nanophotonoics for energy efficiency

D 3.1- Report of Research Activities in nanophotonics

for light harvesting and assessment of partner collaborations & future directions

Nanophotonics for Energy Efficiency

DELIVERABLE REPORT

Grant Agreement number: 248855

Project acronym: N4E

Project title: Nanophotonics for Energy Efficiency · Network of Excellence

Funding Scheme: FP7-ICT-2009-4

Deliverable reported: D 3.1- Report of Research Activities in nanophotonics for light harvesting and assessment of partner collaborations & future directions

Due date: M12

Name, title and organization of partner (task 3.1 leader): Dr. Srinivasan Anand · KTH

E-mail: [email protected]

Name, title and organization of partner (WP3 leader ): Dr. Nikolai Gaponik· TUD

E-mail: [email protected]

Project website address: www.nanophotonics4energy.eu

Reporter and scientific representative of the coordinator: Dr. Srinivasan Anand KTH

Dr. Gonçal Badenes ICFO




D 3.1- Report of Research Activities in nanophotonics for light harvesting and assessment of partner collaborations & future directions

This task 3.1 in WP3 is on light harvesting and addresses how nano-photonic structures with their unique physical properties can enable efficient harvesting of light, in particular for photovoltaic/solar cell applications. Central to this goal are the understanding the key physical phenomena to harness light matter interaction, together with developing appropriate nano-fabrication technologies with a strong emphasis on material properties, and implementation in photovoltaic devices. In addition, cost-effectiveness is also an important factor. To enhance light matter interaction/efficient light management, this task (3.1) on light harvesting in WP3 has been devoted to investigate and exploit technologies and concepts such as meta-materials, nanowires/rods, materials with tailored disorder, semiconductor nanocrystals, hybrid-materials, nanocrystal gels, non-toxic and environmentally friendly nanocrystals, and technologies for contacts. Below a brief introduction to the different research areas is given, followed by a summary (D3.1.2) of partner collaborations and future directions.

D3.1.1 Research Activities in nanophotonics for light harvesting

Plasmonic structures as well as dielectric photonic crystal (PC) structures have advantages for their unique properties for photovoltaic applications. One major goal is to assess these disruptive technologies for enhancement of light and matter interaction, whereby light absorption can be increased for photovoltaic applications. Implementation of plasmon effects, for instance localized surface plasma resonance, could be a key technology to increase the efficiency of semiconductor-based photovoltaic devices. Up to date, use of nanostructured metals in those devices remains disruptive with respect to the mainstream technology. Concerning dielectric photonic crystals, can also be used as diffractive structures to enhance optical path-length in the active layers, and with appropriate design can also provide light trapping. Large surface 3D PCs (Si) are attractive as thermophotovoltaic systems to re-emit the absorbed solar radiation with a spectrum better adapted to the solar cell gap.

Compared to ordered media such as photonic crystals, light propagation in disordered media comparatively less explored, and more so in tailored disorder media an example of which is a newly discovered material called levy glass. Such materials as well as quasi-periodic structures can be attractive for light harvesting to increase efficiency of solar cells and photovoltaic systems. The new optical properties of these materials (super diffusion) could be used to improve efficiency of Silicon and polymer based structures.

Semiconductor nanowire/nanorod solar cells are promising candidates as they can harness the advantages of one dimensional confined structures including enhanced photocurrent owing to increased junction area with a large surface-to-volume ratio and improved optical properties including light trapping, and higher absorption and lower reflectance with respect to their thin film




counterparts. Hitherto there are several semiconductors such as Si, GaN, ZnO, InP and GaAs which are potentially interesting for photovoltaic applications. However, the-state-of-art efficiencies of nanostructured solar cells are lower than their planar versions due to the problems encountered in nanofabrication processes and difficulties related to their poor surface passivation. Thus for the fabrication of such structures, different approaches have to be investigated for high optical quality materials.

