Motivated by recent experiments on nanocrystal superstructures, we study theoretically optical and photo-current responses of hybrid complexes assembled from semiconductor quantum dots (QDs), nanowires (NWs), metal nanoparticles (NPs), and photosynthetic molecules. QDs and NWs can be arranged into light-harvesting complexes [1,2]. In these complexes, nanocrystals are coupled via Förster energy transfer (FRET). Consequently, this coupling creates a flow of excitons from QDs to NWs. Excitons harvested in NWs can be ionized and used to create photo-voltage. Using kinetic equations for excitons, we model exciton transport in QD-NW and NP-NW complexes and explain the origin of a blue shift of exciton emission observed in the experiment [3]. Another system of our interest is a complex composed of natural photosynthetic reaction centers, semiconductor QDs, and metal NPs [4,5]. We show that, by using superior optical properties of nanoparticles and involving energy transfer, one can strongly enhance an efficiency of light harvesting in natural photosynthetic systems [6-8]. Potential applications of hybrid exciton-plasmon systems are in photovoltaic devices and sensors. [1] J. Lee, A. O. Govorov, and N. A. Kotov, Nano Letters 5, 2063 (2005). [2] P. Hernandez-Martinez and A. O. Govorov, Phys. Rev. B 78, 035314 (2008). [3] J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials 6, 291 (2007). [4] A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007). [5] A. O. Govorov, Adv. Mater., 20, 4330 (2008). [6] S. Mackowski, S. Wörmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Bräuchle, Nano Lett. 8, 558 (2008). [7] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V. Zagidullin, V. Pachenko, Y. Rakovich, J. F. Donegan, A.B. Rubin, and A.O. Govorov, Angew. Chemie, 49, 7217 (2010). [8] I. Carmeli, L. Lieberman, L. Kraversky, Z. Fan, A. O. Govorov, G. Markovich, and S. Richter, Nano Letters, 10, 2069 (2010).

Dye-sensitized solar cells (DSCs) are inexpensive photovoltaic devices that can convert sunlight to electricity with relatively high efficiency. They are favorable alternatives to conventional solid-state solar cells consisting of materials such as Si, CdTe and CuIn_1-xGa_xSe₂. However, their use of organic liquid electrolytes seriously limits long-term performance and durability because of their inevitable problems of high volatility, leakage, and complex chemistry. Despite extensive studies to replace liquid electrolytes, the efficiencies of the resulting DSCs remain modest. Here we demonstrate that the p-type inorganic direct bandgap semiconductor CsSnX₃ (X = halogens or their mixtures) with high-hole-mobility can be solution-processable at room temperature to form all-solid-state DSCs and replace the problematic organic liquid electrolytes. CsSnX₃ compounds are made of inexpensive and earth-abundant elements. The resulting solid-state DSCs consist of CsSnX₃, nanoporous TiO₃ and the Ru dye, and exhibit conversion efficiencies up to ca. 10 per cent. References 1. I. Chung, B.-H. Lee, R. P. H. Chang, M. G. Kanatzidis, Nature2012, 485, 486. 2. I. Chung, J.-H. Song, J. Im, J. Androulakis, C. Malliakas, H. Li, A. J. Freeman, J. T. Kenney, M. G. Kanatzidis, J. Am. Chem. Soc.2012, 134, 8579.

