nanocrystalline tio2 photo-sensitized with natural dyes

Solar Asia 2011 Int. Conf., Institute of Fundamental Studies, Kandy, Sri Lanka ( 28-30 July 2011)

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NANOCRYSTALLINE TIO2 PHOTO-SENSITIZED WITH NATURAL DYES

P. ABEYGUNAWARDHANA1, S. PALAMAKUBURA2, C.A. THOTAWATTAGE 2, M.A.K.L. DISSANAYAKE2 AND G.K.R. SENADEERA1,2*

1 Department of Physics, The Open University of Sri Lanka, Nawala, Nugegoda, Sri Lanka.

2 Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka

* Corresponding Author, e-mail: [email protected]

ABSTRACT

Dye sensitized solar cells (DSSCs) are one of the most promising and fast developing photovoltaic technology, specially due to its low cost comparing with the other photovoltaic technologies. Generally DSSCs consist of ruthenium (II) polypyridine complexes as sensitizers of wide band gap semiconductors like TiO2. However the cost in synthesis of these dyes with rare metal complexes is still an expensive process. Therefore, many of the researchers including us have been investigating the possibilities to replace these expensive dyes with various materials like conducting polymers, natural dyes, etc. Therefore, in this work we extended our investigations employing natural dyes extracted from plants widely spread over the Sri Lankan territory.

Among the 65 studied natural dyes, Hibiscus sabdariffa ,Fire fern (Oxalis hedysaroides), Begonia

(Black velvet) ,and Ficus salicifolia sensitized solar cell showed comparatively higher photo responses. The short circuit current densities (Jsc) were ranging from 2.4 to 0.9 mA cm-2, while the open circuit voltages varied from 565 mV to 425 mV. The highest efficiency obtained from Hibiscus sabdariffa, was ≈ 0.5 % . However , dramatic enhancement in efficiency in these cells were obtained from the acid treated dye extracts with con HCl. The DSSCs fabricated with acid treated dye solution obtained from fire fern leaves showed ≈ 1% efficiency showing its promising usage in low cost DSSCs for novel disposable bio sensor applications at room temperature under low light conditions.

1. INTRODUCTION

Dye sensitized photoelectrochemical solar cells (DSSCs) are devices for the conversion of visible light into electricity based on sensitization of wife band gap semiconductors like TiO2. DSSC is assembled with a cathode of mesoporous nanocryastalline TiO2 film on a conductive glass substrate anchored a monolayer of dyes, an anode of conductive glass coated with platinum and an electrolyte of certain organic solvents containing a redox couple such as iodide/triiodide. The adsorbed dye absorbs visible light and injects electrons into the conduction band of TiO2. Since dye plays an important role in absorbing visible light and transferring photon energy into electricity, much attention has been paid to survey the effective sensitizer dyes. No doubtedly, one of the most efficient sensitizers are synthetic dyes, such as ruthenium(ii) polpyridyl complexes with carboxcilic ligands, since these compounds present intense and wide range absorption of visible light as well as suitable ground and excited state energy levels with respect to the band positions of TiO2 and the redox potential of electron donor I-. (1). Apart from that many organic and inorganic dyes including natural dyes have employed in these devices but the performances of them in DSSCs were poor due to the week binding nature to the semiconductor TiO2 (2-6). However, in nature, fruits, flowers and leafs of plants show various colours from red to purple and contain various natural dyes. Because of the simple preparation techniques, widely source and low cost, natural dye as an alternative senstizer for DSCs is promising , specially for some practical applications such as usage of DSSCs in portable, disposable bio sensors (7-9). Therefore, the present work extends our investigations involving natural dyes as semiconductor sensitizers and reports some of the dyes which can be used successfully in DSSCs as sensitizers aiming to use them in fabrication of low cost bio sensors.