Hybrid solar cells represent another disruptive technology to overcome the limitations of conventional technology. An essential ingredient in such hybrid solar cells is the advantage of combining materials and using the respective material properties advantageously. For example, by appropriate design extraction of charge carriers owing to the increased interaction between the substrate and materials exhibiting multiexciton generation (MEG) effects can be achieved. This could enable efficiencies higher than in conventional designs and the Shockley/Queisser efficiency limit may also be overcome. For example, conventional structures can be coated or infilled by spraying or spin-coating or drop-casting suitable colloidal quantum dots. Methods for “carrier multiplication” in such hybrid photovoltaics that utilize non-radiative energy transfer will be have to be investigated to exceed the thermodynamic limit. New technologies such as nanocrystal-based aerogels (3D self-assembled networks) to combine unique optical properties (light-harvesting, plasmonic, etc) of size-confined nanomaterials with charge and energy transporting abilities of the designed network are potentially attractive for such hybrid solar cells.

Nanophotonic concepts can be employed to enhance molecular harvesting of light. The optical response of photoactive materials can be greatly enhanced in the proximity of well-tailored resonant nano-structures. Specifically for multichromophoric complexes (perylenes, thiophenes, etc.) the proximity of resonant optical nano-antennas opens promising routes to control the optical interaction: First, the radiative decay is boosted 10 to 100 times down to picoseconds, far over any loss channel, which improves both photostability and quantum efficiency, with direct potential to generate super-emitters with ps photo-cycling times. Second, the enhanced rate gives rapid energy transfer in π-conjugated (excitonic) complexes. Third the resonant antenna mode determines the direction of emitting and receiving, acting as an angular receiver to capture and funnel the incident light efficiently to dedicated antenna hot spots and thus afford nano-focusing on the active photoactive material. The enhancement of both angular capture and energy transfer in π -conjugated systems due to resonant optical nanoantennas is an attractive route to improve efficiency for light harvesting.

1. Metamaterials for solar cells: (leader: ICFO, partners: LETI, UPC, KTH)

The research activities are focused on plasmonic structures and dielectric photonic crystals. The work carried out range from investigations of fundamental phenomena, technology (materials and processing) development, proof-of-principle demonstration and implementation of concepts in devices.

Plasmonic structures




ICFO has been investigating new strategies based on surface plasmons to increase the efficiency of thin organic photovoltaic cells. The present approach relies on structuring one of the cell electrodes with plasmonic pattern aimed at both (i) enhancing the collection of light and (ii) maximizing its confinement within the thin layer of PV material. Preliminary experiments have given promising results and the optimization of the design and the fabrication process are currently ongoing. In a related work, ICFO has demonstrated unidirectional emission from a nanoscale quantum emitter coupled to a nanofabricated Yagi-Uda antenna [1] The results show the potential of optical antennas to communicate energy to, from, and between nano-emitters.

Photonic crystal structures and Thermo-photovoltaics

UPC has been investigating different photonic crystal technologies (dielectric and metalo-dielectric) with a focus on thermo-photovoltaics. The thermal properties of macroporous silicon photonic crystals with the unit cell gradually varied along the pore axis were investigated [2]. It was shown experimentally that arbitrarily large omnidirectional total-reflectance bands can be produced with such structures. It was also demonstrated that these bands can be effectively used to reduce thermal radiation in large spectral bands. In a related work, UPC has developed 3D metallo-dielectric structures using macroporous Si as templates [3] and proposed that this method could open a route for the fabrication of large-scale 3D-periodic metallic micro-structures. Three-dimensional (3D) periodic nickel micro-structures and a high number of structural periods were fabricated by electrodeposition. Macroporous silicon, consisting of periodic arrays of sine-wave modulated pores, was used as a deposition template. It was prepared by electrochemical etching of silicon and subsequent pore widening by multiple oxidation/oxide-removal steps. It was shown that pore widening allows formation of openings between adjacent pores obtaining a 3D network of interconnected voids embedded in silicon. This structure was then void-free filled with nickel in the electroplating process resulting in a 3D metalo-dielectric periodic structure.