Due to their unique features, semiconductor quantum dots (QDs) are presented as the ultimate frontier as sensitizers for photoelectrochemical solar cells [1],[2]. Up to now, the most interesting results in terms of device performances have been obtained by using polidisperse, in situ generated QDs by means of successive ionic layer absorption and reaction (SILAR) technique [3],[4],[which allows obtaining naked QDs directly grown on the porous structure of the photoanodes, thus guaranteeing an intimate contact between the two interfaces. Moreover, the deposition of networks of QDs presenting absorption features able to collect a wider region of the solar spectrum is easily possible [5]. This study is focused on the application of spray deposition (SD) [6] to the SILAR technique to generate QDs (CdS and PbS) on TiO2 photoanodes. We demonstrate that the use of SD-SILAR systematically results in higher amount of QDs together with smaller nanocrystals as compared with the classical immersion SILAR. Moreover, a reduced amount of chemicals is needed for the preparation of QDs, thus decreasing the environmental impact of the procedure. SD provides for a highly homogeneous coverage of the TiO2 photoanodes for the whole depth of the substrate. Evaluation of the performances of the quantum dot-sensitized solar cells indicates that devices prepared via SD-SILAR present improved functional properties, especially related to photoconversion efficiency and photocurrent density, both of them being almost two-fold the corresponding prepared by immersion SILAR. [1] S. Rühle et al., Chem. Phys. Chem. 2010, 11, 2290 [2] A. Shabaev et al. Nano Lett. 2006, 6, 2856 [3] Y-L. Lee and Y-S. Lo Adv. Func. Mat. 2009, 19 604 [4] H. Lee H et al., Nano Lett. 2009, 9, 4221 [5] A. Bragaet al., J. Phys. Chem. Lett. 2011 2 454 [6] S. Che et al., J. Aer. Sci. 1998, 29 271

By the year 2050, the world&’s energy utilization will have doubled while fossil fuels will be dwindling. Fortunately, if just 0.2% of the earth&’s surface were covered in 10% efficient solar cells by 2050, our energy needs would be met. The only real barrier to such widespread deployment is cost. There are several third generation solar technologies that could make widespread solar cell deployment economically feasible. Amongst those, Dye-Sensitized Solar Cells (DSSCs) show the most promise. With current DSSC efficiencies peaking at over 12.5%, the focus of research needs to shift to mass production. Our research focuses on using inkjet printing to produce the much cheaper DSSCs in a roll-to-roll manufacturing process on flexible substrates with CsSnI3, CsSnI2.95F0.05, and similar molecules as a solid-state electrolyte (SSE) as well as the Z-907 quasi-SSE. Currently, using an aqueous ink containing 10% TiO2 and a liquid I-/I3 electrolyte, we have produced 3.52% efficient DSSCs with a fill factor of 0.668. While this is a good start, there is room for improvement in both the efficiency and the manufacturability of the cells produced with inkjet printing techniques. To further optimize inkjet printed DSSCs, we are investigating TiO2 layer thickness as a function of print speed, drop size, drop spacing, and ink solvent. By maximizing the TiO2 layer thickness the number of printed layers required to produce the ideal DSSC thickness of around 13mu;m can be reduced. This allows cells to be produced with fewer print heads and accompanying lower capital and energy costs. To further reduce cycle times and energy costs, we are investigating the use of fast drying solvents such as low molecular weight alcohols, acetone, and acetonitrile. Additionally, we are optimizing the printability of our ink through the use of various surfactants and viscosifying agents. To ensure the ink will print, a viscosity of at least 7 mPa.s is ideal to ensure the printability of the inks. Currently we are examining glycerol, diethlyene glycol, and polyvinylpyrolidone as viscosifying agents for aqueous inks and polyisobutylene and cyclohexanol for solvent-based inks. In an effort to reduce the variation in efficiency between different cells, improve TiO2 distribution upon drying, and better control film spreading after deposition we are examining the use of Triton 100X, triethanolamine, and Surfynol 465 as surfactants in aqueous inks. To overcome the durability problems associated with I-/I3 while minimizing efficiency loss, we are currently studying the SSEs to investigate potential transport and recombination issues at the P-N junction, ability of the SSE to interact with the dye based on particle size, and the effects of various methods of depositing the SSE on cell efficiency. Also under examination is the effect of various solvents on the SSEs&’ transport characteristics and the potential for inkjet printing.