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2. EXPERIMENTAL 2.1 Extraction of pigments Petals of flowers and barks of trees chosen were cut into small pieces and extracted into ethanol (Fluka, 96%(v/v)), by keeping for overnight. Then residual parts were removed by filtration and the filtrate was washed with hexane several times to remove any oil or chlorophyll present. The ethanol fraction was separated and divided into two solutions and one fraction was used as it was (without any further treatments) and the other fraction was treated with few drops of conc. HCl acid so that the solution color becomes more dark (pH < 1. These solutions were directly used as the dye solutions for preparation of photovoltaic devices. 2.1 Preparation of TiO2 electrodes Fluorine doped tin oxide (FTO) glass slides were cleaned well. The dimension of each glass slide was 2 cm x 0.5 cm. TiO2 (Degussa) powder of average particle size 25 nm was grounded well with Triton, acitic acid and ethanol. A thin film of the resultant mixture was deposited on FTO glass slides using the ‘doctor blade method’. After leaving sometime for drying, only an area of 0.5 cm x 0.5 cm was left by scratching off the rest. Films were then sintered at 450°C for 45 minutes and cooled down to room temperature. 2.3 Fabrication of photo-electrochemical cells (PECs) Above TiO2 electrodes were dipped in each dye solutions for 12 hours. Dyed electrodes were then removed and rinsed with ethanol and dried using an airflow. DSSCs were constructed by introducing the redox electrolyte containing tetrabutylammonium iodide (TBAI)(0.5M) / I2 (0.05M), in a mixture of acetonitrile and ethylene carbonate (6:4, v/v) between the dyed TiO2 electrode and Platinum counter electrode. 2.4 Characterization UV–visible absorption measurements of dyes in ethanol were carried out with a Shimadzu dual wavelength/double beam spectrophotometer (model UV-3000). Current voltage ( I-V) characteristics of the cells at 100 mW cm-2 (Am 1.5) were measured using a home made I-V measuring setup coupled with Keithly 2000 multimeter in to HA-301 Potentiostat via computer controlled software. Xenon 500 W lamp was used with AM 1.5 filters to obtain the simulated sunlight with the intensity of 100 mW cm-2. The I-V curves were used to calculate the short-circuit photocurrent density (JSC), the open-circuit voltage (VOC), the fill factor (FF), and the energy conversion efficiency (η) of DSSC. 3. RESULTS AND DISCUSSIONS

Figure 1 (A) shows the UV-visible absorption spectra of different dye extracts in ethanol. Since the purpose of this study is to use the dye extracts without any pigmential isolation, elucidation of exact structures were not carried out in this stage. However, according to the literature (10,11 ) the red extracts obtained from fire fern, begonia black velvet and hibiscus sabdariffa, which have the main absorption wave bands at 500 to 580 nm, it would be possible to presume that all these dyes contain cyanidin, peonidin, pelargonidin, malvidin, delphinidin, quercetin and flavonol co-pigments. Further, since the red-coloured varieties would contain lower content of delphinidin derived pigments, but a more variable amount of flavonolglycosides, the extracts obtained from above species should contain higher concentrations of above pigments.


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Figure 1. (A) UV-visible absorption curves of (a) Fire fern (b) Begonia black velvet (c) Hibiscus sabdariffa, (d) Ficus salicifolia (B) and (C) Basic chemical structures of the most abundant anthocyanidins .

The other species employed in this study, orange-red coloured Ficus salicifolia , which mainly absorb in the region below 475 nm , would contain a large amount of cyanidin 3,5-O-diglucoside and relatively lesser amount of flavonol-glycosides (7,10,11 ). However, in general, can be seen from the figure, they all absorb in the visible part of the spectrum. In the case of extracts from fire fern the absorption is border than that of the others and it absorbs light from 400-700 nm with an absorption in higher wavelengths.

While figure 2 shows the current –voltage (I-V) curves of the DSSCs employed with

different pigments as indicated, table 1 presents the performances of DSSCs in term of short-circuit photocurrent densities (Jsc) , open circuit voltage (Voc) , fill factor (FF) and the power conversion efficiencies (η). The efficiency of cell sensitized by the fire firm was significantly higher than DSSCs sensitized by the other pigments. This is in good agreement with the broader range of the light absorption of fire firn on TiO2 , and higher interaction between TiO2 and anthocyanin as depicted in the figure 4. The DSSCs employed with Ficus salifcifolia dye has the lowest photoelectric conversion, which again agrees with the weaker absorption spectrum of the dye. Figure 2. Current –voltage (I-V) characteristics of, (i) Fire fern (ii) Hibiscus sabdariffa (iii) Begonia black velvet (iv) Ficus salifcifolia

i

ii

iii

iv

(B)

(A) (C)


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Table 1. photovoltaic performances of DSSCs employed with extracted flower pigments

Name Jsc (mA cm-2)

Voc (mV)