BILKENT’s activities in this area have been on InGaN photonic crystal solar cells including material growth and characterization [4,5], and, on metamaterial absorbers where in they designed and experimentally verified metamaterial based microwave absorbers [6]. In particular, the possibility to increase the absorption of a thin InGaN film compatible with a solar cell structure by incorporating a photonic crystal type of array on the surface has been investigated. Current research in this topic has been mainly on the mainly on the fabrication of an InGaN p-i-n type photovoltaic device, and on the absorption properties of an InGaN/GaN based photonic crystal implemented structure investigated by appropriate numerical calculations (FDTD). Preliminary results were promising showing an increase in the absorption. The work on InGaN PC solar cells will be continued and more numerical calculations will be carried out in order to optimize our design.

KTH has developed high-aspect ratio etching of nanostructures in InP, both nano-slots and nano-holes, primarily for photonic crystal devices. This technology is directly applicable for implementing hole-type photonic crystals in window layers of solar cells (e.g. GaInP/GaAs solar cells) for increasing absorption in GaAs. In particular a new post-etch method – thermally driven




reflow of material – has been developed to reshape nanostructures [7]. In addition it was shown that material reflow significantly reduces hole depth and shape irregularities which are inherent (or inevitable) to the etch-process. Although this work was on InP-technology the demonstration of material reflow as a method to reshape nanostructures is applicable to other materials as well.

CSIC has investigated 2D photonic crystals to obtain large emission enhancement from Chromophores (Section 5).

At CEA, 2D photonic crystal have been realised on the backside of silicon wafer-based solar cells. Integration of such technology showed some difficulties, especially with respect to the interaction with passivation layers and contact layers. Another important showstopper is the fact that most of silicon substrates used for building industrial solar cells are not polished : a strong and random texturation exists on both front and back surfaces.

2. Nanowires/nanorods for photovoltaics: (leader: LETI, partners: KTH, CSIC, TUD, UPC)

The research activities on nanowires/nanorods have been on technology of nanowire/rods fabrication/synthesis in different materials and on new combinations of structures, in particular quantum dots with nanowire/rods. The research also addresses cost-effective fabrication in conventional semiconductors by top-down method and by solution synthesis.

BILKENT has investigated the integration of CdSe nanocrystals intimately on radial p-n junction based Si nanopillar solar cells [8]. The proposal was to use these nanocrystals which strongly absorb incident UV and blue light, but emit at a longer wavelength which matches the higher spectral response of the silicon solar cell. Through hybridization of these colloidal quantum dots on Si nanopillar solar cells, approximately 13% enhancement of overall solar conversion efficiency under AM1.5G conditions was demonstrated. Additionally, a maximum responsivity enhancement of more than four-fold in UV using the nanopillar solar cells compared to the planar cell case was obtained. Thus performance enhancement via the hybridization of optical wavelength upconverting nanocrystal quantum dots around the pillars together with light trapping is very promising for high efficiency photovoltaics.

TUD has been developing nanowires and gels made of semiconductor quantum dots and together with BILKENT is currently applying these technologies for light harvesting (and light generation) applications. TUD is investigating new approaches for the formation of optically active nanowires and gels as well as to widen the list of materials suitable for gelation. A quick and reproducible formation of the gels from the nanoparticles capped with tertazole-derivatives has been developed [9]. TUD together with BILKENT will further develop and exploit this technique for the controllable preparation of nanowires and gels from different materials including strongly absorbing IV-VI (e.g. PbS, PbSe) semiconductor quantum dots promising as synthesizers for light harvesting applications; and for fabrication of hybrid semiconductor1-semiconductor2 and




semiconductor-metal 3D structures which have potential applications as light-harvesting and energy-transporting systems in nanophotonics and photovoltaics.