Efficiency of dye-sensitized solar cells (DSC) reached 11 % (certified efficiency of cells with more than 1 cm2). One of problems remaining is encapsulation of the cell. We focused on cylindrical solar cells because the shape allows easy and perfect encapsulation. In addition, it has been reported that CuInGaSeS (CIGS) type cylindrical solar cell has some advantages over flat type solar cells from the view point of the total amount of solar light harvesting in a day and light weight modules. We have reported cylindrical DSCs with 5.6 % efficiency. The cell was back contact type solar cells which do not need expensive and awkward transparent conductive oxide layered glasses (TCO). The TCO-less structure actually made the fabrication of the cylinder DSC possible. The cell consists of round-shaped glass/TiO2-dye layers fabricated on Ti protected metal mesh (working electrode)/gel electrolyte sheet/Pt-Ti working as an counter electrodes, from the outside to the inside. The fabrication process is precisely explained in the presentation. The photo active area was experimentally measured by a laser beam induced current (LBIC) method. It was found that the area where light does not reach directly also caused photoconversion. Projected photoactive area against the total area was calculated to be 66 %. However, the actual photoacitive area obtained the LBIC method was 93% of the total projected area. This was explained by the fact that the glass wall act as an optical wave guide. The optical wave guide effect was simulated by using ZEMAX software and these results were consistent with experimental results. The optical wave guide effect was largely affected by dielectric constant of electrolyte compositions. Finally, we report comparison of totally generated electricity in a day between cylindrical DSC and flat DSC. The former was 1.3 times larger than the latter which proved the effectiveness of the cylinder type DSC.

A key challenge in photovoltaics is design optimization accounting for the interdependent optical and electrical properties of solar cells together with their material microstructure. We present a simulation-based tool for simultaneously evaluating these factors and illustrate its use for thin-film amorphous Si solar cells with advanced light-trapping structures. Light-trapping strategies generally include both random structuring and controlled nanostructuring. This study focuses on periodic nanostructures, which enable improved control of light absorption through coupling of light into localized resonant and guided wave modes. These light-trapping structures may be metallic or dielectric, implemented on the front, back, and/or internal surfaces of the device, and can include structuring the active material itself. Our method begins with a full wave optical simulation from which we calculate an optical absorption profile within the device. This is used as input into a finite element device physics simulation to model the full electrical performance of the structured device under illumination, accounting for imperfect carrier collection from the active layers. The structures investigated here are based on a Ag back reflector and contact, a ZnO:Al buffer layer, an n-i-p amorphous Si active region, and an ITO top contact. Among the structures studied, we find that the highest efficiency is achieved when all the layers are conformally textured. The inclusion of texturing in the Ag layer increases optical coupling of 650 nm light to a waveguide mode confined in the i-region. This improves carrier collection by avoiding parasitic absorption in the n- and p-layers, which exhibit higher levels of trap-mediated carrier recombination. This effect increases the internal quantum efficiency of charge collection from 73.8% to 75.3% and accounts for approximately one quarter of the relative efficiency improvement of 5.7% compared to a device with a flat back reflector; the remainder comes from the increase in overall light absorption. Material deposited on highly structured substrates is known to exhibit defects that can degrade electrical performance. We account for this effect by introducing a functional dependence of the material parameters used in the electrical simulation on the device morphology. We consider both homogeneous material degradation and localized defective regions correlated with specific geometric features. In the presence of morphologically induced defects, we find that improvements in current are offset by declines in open-circuit voltage and fill factor. However, thinner cells benefit from a stronger drift field in the i-layer making them more robust to such performance degradation. This modeling framework, based on coupled optical and electrical simulations, enables full optoelectronic device optimization simultaneously accounting for the interplay and trade-offs between the optical, electrical, and material properties of solar cells.

It is well known that the cadmium chloride surface treatment is an essential step in the manufacture of efficient thin film CdTe solar cells. The precise mechanisms involved have never before been identified. It has been recognized that the combination of annealing at ~4000C together with adding cadmium chloride at the surface induces re-crystallisation of the cadmium telluride layer and also affects the n-type cadmium sulfide. In this study we have been able to reveal the precise mechanisms at work. We have applied advanced micro-structural characterization techniques to study the effect of the cadmium chloride treatment on the physical properties of the cadmium telluride solar cell deposited via close space sublimation (CSS) and magnetron sputtering and relate these observations to device performance. A range of analytical techniques has been used to observe the morphological changes to the microstructure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Structural and morphological properties have been studied using a Field Emission Gun Scanning Electron Microscopy (FEG-SEM); Transmission Electron Microscopy (TEM) has been used for sub-grain analysis. Chemical analysis has been carried out using a Scanning Transmission Electron Microscope (STEM), along with Energy Dispersive Spectroscopy (EDS) to examine elemental distribution in the multilayer structured cell. Grain orientation data has been obtained using Electron Backscatter Diffraction (EBSD) on Focused Ion Beam (FIB) prepared cross-sections and planar sections providing grain to grain orientation. X-ray photoelectron spectroscopy (XPS) depth profile chemical analysis indicates chlorine rich regions appearing at the cadmium telluride/cadmium sulfide interface. A change in grain size and change in texture is observed in EBSD and XRD, which confirms grain re-crystallization. Stacking faults observed within grains in TEM diffraction are removed via the cadmium chloride treatment; these findings show the chlorine plays an important role in the formation of efficient cadmium telluride solar cells and therefore the mechanism which this occurs and the influence this has on the device characteristics

