with intermittent cocrystallite structure. If continuous cocrystallite structures form perpendicular to the thin films, it is believed that the recombination of electrons and holes could be suppressed, which would lead to improvement of conversion efficiency.

CONCLUSION

C60-based solar cells with copper oxides, CuInS2, phthalocyanine, diamond, porphyrin and exciton-diffusion blocking layer were fabricated and characterized.

-5.0eV-4.5eV

-6.2eV

-4.1eV

-5.6eV

ITO PEDOT:PSS

ZnTPP C60

hν-4.7eV

Nanocomposite

e- e-

h＋

h＋

-4.7eV

-6.9eV

Al

-4.3eV

e-

PTCDA


The results are summarized as follows. Cu2O/C60 and CuO/C60 solar cells were produced and characterized. Devices based on the Cu2O/C60 structure were fabricated by electrodeposition and spin-coating methods, which provided better η of ~4×10-3% compared to CuO/C60 solar cells. XRD and TEM results indicated the presence of Cu2O and CuO nanoparticles. Energy level diagram was proposed, and separated holes could transfer from the valence band of the Cu2O to the ITO, and separated electrons could transfer from the conduction band of the Cu2O to the Al electrode, respectively. Formation of higher quality copper oxide thin films, introduction of a new electrode and optimization of deposition condition would improve the efficiencies of the solar cells.

Chalcopyrite/fullerene solar cells were fabricated and characterized. A device of bulk heterojunction structure based on CuInS2 and C60 provided η of 8.0×10-4%, FF of 0.28, Voc of 0.18 V, which is better than those of the heterojunction structure. The present solar cells showed a high optical absorption in the range of 300 nm to 800 nm. Microstructures of the solar cells were observed by TEM and XRD, which indicated average particle sizes of CuInS2 and C60 of 5 nm and 13 nm, respectively. A carrier transfer mechanism was discussed by the energy level diagram.

Nanodiamond-based solar cells were fabricated and characterized. J-V characteristic was investigated under illumination of 100 mW/cm2 to confirm the solar cell performance. Diamond:C60 nanocomposite structure provided photo-absorption in the range of 350 to 500 nm, and provided η of 4.3×10-5%, FF of 0.35, Jsc of 5.3 µA/cm2 and Voc of 0.023V. Nanostructures of the thin films were investigated by TEM and X-ray diffraction, and the grain size of diamond and nanodiamond were determined to be 12 nm and 4-6 nm, respectively. Optimization of blended structures with diamond would increase the efficiencies of the thin films.

GaPc/fullerene solar cells were fabricated and characterized. The device based on the GaPc dimmer provided η of 4.2×10−3%, FF of 0.27, Voc of 0.14 V, which are better than those of the cells based on GaPc monomer. The present solar cells showed a wide optical absorption ranging from 320 nm to 800 nm. Geometry and electronic structure of phthalocyanine dimer-fullerene molecules were investigated by using ab-initio molecular orbital calculations, which indicated that the HOMOs were localized on the donor site and LUMOs were localized on accepter site. The energy gap of GaPc dimer was ~2.1 eV. A carrier transfer mechanism was discussed based on energy level diagram.


ZnTPP:C60 bulk heterojunction solar cells were fabricated and characterized. A device based on ZnTPP:C60(3:7)/PTCDA provided η of 7.8×10-2%, FF of 0.38, JSC of 0.62 mA/cm2 and VOC of 0.33 V, which was better than those of devices with other ratios of ZnTPP:C60. Conversion efficiency was increased by introduction of PTCDA layer because the exciton migration of C60 can be efficiently suppressed by use of PTCDA.

Photovoltaic behavior including charge transfer and mobility can be described on the basis of the energy diagram of the bulk heterojunction solar cells from the present J-V measurements, optical absorption and microstructure analysis. Optimization of blended structures with C60 would increase the efficiencies of solar cells.

ACKNOWLEDGEMENTS

None declared.