KTH has developed fabrication of nanopillars in InP [10], GaP, and Si using nanosphere lithography (NSL) and dry etching using techniques such as ICP-RIE and chemically assisted ion beam etching. Here we briefly report on fabrication of InP-nanopillars. Using ICP-RIE with Ar/Cl2/H2/CH4 chemistry InP-Pillars with varied sizes were fabricated by depositing colloidal SiO2 (dispersed in water) particles with different sizes on the sample and/or by reducing sizes of the deposited SiO2 particles by dry etching. Nanopillars with different heights and shapes from near cylindrical to conical are obtained by varying different etch parameters such as ICP power, rf power, etch time and progressive erosion of colloidal SiO2 mask. The lateral size reduction of the fabricated pillars was also performed using a Sulphur-based wet chemical etch step and a five-fold enhancement of PL intensity at RT was observed due to surface passivation. Dense array of conical InP pillars have been fabricated which allow continuous grading of refractive index which is beneficial for antireflection in solar cells and for light extraction in InP-based NIR LEDs. In parallel KTH has been investigating self-organized formation of III-V nanopillars primarily on the physical mechanism of pillar formation, and optical properties. Future work will focus on quantifying the total reflectivity, transmission and absorption in these structures; and on fabrication and characterization of pn junction (both radial and axial) solar cells.

3. Hybrid solar cells (leader: US, TUD, BILKENT, UPC)

The research activities are focused on novel solutions to overcome limitations of conventional technology. In this regard, the primary research focus has been on solar energy conversion hybrid devices using multi-exciton effects, new composites such as nanocrystal based aerogels for efficient charge and energy transfer, quantum dot solar cells made by solution synthesis and on fundamental investigations of these phenomena. By complementing the advantages of different classes of materials to overcome their drawbacks, highly efficient hybrid optoelectronic devices can be achieved. Semiconductor colloidal quantum dots (QDs) offer the possibility of flexible, low cost, large area, and simple-processed optoelectronic devices. However, they have inherently low carrier transfer compared to, for example, III-V semiconductors.

US has been investigating energy transfer mechanisms, materials, and device concepts based on these [11,12,13]. In particular hybrid colloidal QD/patterned pin GaAs heterostructures that utilise nonradiative energy transfer from highly absorbing colloidal QDs to a high carrier mobility patterned semiconductor slab has been extensively studied. Semiconductor colloidal quantum dots (QDs) offer the possibility of flexible, low cost, large area, and simple-processed optoelectronic devices. However, they have inherently low carrier transfer compared to, for example, III-V semiconductors. The hybrid configuration investigated by US releases the potential of colloidal QDs as luminophores with large oscillator strength in efficient light harvesting devices, while overcoming altogether the limitations imposed by low carrier transfer.




BILKENT together with TUD has investigated light harvesting in solution with quantum dot antennas [14]. Nonradiative energy transfer based light harvesting of aqueous colloidal CdTe quantum dot antennas for dye molecules in water was studied. The experiments show that these quantum dots used as donors need to be carefully optimized to match Rhodamine B used as acceptors. Strong lifetime modifications of these CdTe quantum dots from 25.3 to 7.2 ns have been observed. The energy transfer efficiency was tuned up to 86% as the acceptor-donor concentration ratio is varied. These experiments indicate that nonradiative energy transfer mediated light harvesting using aqueous quantum dots leads to enhanced emission of dye molecules in water at wavelengths beyond the absorption range of the dyes. The work also notes that a good operating point in the A/D concentration ratio for a specific donor-acceptor pair has to be set to provide both reasonably high efficiency and high light harvesting of the acceptor emission. This nonradiative energy transfer assisted light harvesting holds great potential for future quantum dot multiplexed biological and optoelectronic applications.

TUD together with BILKENT are investigating nanocrystal based aerogels for photovoltaic applications with properties such as optical absorption and efficient charge and energy transport. (Also see Section 2).

ICFO has been working on the development of solution processed colloidal nanocrystals materials for solar cell cells. The first solution processed solar cell based on colloidal quantum dots with power conversion efficiency (PCE) exceeding 5% has been demonstrated [15] and represents a major step in bringing this disruptive technology closer to the commercialization target of PCE ~10%. The fabricated colloidal quantum dot (CQD) photovoltaic devices on transparent conductive oxides (TCOs) were shown to rely on the establishment of a depletion region for field-driven charge transport and separation. These devices also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. The highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety were reported. ICFO has established a new collaboration with US to study energy transfer from colloidal quantum dots to crystalline Si, aiming in developing novel hybrid solar cells based on ultra thin film Si.