CIGS solar cells modeling is problematic due to the microcrystalline structure of the absorber (Burgelman, Nollet et al. 2000; Werner, KOLODENNY et al. 2011). Development of a ISE-TCAD based realistic absorber model is proposed, with the specific objective to take into account, among several effects, this challenging aspect. The CIGS/CdS solar cell is modeled as an array of columnar microcells, in electrical parallel, mimicking the polycrystalline nature of the absorber. Charge transport and recombination are affected, in each microcell, by the grain boundary presence and the correspondingly associated defect states. The carrier mobility itself depends on their distance from the grain separation interfaces. In each microcell the model incorporates: the energy profile of defects states in both CuInxGa1-xSe2 and CdS, SRH recombination at grain boundary and at the interface with CdS, electrical conduction by thermionic emission at the interface (Niemegeers, Burgelman et al. 1995), ideal behavior of the transparent conducting oxide. The model takes also into account the not-uniform elements (Cu,In, Ga, Se) distribution inside the material where, in particular, Cu is known to migrate towards the central regions of the grain with a reduced majority carriers population at the grain boundary, (Herberholz, Rau et al. 1999) potentially leading to the formation of “weak diodes” cells. Initially, the model optical and electrical parameters are optimized based on a review of available experimental material characterization and realization results and, subsequently, the model is used in the framework of a design and optimization process of a low bandgap CGS like solar cell to be used under high intensity monochromatic radiation coupled to a solar pumped laser. The experimentally realized component tests and their comparison with the simulations will be presented to validate the model and open it to further improvements for the development of bandgap tuned CIGS cells to be used in spectral splitting concentrating solar systems.

Thin-film solar cells are sensitive to the processing conditions under which they are synthesized. CuIn1-xGaxS2 (CIGS2) chalcopyrite thin-film absorbers can be synthesized both with copper-rich and copper-poor compositions. When grown in copper-rich regime, the formation of excess, liquid-like CuxS phase acts as a fluxing agent for the growth of absorber films resulting in improved grain growth. However, when starting with a copper-poor composition, it is difficult to produce device quality films in this regime due to the unavailability of CuxS. Therefore, a sodium precursor such as NaF is used as a fluxing agent for grain growth and development in addition to other improvements in electronic properties realized by its presence. In this study, CIGS2 thin-film absorbers were synthesized in copper-rich and copper-poor regimes using a two-step process under identical parameters of sulfurization temperature time and partial pressure of hydrogen sulfide gas. Structural and morphological characterization of the resulting absorbers was carried out using secondary electron microscopy (SEM), x-ray diffraction (XRD), atomic force microscopy (AFM). Photovoltaic devices were also completed with the synthesized absorbers and the device characterization was carried out with current-voltage measurement under light and dark conditions. The various PV parameters obtains form the light and dark I-V analysis namely, open-circuit voltage, short-circuit current, fill-factor , efficiency, reverse saturation current, diode quality factor and other results obtained from this study are compared and presented.

The DOE SunShot Initiative (within the Solar Energy Technology Program) aims to reduce the installed costs of solar energy systems by 75% by the end of the decade, achieving grid parity for subsidy-free solar energy. SunShot drives American innovation through advanced research and development, strengthening domestic manufacturing and cutting-edge technology. If successful, the SunShot Initiative will ensure solar energy is a viable and economic source for the nation&’s power needs and will significantly contribute to U.S. prosperity in the 21st century. For solar energy to become competitive with other energy resources, the installed cost for utility-scale photovoltaic (PV) solar systems must reach $1 per watt or, equivalently, 5-6cent; per kilowatt hour (kWh). While much progress has been made in achieving these goals through recent cost reductions, learning curve analysis indicates that the SunShot goals can only be realized through gains due to innovation . For the solar industry to remain competitive in this changing market, there must be mechanisms for incorporating innovation into existing products and systems and for transitioning to new products as technology advances. SunShot currently offers a number of materials related funding opportunities that create pathways for feedback between applied researchers and basic science researchers so that fundamental insights and discoveries can be rapidly developed and/or transitioned to applied research, existing product lines and projects. Current and forthcoming programs to support materials science and research that develops new capabilities that enables optimum design and/or performance of Photovoltaic and Concentrating Solar Power devices and systems will be described.