DISCLOSURE

Part of information included in this chapter has been previously published in ‘Energies 2010, 3, 671-685; doi:10.3390/en3040671’ and 'Materials Technology 2013, 28, 21-39; doi: 10.1179/1753555712Y.0000000042.

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Index

A Absorbance 4, 106, 112, 118, 120, 133 Absorption 3, 7, 21, 26, 28, 30, 33, 35, 38, 39, 46-

48, 65, 78, 80, 83, 85, 124, 25 Absorption bands 65 Absorption intensity 85, 93 Absorption peaks 105, 117 Absorption phthalocyanine 47 Absorption spectra 27, 32, 46 Absorption spectrum 31, 34, 38, 48, 65, 81 Acceptors 27, 56, 57, 101 Acetone 87, 88 Admicelle 95 Adsolubilization amounts 94, 95 Aerobic conditions 83 Aggregation 21, 24, 26, 31, 39, 43, 44, 46-48, 61 Ag nanoparticle array 15 Ag nanoparticles 12, 13, 16-18 Ag volume fraction 16-19 Alcohol oxidation, cinnamyl 88, 90 Alcohols 79, 90, 94-96 Aluminium 39, 70 American society for testing and materials (ASTM)

66, 67 Amorphous structures 120 Anchoring groups 21, 23-26, 28-30, 32, 33, 39, 42,

43, 45-47, 49 Angle, diffraction 113, 134 Annealing 10, 103, 104, 109, 111, 123, 131 Anti-reflection coatings, embedded SiN 16, 17 Anti-reflection coatings (ARC) 3, 6, 9, 14-16, 18 Application of polymer nanocomposite based

devices 67 Applications, optoelectronic 48, 56, 57 A-Si 5 A-TiO2 85, 90, 91, 94 Au nanoparticles 79 Au NP surfaces 94, 95 Au particle size 90, 93, 94, 96 Axial positions 39, 41 B Band gap 58, 69, 120, 122 Bands, valence 78, 79, 109, 137 Based devices 58, 74 Based sensitizers 26, 28, 31-33, 45, 47 Benzene 88, 96 Benzoquinone 88, 89 BHJ solar cell 116

Blended structures 137, 138 Building blocks 37, 38, 56 Bulk heterojunction 101, 106, 112, 131 phthalocyanine-based 115, 116 Bulk heterojunction layer 107, 108, 113, 114, 133,

134, 136 Bulk heterojunction solar cells 100, 110, 112, 115,

130, 133, 136, 138 C Capping of nanoparticles 61 Carboxylic acid groups, anchoring 31, 32 Carrier transfer mechanism 137 C-CHO concentrations 94, 95 Cd-free solar cells 110 CdSe nanoparticle 75 CdS nanoparticles 61, 71, 73 CdS nanoparticless 71 fabricated 4, 12 sensitized TiO2 32 Chains, olefinic side 31, 32 Chemical vapor deposition (CVD) 12, 101 Chromophores 25, 56, 57 Cinnamyl alcohol 88, 94 CIS, electron diffraction pattern of 112, 113 CIS layer 112 CIS particles 112 Co-condensation 62 Cocrystallites 133 Commercial silicon solar cells 16, 18 Composite layer 9, 10, 16, 118-20, 122 Compounds organic 79, 80 tetrapyrrolic 21, 23 Concentrations of PySH 83, 84 Concept, push-pull 21, 44, 45 Conduction band (CB) 39, 78, 79, 83, 84, 101, 109,

128, 137 Constant, dielectric 82 Conversion efficiency 25, 29-31, 35, 37, 42, 44-49,

133, 136, 138 power 32, 72, 117, 130, 132 Copper oxides 100-102, 104, 105, 109, 136 Core shell 61 Core-shell nanoparticles 61 Corroles 21, 23, 48, 49 Co-sensitization 28, 47, 48 Crystallographic structure 130 CuO nanoparticles 108