ICFO and TUD have initiated work to investigate colloidal nanocrystals (NCs) of SnS as a potential non-toxic and environmentally friendly material for solar cells. SnS was chosen as it is a p-type semiconductor based on non-toxic and earth-crust abundant materials and has an extremely high absorption coefficient on the order of 105-106 cm-1 in the visible allowing for full light absorption using ultra thin films of 100-500 nm. SnS also has the optimal band gap of 1.4 eV for maximum power harnessing which can reach the Shockley-Queisser limit of ~33%. The partners have taken the initial steps to develop the first ever solution-processed non-toxic inorganic solar cell based on a p-n heterojucntion formed from p-type SnS and n-type nanoporous TiO2. A bulk-heterojunction approach whose designed purpose is to break the absorption-carrier extraction trade-




off through the formation of fully depleted zones within the pores of the structure at the interface of TiO2 and SnS has been employed. An in depth characterization of the system has been undertaken: initial characterization of the charge transfer processes in the TiO2-SnS interface by electrochemistry, C-V measurements; photoconductivity and FET measurements to measure the charge transport properties of the SnS medium. Independent investigations by cap-voltage and impedance spectroscopy in solid-state devices measure the carrier density and associated depletion widths in our bulk heterojunction structures are also planned. Optimization of the adhesion of SnS nanocrystals to the TiO2 substrate and development of strategies for cross-linking the nanoparticles to increase the carrier mobility using surface chemistry (ligand exchange processes) have been initiated. ICFO has received SnS NCs synthesised specifically for this investigation, from the TUD group and had performed the first attempts to fabricate p-n bulk heterojunctions by infiltrating p-type SnS NCs into mesoporous TiO2 substrates. To date the major achievements are: A strongly rectifying junction with rectification ratios on the order of 104 ; a photo-response from the developed device under external applied bias ; and a first demonstration of a solution processed non-toxic photodiode with a quantum efficiency on the order of 1-5%.

4. Disordered systems (leader LENS, CSIC, UPC)

Disordered systems (random and quasi-random) offer interesting possibilities such as diffusive transport of light and light trapping, both of which can be used for light harvesting. The research activities in this area have been on theory, design and optical characterization of such systems for efficient light trapping. Fabrication, primarily, developed using conventional semiconductor technology for photonic crystals in thin films could be one possible system for experimental realization of such systems for light harvesting.

LENS has studied the behavior of light in disordered media and has developed random and quasi random patterns useful for light harvesting and light generation [16-22]. In particular, LENS has studied theoretically how a random pattern applied in a thin film waveguide can be used efficiently to trap light waves. It has been shown that the concept can be scaled to different wavelength ranges and is very broadband (>500 nm) and angular independent (0-180 degrees). It has been found that increase of efficiency can be as high as one order of magnitude for ultra thin film Silicon based solar cells. This concept is now in the experimental testing phase and has been patented (patent pending). The above concept will be applied in lighting to design efficient diffuse light sources in which the light cone can be tuned. That is, in which the extraction from e.g. OLED sources can be tuned over an angular spreading, with specific spectral requirements. On the topic of light trapping by disordered photonic structures, LENS has started a new collaboration with KTH.

In parallel LENS has worked on random lasing, in particular the use of Levy glasses as basis material for novel random laser sources. The initial experimental results show that it is possible to obtain random lasing in activated Levy glasses. The mode structure of the random laser seems an extremely interesting point of further study that we will address in the coming year. These concepts also have a bearing on light harvesting.




KTH has developed nano-pattering of SOI and InP/GaInAsP membranes. The InP/GaInAsP layers are grown by MOVPE and also includes optical emitters in the 1.2 to 1.55 µm range. Thin films of InP/GaInAsP have been fabricated by selective wet-chemical etching, and investigations for suitable bonding methods are planned in the near future. Together with LENS, KTH will investigate the application of this technology for fabrication of light trapping structures in thin film Si and InP.