Dislocations are a primary performance limiting defect for multicrystalline silicon (mc-Si) solar cells, which currently comprise the largest share of the industrial market. To reduce dislocation density in as-grown and processed materials, we aim to develop techniques that can enable single-crystal silicon performance out of the lower-cost multicrystalline material. In this contribution, we focus on the application of mechanical stress during high-temperature annealing to promote enhanced dislocation density reduction relative to stress-free thermal annealing. The presence of mechanical stress is generally thought to ensure dislocation multiplication, given research findings related to crystal growth [1, 2]. We have explored the application of mechanical stress during high-temperature annealing to promote dislocation motion, increasing the probability of pairwise annihilation and sinking at grain boundaries and/or surfaces. Our initial results are based on a 3-point bending arrangement which applied force to the sample via relative thermal expansion [3]. With this experiment, we hypothesized that a low magnitude of stress is effective in stimulating enhanced dislocation density reduction; preliminary results indicate strong dislocation density reduction (>90%) in regions with moderate stresses. We argue that certain types of stress in moderation may actually be beneficial, i.e., a possible tool to reduce dislocation density and increase solar cell efficiency. As a second experiment, we designed and built a compression-testing fixture that applies a controlled stress (up to 25 MPa) at multiple temperatures (up to 1400 °C). The objective of this experiment is to decouple the influences of different stress-tensor elements, elucidating the fundamental driving forces of dislocation density reduction. With this work, we will further challenge the traditional notion that net dislocation growth is purely positive while under the influence of stress. We will conclude by highlighting pathways to translate these findings to industrial growth systems. With the continued importance of mc-Si material, and increased interest in quasi-mono and advanced kerfless techniques, we believe that our fundamental findings hold strong potential for future application. [1] D. Franke et al., “Silicon ingot casting: process development by numerical simulations,” Solar Energy Materials & Solar Cells 72, 83-92 (2002). [2] M. M&’Hamdi and E. Olsen, “Analysis of dislocation multiplication during multicrystalline silicon ingot casting,” Proc. 21st European Photovoltaic Solar Energy Conference, Dresden, Germany, 4-8 September 2006. [3] M.I. Bertoni et al., “Stress-enhanced dislocation density reduction in multicrystalline silicon,” Physica Status Solidi, RRL, 5, 28-30 (2011).

Modern solar cell front face contacts are created by screen printing a silver and glass frit paste on to the antireflection coating (ARC) surface of the cell emitter. Upon heating to ~800 C, the glass etches through the SiNx ARC, and in the ideal situation, dissolved silver particles precipitate at the Si interface forming favorable contact and a highly conductive electrode. Contact quality is potentially inconsistent since there is no guarantee that the silver particles fully populate the interface. In fact, estimates suggest only 5% substrate coverage by metal, the rest of the area being filled in by glass. Here we report a transformational approach to contact formation based on nitrogen gettering is presented. The contact formation process relies on the local reduction of a SiNx ARC by a reactive base metal, and its subsequent conversion to a metallically conducting nitride. Example metals include Ti and Zr. This concept is particularly attractive because: 1) there is no residual glass; 2) (Ti,Zr)N exhibit some of the lowest contact resistivity values to Si; and 3) this contact does not rely on noble metals. We will present first the results of powder processing experiments to pelletize various reactive metal alloys combined with amorphous SiNx to identify the times and temperatures required to convert between SiNx and TiN. X-ray diffraction and calorimetry data shows that this conversion begins in the vicinity of 900 C for Ti, but can be lowered to ~ 750 C with the addition of Sn. Several Ti:Sn compositions were evaluated, and the Ti6Sn5 stoichiometry provided the optimal solid state reactions. To evaluate contact formation contact resistivity values were measured by the circular transmission line method (CTLM). Sputtering targets with the Ti6Sn5 composition were prepared by conventional power processing methods from Ti and Sn powders. Lithogrpahically patterned CTLM films were deposited on commercial monocrystalline solar cells with industry standard ~90 nm PECVD SiNx antireflection coatings. Heating these wafers to 875 oC for 1.5 minutes was sufficient to penetrate the ARC and to achieve consistently specific contact resistance values of 0.19 mOmega;/cm2; this is a factor of 10 lower than commonly observed for paste-based contacts. Interfacial TEM imaging reveals a nitride/Si interface, as opposed to Ag-frit pastes that are characteristically non-uniform with respect to electrical contact. This represents a completely new approach to solar cell contact formation, with superior contact properties and a potential for enhanced cell efficiency and reduced costs. Prototype solar cells were prepared in collaboration with the Georgia Tech University Center of Excellence for Photovoltaics. Initial measurements show promising cell efficiency values, however, additional optimizations are needed, with particular attention to preserving the hydrogen content of the ARC upon contact formation.