Index Light Harvesting Nanomaterials 143

D DA bulk-heterojunction solar cells 101 Damping coefficient 66 Dash-dot line 14, 15 Dashed line 14, 15, 117 Debye-Scherrer rings 107, 112 Degradations 87, 88 Density, electronic 124, 127, 128 Density functional theory (DFT) 29, 90, 124 Deposition, chemical vapor 12, 101 Designing metal nano-particle layer 5 Design of solar cell 4 Device applications 55, 59, 69 Devices optoelectronic 60, 67 single-nanoparticle 75 Devices sensitizer 22 Diameters, equivalent 14 Diamond, diffraction peaks of 120 Diamond-based solar cells 117 Diamond particles 101, 115, 117, 123 Diamond powder 116, 117, 119 Diffraction pattern, selected area electron 107 Diffraction peaks 105, 112, 117, 120, 133 Diffusion 58, 71, 72 Dimerization 130 Directionality 25, 43, 44 Disordered systems 74 Distributed Bragg reflector (DBR) 71 Distribution of nanoparticle 61 Disulfide 82, 85, 96 Donor 56, 57, 60, 101, 122 Donor-acceptor (DA) 101 Donor chromophores 56, 57 Doped nanocrystals 60 DSSC applications 21, 23, 33, 38, 41, 43 DSSC devices 21, 32, 33, 43, 45, 46 based 44, 46 DSSC device structure 22 DSSC performance 25 Dye-sensitized solar cells (DSSC) 21-23, 25, 32,

33, 37, 48, 49 Dynamic mechanical analysis (DMA) 66, 67 E Elastic modulus 65, 66 Electrode 22, 67, 69, 102, 109, 122, 129, 135, 137 composite 27 sensitized TiO2 42 Electrodeposition 102-8, 137 Electron acceptor 81, 83 Electron beam 64, 75

Electron diffraction pattern 107, 108, 118-21, 133, 136

Electron diffraction pattern of C60 120, 121 Electronic energy transfer (EET) 56, 57 Electronic structures 31, 100, 124, 129, 131, 133,

137 Electron injection 24, 69, 70 Electrons 4, 44, 61, 64, 67, 69, 70, 79, 83, 84, 93,

96, 101, 115, 116, 122, 123, 129, 131, 133, 135, 136

conductive 80, 81 Electron system 130 Electron transfer, interfacial 79, 82, 92 Electron transistors, single 75 Elongation data 65 Energy level diagram 69, 110, 115, 120, 122, 129,

135, 137 Energy levels 32, 100, 109, 115, 120, 122, 126-29,

133, 135 Energy transfer 56, 57 electronic 56, 57 EQE enhancements 14, 15 experimental 15 Escape cone 8 Exciton annihilation 24 Exciton-diffusion 100, 101, 131, 136 Excitonic coupling 35, 37 External quantum efficiency (EQE) 14, 15, 68 F Fabrication and characterization 3, 12, 19, 101 Fano interference term 86 Flexible organic solar cells 70 Formation, reversible 84 Frequency 66, 81 Fuel cells, catalyst of 101, 115 Fullerene solar cells 101, 137 Fullerenol 101, 115, 116 Function, complex dielectric 82 G GaPc dimer energy level of 128, 130 using 124 GaPc monomer 124, 127, 137 Glass plates 103, 104, 111, 116, 123, 131 Group, sulfonic acid 25, 38, 39 H Halogens 89 Heterojunction interface 122 Heterojunction solar cells 112, 115 Heterojunction structure 112, 137

144 Light Harvesting Nanomaterials Surya Prakash Singh

bulk 104, 111, 131, 137 Highest occupied molecular orbitals (HOMO) 24,

70, 124, 127-29, 131, 133, 135, 137 I Incident light 7, 8, 14, 22, 23, 81, 82 Incident radiation 3, 16, 18 Indium tin oxide (ITO) 68-71, 103, 104, 106, 107,