5. Molecular harvesting (leader: ICFO, partner CSIC)

The research theme has been on investigating methods to enhance the optical response of photoactive materials using tailored resonant nanostructures and on fundamental processes in excitation energy transfer in molecular assemblies.

In this area, ICFO has been working on electronic coherences in single molecules studied with femtosecond phase-controlled spectroscopy [23,24]. Excitation energy transfer in molecular assemblies is the main mechanism behind energy transfer in photosynthetic assemblies. Evidence is growing that quantum coherences play a significant role in the initial ultrafast steps of excitation energy transfer in such photosynthetic assemblies, just as in conjugated polymers, and dendritic systems, all at room temperature. To observe the role of coherence it is essential to remove any conformational inhomogeneous disorder, i.e. detect at the level of single units. These processes typically occur on (sub)picosecond time scales at room temperature, whereas the time-resolution of single-molecule techniques was limited to some tens of picoseconds, as achieved with time-correlated single-photon counting. To this end, ICFO has investigated such typically highly heterogeneous molecular assemblies with an ultrafast single-molecule technique that allows both coherent and incoherent processes to be addressed. ICFO has demonstrated unidirectional emission from a nanoscale quantum emitter coupled to a nanofabricated Yagi-Uda antenna [1], and such optical antenna concepts can be applied for single molecules.

CSIC has investigated chromophores in 2D photonic crystals. Mixed structures formed by self-assembled photonic crystals based on monolayers of spheres and metallic substrates provide an easy-implementation fabrication method to obtain large emission enhancements as well as controllable angular emission patterns [25]. In addition, by continuously reducing the diameter of polystyrene spheres deposited on gold substrates while keeping the lattice constant unchanged, we show that the modal position can be strongly modified [26]. In a related work on Si-nanocrystals they demonstrate magnesiothermic reduction of SiO2 opals yielding porous silicon structures mainly composed by silicon nanocrystals (Si-NCs). By this means, self-supported ensemble of emitting Si-NCs has been prepared and passivated with Al2O3 grown by ALD [27]. FRET in chromophore modified DNA has been investigated. In particular, DNA-CTMA used as dopant in polymeric matrices has been studied for FRET improvement and tuning. The final goal was to incorporate these materials in photonic nanostructures [28].

6. Modeling light behaviour (leader: CSIC, partners ICFO, TUD, US, LENS)




This activity is on modelling the behaviour of light in nanostructured media for enhanced light harvesting. The aims of this task are (1) to provide a database of the models developed in the different topics mentioned above and (2) to make recommendations for further development of the modelling tools. To this end the different partners have been using electromagnetic simulation tools to model behavior of light in plasmonic structures, meta-materials, photonic crystals, random and quasi-periodic structures and nano-wire/rod arrays. During next year, the plan is to collect more detailed information on the available tools and their respective applications, and implement them in the database of resources and facilities in the network (WP1).

7. Platform for solar cells (leader UPC, partners LETI, ICFO, KTH, TUD, CSIC)

This activity is two-fold: (1) Development of solar cell technology-materials and processing such as for example new approaches for transparent contacts, etc. and (2) to provide a bench-mark for new concepts based on disruptive technologies [eg. as in research activities:1-5, above]

UPC has developed Silicon Solar cell technology with > 20% efficiency achieved on 4 cm2; and has been working on laser processing of back contacts and for emitter diffusion in solar cells. In organic Solar Cell technology UPC is building a set up for small molecule sublimation for multilayer solar cell fabrication. UPC has been investigating LASER fired contacts (LFC) on silicon heterojunction solar cells for the application to rear contact structures [29,30]. Optimization of the rear contact scheme of p-type c-Si LFC-PERC high efficiency solar cells minimizing base ohmic losses without jeopardize rear passivation has been performed. This was carried out by optimizing on one hand the LFC laser conditions for minimum point resistance and on the other hand through a proper design of the contact grid layout finding the optimum trade off for a given base resistivity between rear passivation and base resistance. LFC process was carried out through 110 nm thermal SiO2 passivation layer using IR and green lasers. Very low specific contact resistances, 0.1 mΩ cm2 have been achieved independently of the laser used. For optimum rear contacted area fraction efficiencies over 21.5% and 22%, for IR and green lasers respectively are expected in the 0.5-5 Ω cm resistivity range. UPC has also investigated optical and electronic properties of organic molecules (e.g. Alq(3) and Pentacene) relevant for organic light emitters and electronic devices [31,32].