Shockley and Queisser showed that there is a fundamental limit to the fraction of incident solar energy that can be harvested by a photovoltaic device with a single bandgap. A variety of approaches to exceeding this limit have been explored, including multijunction (tandem) and intermediate band solar cells. These approaches improve efficiency by harvesting different portions of the solar spectrum with optical transitions tailored for each portion. Quantum Dots have been extensively considered for use in such devices because their confined states can be tuned to tailor absorption energies and because the absence of a continuum of states may provide suppression of nonradiative relaxation. We discuss the feasibility of using epitaxial InAs self-assembled quantum dots to harvest low-energy photons. We show that the inhomogeneous distribution of energy levels present in realistic ensembles of quantum dots prevents the formation of delocalized bands and discuss the implication of these results for intermediate band solar cells. We then show that new nanostructured materials based on quantum dots may enable efficient upconversion of two low-energy photons into a single high-energy photon. We show how this upconversion can enable dramatic enhancements in the efficiency with which solar energy is harvested and discuss our progress toward realizing efficient quantum dot-based upconversion materials.

Metallic impurities are detrimental to the performance of silicon devices, even at concentrations as dilute as parts per billion. Silicon photovoltaics are particularly sensitive because of the need to transport minority carriers across the thickness of the device. Due to the rapid solid-state diffusion of many metals in silicon, the high temperatures involved in the manufacturing of silicon solar cells not only present a risk for contamination, but they can also significantly change the distribution of metal impurities already present inside the wafer. While the control of metal point defect density has been the focus of much research in solar and in the semiconductor industry, the high total metal concentrations found in solar-grade silicon generally exceed the solid solubility at processing temperatures and thus control of metal precipitates is requisite. In the high-density point defect state, metal impurities severely degrade the minority carrier lifetime, but low-density metal-silicide precipitates are associated with the presence of device shunts and generally act as semi-infinite sources of point defects during processing. Thorough understanding of metal impurity kinetics, especially the interplay between metal point defects and precipitates, is required to achieve effective defect engineering that will enable high device efficiencies at low cost. Through the application of kinetics-based processing models supported by bulk electronic characterization and microscopic X-ray fluorescence (XRF) experimentation, we find that the high-temperature solar cell processing regimes can be re-designed to reduce the final impact of impurities across a wide range of input material quality. Synchrotron-based nano-XRF measurements are conducted of silicon samples at varying states of solar cell processing to examine on the nano-scale the efficacy of the applied processing schemes. When viewed in combination with the kinetics modeling, our results show that depending on the nature of the starting material, and weighing the tolerance for additional processing cost and time, different defect engineering strategies should be adopted than those currently industrially employed. In the end, we extract processing guidelines dictated by the material itself that should improve device performance.