109, 110, 116, 122-24, 131, 137 Interactions, synergistic 55, 56 Interband transition modes 87, 90, 92 Interface 3, 111, 113, 115, 123, 129 Interfacial electron transfer (IET) 78-80, 82, 87, 92 Inverse Fourier transform 113, 114 IPCEQ band 24, 27 IPCESoret 24, 27 Irradiation, visible-light 81, 82, 88, 96 IR region 4, 33 ITO layer 69, 71 ITO substrate 120, 122, 129, 135 L Layers, silicate 63 Light emitted 69 path length of 14 Light absorption 3, 4, 115, 129 improving 3, 19 Light emitting diodes (LEDs) 55, 67, 69, 74 Light trapping 3, 5, 10, 12, 13 Local fields, high 71 Localized surface plasmon resonance (LSPR) 78,

79, 81, 82, 85, 90-92 Lowest unoccupied molecular orbitals (LUMO) 24,

33, 44, 70, 124, 127-29, 131, 133, 135, 137 LSPR absorption 85, 91, 93 LSPR-life time 93, 94 LSPR-lifetime 85, 87 LSPR mode 86, 87 LSPR-PC oxidations 81, 87, 94 LSPR-PCs 81, 87, 88, 90, 92, 96 LSPR peak 82, 86, 90 M Macrocycle 21, 24, 28, 32, 47, 49 Magnetic force microscopy (MFM) 74 Magnetic materials 73, 74 Materials clay 62, 63 generation solar cell 110 hole transport 29, 30 magnetic storage 55, 73 Metal layer 100, 111, 131

Metal nano-particle layer 6 Metal nanoparticles 3, 4, 10, 19, 62 characterization of 3, 19 depositing 5 embedded 5, 9 spherical 8 subwavelength 5 Metal particles, depositing 10 Metal phthalocyanine 101, 115, 123 Metal scatterers 6 Methods, standard test 66, 67 Micrographs 13, 14 Microstructures 100, 104, 115, 129, 136, 137 Mirror, detached Ag 14, 15 Mixed solution 104, 116 Model, baseline 17, 18 Modes, interband 85-87 Molecular orbitals highest occupied 24, 70, 101, 124, 131, 133 lowest unoccupied 24, 44, 70, 124, 131, 133 Molecular orbitals (MOs) 28, 29, 123 N Nanocomposites 55, 56, 61, 70-72 Nanocrystalline TiO2 sensitization of 34, 38, 39, 42, 49 surface of 25, 26, 31, 41 Nanodiamonds 101, 115-18, 120, 123, 137 Nanoindentation 66, 67 Nanometers 66, 67 Nano-particle, modeled metal 16, 17 Nanoparticle arrays 14, 15 random 14 rear-located 13 Nanoparticle chains 72 Nanoparticle deposition 14 Nanoparticle polymer nanocomposite 60 Nanoparticles 5-8, 12, 14, 15, 55, 56, 58-63, 65, 74,

75 core shell 61 functional 75 gold 78 isolated 8 method 59 mixed iron oxide ferromagnetic 73 n-doped 71 porphyrin-modified TiO2 27 positioned 14 p-type Cu2S 72 reagents 61 rear-located 14, 15 semiconductor 61, 73 silver 4, 10


smallest 7 spherical 7 sulfide 60, 61 Nanoparticle size 7, 14, 59, 69, 74 Nanoparticle surface 61, 74 Nanoparticle surface modification 61 Nanostructure 101, 116, 117 Naphthalocyanines 47 Net absorption 3, 5 N-type nanoparticle 59 N-type semiconductors 100-102, 111, 115, 116, 132 O Optical absorption 26, 101, 103-5, 109, 112, 116,