ICFO has been investigating different contact schemes including new materials and ultra-thin metal layers with optimized conductivity [33-35]. In particular the focus has been on the development of Indium free organic photovoltaic cells that incorporate an ultrathin metal film as a semitransparent anode [33,34]. In the proposed device structure, the indium tin oxide electrode is replaced by an ultrathin Cu-Ni bilayer. When NiO is used as the hole transporting layer, the characteristic photovoltaic parameters of the cell fabricated with the metal electrode are similar to those of the device fabricated with the indium tin oxide (ITO). Despite the fact that the metal electrode exhibits a transparency that is 65% of the ITO electrode, the short circuit current for the metallic anode based cell is 77% of the ITO based one, indicating that photon absorption could be enhanced by the optical microcavity formed between the Cu-Ni and Al electrodes. The overall photo conversion efficiency for the metallic electrode based cell is 76% of the ITO based one,




which was measured to be 3.3%. The obtained performances of ultrathin metals when included in the cell architecture used here, combined with their low cost, high compatibility with other materials, and mechanical flexibility, confirm their potentials for organic photovoltaics.

CEA operates a technology platform for the development of new fabrication processes and basic roadblocks of silicon solar cells technology. The RESTAURE platform is installed in a clean room near Chambéry in France. Standard equipments are available for fabricating silicon wafer-based solar cells, from any kind of silicon material, from electronic grade mono crystalline wafer to said metallurgical grade multi crystalline ingots. Wet benches, diffusion furnaces, PECVD deposition, screen printing for metallization and annealing are used in the standard process. Other equipments are used for the development and research of new technology steps or completely new integrated processes: PVD and CVD specific deposition (dielectric, TCO, semiconductor), lithography... Characterization tools and control tools are used to validate raw material performances, technology process steps and final evaluation of the performances of various PV devices. Solar cells design is supported by expertise and simulation tools. We work on cell design : various homojonction concepts, heterojunction built from amorphous silicon deposition and more advanced architecture based on new materials. PV module encapsulation is also studied to develop new solutions to enhance efficiency and durability of these finalised products.

D3.1.2 ASSESSMENT OF PARTNER COLLABORATIONS AND FUTURE DIRECTIONS:

As described above, all partners of the consortium are actively involved in the topic of light harvesting. Many of the partners have several research groups integrated in the network and have internal collaborations. Concerning the collaborations between partners, several explicit joint research activities have been established (see list below). And more such focused joint research activities are expected to emerge in connection with the Seed Project Scheme, both at the proposal formulation and project execution stages. The database of resources and facilities available in the network (WP1) could be used effectively by the partners to identify new collaborations and research exchanges, both within and outside the Seed projects.

NEW JOINT ACTIVITIES:

TUD together with BILKENT are investigating nanocrystal based aerogels for photovoltaic applications.

LENS has established collaboration with KTH on the topic of light trapping by disordered photonic structures.

BILKENT together with TUD has investigated light harvesting in solution with quantum dot antennas.

ICFO has established collaboration with US to develop novel hybrid solar cells based on ultra thin film Si.




ICFO and TUD have initiated work to investigate colloidal nanocrystals (NCs) of SnS as a potential non-toxic and environmentally friendly material for solar cells.