In developing photovoltaic (PV) systems with reliable lifetime performances, it is critical to have quantitative knowledge of not just initial properties and performances, but also their performance over the warrantied 25 year lifetime. In 2010, the Science for Energy Technology Workshop, convened by U.S Department of Energy (DOE) Basic Energy Science, prioritized photovoltaic module lifetime and degradation science (L&DS), which serve as the basis for quantitative and mechanistic understanding of lifetime performance. In order to better understand degradation rates and mechanisms of PV systems in the real-world environment. The SDLE SunFarm at Case Western Reserve University is a highly instrumented outdoor test facility with 148 PV modules and > 8000 samples on sun for weathering and degradation studies of materials components and systems designed for long-lived energy systems. I-V and power performance of 10 multi-crystalline silicon PV modules from different manufacturers using baseline and continuous power monitoring and comprehensive weather and solar resource monitoring to enable time series analysis for insights into performance characteristics and initial degradation. Five modules from each manufacturer were exposed using mirror augmentation in typical (Cleveland, OH) climatic conditions. The mirror augmentation used geometric concentration factors of 1X, 1.5X and 1.9X of the nominal 1 sun. The effect of mirror augmentation on the modules&’ performance is reported. A Daystar multi-tracer was used to measure I-V curves of individual modules each 15 minutes while power output under maximum power point tracking was monitored continuously. Monitoring environmental factors (wind speed, wind direction, rainfall, and humidity), solar resource, and module temperatures allow for determination of the effects of these conditions on module power production. Power data was corrected to standard test condition (STC) according to climatic and solar irradiance. Changes in fill factor, short circuit current, open circuit voltage and maximum power are reported for each module are reported. With time series analysis, a better understanding of the module&’s performance over time and under environmental conditions can be developed.

Conventional single junction photovoltaic devices have made impressive advances in efficiency in recent years, but still operate far below the intrinsic thermodynamic efficiency limits for solar energy conversion. To reach significantly higher efficiencies (>30% for single junction cells and >50% for multijunction cells), the solar-cell architecture must be radically modified to minimize thermodynamic losses due to i) photon entropy gain and ii) carrier thermalization arising from the ‘quantum defect&’ between the absorbed photon energy and bandgap energy . The first key factor, photon entropy gain, can first be reduced by integration of photonic light directors at the solar cell front surface to limit dark current in the radiative emission limit to only that solid angular range corresponding to the disk of the Sun. Second, perfect light trapping must be achieved in thin film cells by using wavelength-scale structures that increase the photonic density of states up to and some cases beyond the 4n^2 statistical ray optics limit. Third, the internal radiative efficiency much reach to near-unity, as is possible for example in GaAs. However other photovoltaic materials such as crystalline Si, copper indium gallisum diselenide and cadmium telluride have internal radiate efficiencies much less than unity. Light management can potentially be used to enhance internal radiative efficiency by enhancing the photonic density of states, for example by engineering the modal dispersion in a thin-film solar cell absorber layer. The second major factor limiting solar-cell performance is carrier thermalization. Conventionally, multi-junction solar cells are made in a series-connected architecture, with each of 3 or 4 subcells that reduce carrier thermalization losses, but these thermalization losses are still substantial. Alternatively, an optically-in-parallel array of high-efficiency single-junction cells that form the receiver of a spectrum-splitting photonic structure can easily accommodate a larger number (e.g., 8-10) of subcells limiting carrier thermalization to approximately 10%. Spectrum-splitting photonic structures and system architectures than can enable >50% efficiency will be discussed.