132, 138 high 101, 102, 110, 115, 131, 137 measured 112, 124 Optical absorption ranging 124, 137 Optical absorption spectra 117, 118 Optimization, structural 126, 127 Optimization of blended structures 137, 138 Organic molecules 48, 57, 61 various assemblies of 56 Organic pollutants 87, 89 Organic solar cells 101, 135 Organic synthesis 78-81, 87, 96 Organometallic bonds 62 Orient chemical industries Co 116 Oxidations 79, 87, 90, 92, 94-96 selective 87, 88 Oxide layer 12, 61 P Paramagnetic metalloporphyrins 29 Parameters 17, 73, 74, 81, 124 lattice 105, 119 Particle polarization 81, 82 Particles 7, 8, 10, 13-15, 61, 63, 64, 71, 73, 82 dielectric 7 Particle size distribution 13, 14 Particle sizes 7, 12, 13, 18, 64, 69, 74, 82, 105, 125 Particle surface 80, 82 Path length 5, 6 PEDOT, thin layer of 116 PEDOT layer 104, 116, 131 Penetration, depth of 66, 67 Percolation threshold 74 Permittivity 7, 90 Phenanthrenylethynyl 28 Phenol 88, 89, 96 Photo-absorption 117, 137 Photocatalysis 78, 79 Photocatalyst, plasmonic 78

Photoconduction, high 123, 127 Photocurrent 13 short-circuit 42 Photovoltaic cells 7, 71, 72 Photovoltaic efficiencies 101 Photovoltaic properties 31, 35, 48, 100, 101, 115 Phthalocyanine dimers 123 covalently-bridged 123, 127 Phthalocyanine macrocycle 43, 44, 46, 48 Phthalocyanine monomers 129 Phthalocyanines 21, 23, 38, 39, 41-48, 100, 101,

115, 123, 124, 127, 136 aggregation of 39 copper 70, 101 unsymmetrical 21, 42, 44-46 Phthalocyanine sensitizer 38, 44 Plasma 62, 81, 138 Polarizability 7 Polymer blends 57 Polymer chain 56, 57 Polymerization 62 Polymer layer 60, 63, 69, 71 Polymer materials 55, 58 Polymer matrix 55, 56, 58, 59, 61-63, 65, 72-74 Polymernanocomposite 55 Polymer Nanocomposite material 69 Polymer nanocomposites 55, 59, 62, 67, 69, 72-75 Polymers, conjugated 57 Porphyrin arrays 33 Porphyrin chromophore 27 Porphyrin dimers 35, 38 Porphyrin light absorbers 26 Porphyrin macrocycles 25, 34, 35 Porphyrins 21, 23-34, 38, 48, 49, 100, 101, 130,

131, 133, 136 monomeric 33, 35 substituted 24 Porphyrin sensitizers 23, 24, 32 based 33 Porphyrins sensitizers 33 Position, substitution 25 Power, mild oxidation 80-82 Properties electronic 101, 102, 116, 124 magnetic 61, 73, 74 Properties of disordered systems 74 PTCDA addition 132 PTCDA layers 136, 138 PTCDA solar cell 135, 136 P-type nanoparticle 59 P-type semiconductor layers 116 P-type semiconductors 100-102, 110, 111, 115,

116, 123, 131

146 Light Harvesting Nanomaterials Surya Prakash Singh

PVA matrix 71-72 PZn moiety 34 R Radiative recombination 67, 68 Reactant Product 88, 89 Reaction field 94, 96 Rear surfaces 12 Redox electrolyte, using liquid 28, 30 Redox properties 21, 23, 38 Redshift, clear 10, 12 Reductions 14, 28, 59, 79, 90, 91 Reflectance 16, 17 Reflection 3, 6, 9, 10, 12, 63 Reflection coefficient 9, 16 Refractive index 9, 12, 59 complex 10 Region, near-IR 29, 37, 38 Restricted Hartree-Fock (RHF) 124, 131 Ruthenium 21-23, 33, 39, 42, 43 S Scanning electron microscope (SEM) 10, 55, 64 Scanning Electron Microscopy 64 Scattered light 6 Scattering objects 5, 6, 8 Selectivity, high 79, 87, 88 Semiconductor interface 4, 6, 101, 122, 135 Semiconductor layers 9, 103, 111, 116, 131 Semiconductors 6-9, 23, 39, 56, 62, 75, 122, 129,