FF η% Flower/leave/bark

(a) Fire fern 2.354 422 49 0.49

(b) Hibiscus sabdariffa 1.968 457 50 0.45

(c) Begonia black velvet 1.140 408 56 0.26

(d) Ficus salifcifolia 0.864 565 52 0.25

The effect of pH of these dye solutions were also investigated. The original pH of these extracted were found to be in the range from 3.00 to 4.5. However, in this preliminary studies pH of these extracts were changed to the values lower than 1 (pH <1) by adding trace amounts of conc HCl. Figure 3 (A) shows the chelation mechanism of anthocyanidins with TiO2 while figure 3(B) presents the possible reaction of the pigments with HCl as proposed by Hao et al (7 ). According to the literature, in an acidic solution, the oxonium ion results in an extended conjugation of double bonds through three rings of the aglycone moiety, which helps in the absorption of photons in the visible spectra. Addition of a base disrupts the conjugation of double bonds between the second and third rings and results in absorption of photons in the UV range, rather than in the visible range (7,10,11 ). Therefore, the change in pH on increasing the number of conjugated double bonds in the molecule, lowers the energy level of the electronic transition between the ground state and the excited states and in turn results in the absorption of photons at greater wavelength (Figure not shown).


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Figure 3 (A) Chelation mechanism of anthocyanidins with TiO2, (B) Two chemical structures of anthocyanidins in acidic and basic media.

Figure 4 presents the I-V characteristics of the DSSCs employed with acidified pigment

extracts. The photoelectric parameters of DSSCs sensitized with acidified extracts under the irradiance of 100 mW cm-2 were summarized in table 2. As can be seen from the table, the acidity of dye solution is found to affect the resulting photocurrent values. Again, it can be seen from the data presented in the table 2, fire firn exhibits the best photoelectrochemical performances among the others. The photoelectric power conversion efficiency ( η) of DSSCs sensitized with acidified fire fern is two times higher than that of the non acidified dye. Figure 4. Current –voltage characteristics of pigments after acidification (i) Fire fern (ii) Begonia black velvet (iii) Fiscus salifcifolia (iv) Hibiscus sabdariffa

(i)

(ii)

(iii) (iv)

(A)

(B) Basic Acidic


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Table 2. photoelectrochemical parameters of the DSSCs sensitized by acidified pigment extracts

Jsc (mA cm-2) Voc (mV) FF η% (i) Fire fern 3.09 438 61 0.82 (ii) Begonia black velvet 2.03 426 59 0.506 (iii) Ficus salifcifolia 1.48 442 52 0.339 (iv) Hibiscus sabdariffa 1.12 443 56 0.298

The Highest Occupied Molecular Orbital (HOMO) level and the Lowest Unoccupied Molecular Orbital level (LUMO) of three extracts were calculated using cyclic voltammograms and the UV-visible absorption spectra, according to the method described elsewhere (12 ) and tabulated in the table 3. Table 3. Estimated physical parameters of the dye extracts obtained from cyclicvoltametry and UV0visible absorption spectroscopic techniques.

Name of the dye solution Band Gap (eV) HOMO (eV) LUMO (eV)

Fire Fern 2.13 -5.01 -2.88 Begonia(Black velvet) 3.10 -4.90 -1.80

Hibiscus sabdariffa 3.43 -4.99 -1.57

As it is evident from the , table 3, the excited state energy levels for the extracts are higher

than the energy level of TiO2 conduction band edge (–4.40 eV) ( 1,2,19) showing that the electron injection should be possible energetically. Therefore, upon illumination of the cell, dye extracts absorb light and get excited from the HOMO level to the LUMO level and eject electrons to the conduction band of TiO2, which transports these electrons to the transparent bottom electrode (FTO) and then to the other end of the cell via external load and being received by the redox mediator (I3

– /I –) in the electrolyte. Finally the electrolyte (I3– /I –) regenerates the dyes to their

ground state, completing the cell reaction 4. CONCLUSION Under the present preparation and irradiation conditions, it was found that the DSSCs fabricated with fire fern extracts possesses the best photosensitized effect in the extracts studied in this study. Therefore, fire fern extracts should be an alternative anthocyanin source for DSSCs fabrication in geographical regions that fire fern is widely available. Moreover, the DSSCs fabricated with acidified fire fern extracts having ≈1% efficiencies, could easily employed in low cost , disposable DSSCs which can be employed in DNA biosensors specially under room temperature low light intensity conditions. Therefore, the use of a natural dye for the semiconductor sensitizer with a straight forward preparation would provide alternative to commonly used synthetic dyes in many environmentally friendly applications.


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nanocrystalline tio2 photo-sensitized with natural dyes

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