Concerning the research topics, as described above and as also documented by the publications, the partners of the consortium have made considerable efforts on the investigation of technologies and concepts for light harvesting such as meta-materials, nanowires/rods, materials with tailored disorder, semiconductor nanocrystals, hybrid-materials, nanocrystal gels, non-toxic and environmentally friendly nanocrystals, and technologies for contacts. Notably during year 1 a number of new joint research activities have been initiated on the topic of light harvesting. Although the research efforts in these disruptive technologies for light harvesting, both within the consortium as well as internationally, indicate appreciable progress the research is far from maturity. Similarly, it is too early to identify/select out most promising ones from the various approaches/technologies investigated here. Thus it is recommended that all the research topics listed above for light harvesting be continued within the network. In the activity Platform for Solar Cells (7) specifically with respect to bench marking devices using new technologies/concepts the platforms at UPC and CEA should be available during the entire duration of the network. Concerning the activity Modeling light behavior in nanostructured materials (6), the objective to provide a data-base of the models developed /available at partner sites should be moved to WP1 (Task: database of resources and facilities in the network ).

PUBLICATIONS

1. G. Curto, G. Volpe, T. H. Taminiau, M. P. Kreuzer, R. Quidant, and N. F. van Hulst, Unidirectional emission of a quantum dot coupled to a nanoantenna”, Science 329, 930-933 (2010).

2. M. Garin, T. Trifonov, D. Hernandez, A. Rodriguez, and R. Alcubilla, Thermal emission of macroporous silicon chirped photonic crystals, Optics Letters 35 (20):3348-3350 2010.

3. D. Hernandez, D. Lange, T. Trifonov, M. Garin, M. Garcia, A. Rodriguez, and R. Alcubilla, 3D metallo-dielectric structures combining electrochemical and electroplating techniques, Microelectronic Engineering 87 (5-8):1458-1462 2010.

4. M. K. Öztürk, Yu Hongbo, B. Sarıkavak, S. Korçak and S. Özçelik, et al Structural analysis of an InGaN/GaN based light emitting diode by X-ray diffraction, J Mater Sci: Mater Electron 21:185-191, (2010).

5. A. Yildiz, S.B. Lisesivdin, P. Tasli, E. Ozbay, and M. Kasap, Determination of the critical indium composition corresponding to the metal-insulator transition in InxGa1-xN (0.06 ≤ x ≤ 0.135), Current Applied Physics 10, 838-841, (2010).

6. K. B. Alici, F. Bilotti, L. Vegni, and E. Ozbay, Experimental verification of metamaterial based microwave absorbers, Journal of Applied Physics 108, 083113, (2010).




7. N. Shahid, S. Naureen, M-Y. Li, M. Swillo, and S. Anand, A novel post-etch process to realize high quality photonic crystals in InP, submitted to J. Vac. Sci. Technol. B, (2010).

8. B. Guzelturk, E. Mutlugun, X.Wang, K. L. Pey and H. V. Demir, Photovoltaic nanopillar radial junction diode architecture enhanced by integrating semiconductor quantum dot nanocrystals as light harvesters, Applied Physics Letters 97, 093111 (2010).

9. V. Lesnyak, S. V. Voitekhovich, P. N. Gaponik, N. Gaponik, and A. Eychmüller, CdTe Nanocrystals Capped with a Tetrazolyl Analogue of Thioglycolic Acid: Aqueous Synthesis, Characterization, and Metal-Assisted Assembly, ACS Nano, 2010, 4, 4090-4096.

10. M. Y. Li, S. Naureen, N. Shahid and S. Anand, Fabrication of sub-micron InP nanopillars by colloidal lithography and dry etching, J. Electrochemical Society 157, H896 (2010)

11. J.J. Rindermann, Y. Akhtman, J. Richardson, T. Brown, and P.G. Lagoudakis, Gauging the flexibility of fluorescent markers for the interpretation of fluorescence resonance energy transfer, Journal of the American Chemical Society (2010), Article ASAP, DOI:10.1021/ja105720j.

12. S. Chanyawadee, P.G. Lagoudakis, R.T. Harley, M.D.B. Charlton, D.V. Talapin, and S. Lin, Increased color conversion efficiency in hybrid light emitting diodes utilizing non-radiative energy transfer, Advanced Materials 22, (5) 602 (2010).

13. Galia Pozina, Lili Yang, Qingxiang Zhao, Lars Hultman, and Pavlos Lagoudakis, Size dependent carrier recombination in ZnO nanocrystals, Appl. Phys. Lett. 97, 131909 (2010).

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