135 SEM images 4, 10, 14 Sensitizers 21-23, 26-33, 39, 42, 44-46, 48, 49 cocktail 46, 48 light-harvesting 37 metal-free 23 organic 46, 47 Short circuit 18, 124, 132 Sigma Aldrich 103, 104, 111, 116, 131 Sigma Aldrich Corp 111, 123 Silicon nitride ARC 18 embedded 16, 18 Solar cell absorption 5 Solar cell material 123 Solar Cell Parameters 17 Solar cells based 71, 73 bulkheterojunction 132 carbon-based 123 chalcopyrite 110 chalcopyrite compounds 110 c-Si 14, 15 dyesensitized 102

dye-sensitized 21 finished 12 fullerene heterojunction 123 generation 21 high efficiency 102 inorganic-organic hybrid 110 large-scale 101 multi-crystalline silicon 18, 19 nanocrystalline 29 nanodiamond-based 120, 123, 137 organic thin film 103 patterned 4 plasmonic 5 present bulk heterojunction 115 present C60-based 101, 102 semiconductor 34 sensitized TiO2 29 silicon 4, 101, 110 silicon-based 102 thin c-Si 13 thin film 102 Solar cells based 70 Solar photovoltaic cell 3, 6, 12, 19 Solvents, common organic 21, 43, 44 Soret band 23, 26, 31, 33, 37 Spectrum, solar 29, 38, 78, 80, 102 Spin coating 60, 104, 111, 116, 131 Spin-coating 103-8 Spin-coating methods 102, 105, 137 Stationary fixtures 65, 66 Support effect 78, 90-92 Surface coverage 14 Surface plasmon polaritons (SPPs) 4, 81 Surface plasmon resonances, localized 78, 79, 81 Surface plasmons 4, 11, 81 Suspension 83, 103 Synthesis of core-shell nanoparticles 61 Synthesis of p-type nanoparticle 59 T Technologies, green 78 TEM image 107, 108, 114, 118, 120, 121 TEM image of CIS nanoparticles 113 TEM images of polymer samples 64 Tetramethyl ammonium hydroxide 103 Thermal evaporation 10, 12, 69 Thermal stability 45, 56, 63 Thin films, nanodiamond-based 117 Thin-film solar cell structures 11 Thin films structure 117 Thin film structure 117 Thiophenolate 61 TiO2, conduction band of 24, 26, 33, 42, 44


TiO2 coating 12 TiO2 nanoparticles 25, 27 deposited porphyrin-modified 26 TiO2 oxidation processes 78 TiO2 photocatalytic reactions 79 TiO2 surface 25, 47 Transmission electron microscopy (TEM) 55, 64,

100, 104, 113, 137 Treatment, theoretical 9, 16 U Unoccupied orbital levels, lowest 128, 129 UV-light-activity 78-79, 96 UV-Vis absorption spectra 26, 90 V Vacuum, high 115 Visible-light-activity 96 VOC of organic solar cells 135 Voltages open circuit 124, 129, 132 open-circuit 105, 106, 111, 117, 130, 132

open-circuit photo 42 operating 104 Volume fraction 10, 16, 18 increasing Ag 18 W Water, de-ionized 116 Water phase 95, 96 Water splitting 78, 87, 89, 96 Wavelength 9, 12, 14-17, 63, 82, 90, 106, 112, 118,

125, 133 Wavelength of incident light 7 Weight, low molecular 57, 58 Weight losses 63 Wideband-gap semiconductors 21, 22, 24 X X-ray diffraction pattern of CIS 112, 113 Z Zinc porphyrins 26, 33

light harvesting nanomaterials

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