For the first time wet alkaline texturing of both surfaces of ~25mu;m thin monocrystalline (100) silicon substrates produced by a novel Semiconductor on Metal (SOM®) process has been demonstrated. This SOM® process is a novel kerfless exfoliation technology capable of producing ultra-thin flexible monocrystalline silicon substrates from a thicker (>450mu;m) parent wafer. One of the many advantages of this process is that it allows completion of process steps such as texturing, junction formation, passivation, and contact formation on the rear surface of the solar cell while it is still at a thick wafer form. After exfoliation, the metal backing enables handling of the thin silicon substrate during the wet texturing, thin film depositions, and silver screen printing processes on the front surface. The metal backing also acts as both a back surface reflector (BSR) and a rear electrode for the PV cell. The reflectance characteristics of the solar cells fabricated with this process have been analyzed for various texturing options and back side metal/dielectric stacks. An improvement of absorption by 58% in the measured range of near IR (740-1120nm) region is observed on ultra-thin ~25mu;m monocrystalline silicon substrates with the use of antireflective coating, texturing, and back surface dielectric. Solar cells with plasma assisted chemical vapor deposited doped amorphous silicon (a-Si) to form p+ front heterojunction emitter without intrinsic amorphous silicon (i-a-Si) layer passivation and difused n+ back surface field (BSF), are fabricated with these thin substrates with n-type base using a metal/dielectric stack BSR. External Quantum Efficiency (EQE) measurements show an increase in long wavelength response due to rear surface texturing in fully fabricated solar cells resulting in an improvement of ~2mA/cm2 of integrated current density. Optimizing a-Si deposition process on textured front surface to enhance ultraviolet (UV) and blue response and thereby improving overall current density is currently in progress. Our champion cell with i-a-Si passivation and only rear surface textured shows an efficiency of 14.9%, and a current density of 33.6mA/cm2. Simulations suggest that with optimized light trapping and surface passivation, such thin monocrystalline silicon solar cells can reach efficiencies >20%.

Thin film silicon solar cells, commonly made from microcrystalline silicon (mu;c-Si) or amorphous silicon (a-Si), have been considered inexpensive alternatives to wafer-thick polycrystalline silicon solar cells. However, the low solar cell efficiency of these thin film cells has become a major problem, which prevents thin film silicon cells from being able to compete with other solar cells in the market. One source of inefficiency is the light reflection off the interface between the thin film cell&’s top transparent conducting oxide (TCO) and the light absorbing silicon, which is mainly caused by the steep change of refractive index between the two materials. In this work, we demonstrate the use of nanocone textured ZnO as the anti-reflection surface that mitigates this problem. The tapered structure of the nanocone forms a smooth transition of refractive index on the interface between the TCO and the silicon, effectively acting as a wideband anti-reflection coating. Finite Difference Time Domain simulation is used to estimate the optimal ZnO nanocone geometry (periodicity and height) to be applied on a single junction microcrystalline silicon (mu;c-Si) solar cell. Relative improvement over 25% in optical performance is achieved in the simulated structure when compared to state-of-the-art mu;c-Si cell structure. Afterwards, we develop a scalable and inexpensive fabrication process for the nanocone structure using colloidal lithography combined with Langmuir-Blodgett process. The nanocone structure was fabricated on 4” dia. fused silica substrate by dry etching after closely packed nanosophere assembly [1]. Aluminum-doped ZnO (AZO) is then deposited on the textured glass, which results in the formation of AZO nanocones. Since the ZnO texturing technique works by depositing ZnO on nanocone-textured glass substrate, the technique is potentially applicable to transparent conducting oxides other than ZnO as well, making it a useful TCO texturing technique for various thin film solar cell applications. [1] C-M. Hsu, S. T. Connor, M. X. Tang, Y. Cui, “Wafer-scale silicon nanopillars and nanocones by Langmuir-Blodgett assembly and etching”, Applied Physics Letters93, 133109 (2008).

There is growing interest in nanostructured thin film designs for high efficiency thin film solar cells. Significant absorption enhancements can be achieved using resonant dielectric nanostructures by trapping the light in the active layer of the thin film. Here we report on a computational and experimental effort to design polarization-independent, angle-insensitive, broadband spectral response by direct coupling of incoming light to the resonant modes of subwavelength-scale nanoresonators incorporated into the active layer of thin film crystalline silicon solar cells. Our prototype structure consists of a two-dimensional periodic array of 150 nm thick Si nanoresonators on a silica substrate. A crossed trapezoid shape[1] is used of rectangular cross section absorbers in order to excite broadband Mie resonances across the visible spectra to achieve broadband and polarization-independent light absorption. Full-field electromagnetic simulations were used to design parameters and maximize broadband absorption, with a 420% overall enhancement relative to planar 220 nm thick Si films. This design featured trapezoidal Si resonators with 200 nm and 300 nm long bases, 150 nm height and periodicity of 600 nm. (18.5 mA/cm2 for 220 nm thick Si film) We have experimentally tested our predictions by optical absorption spectroscopy and spectral response photocurrent measurements in planar and nanoresonator-patterned 220 nm thick Si-on-insulator (SOI) films. Nanoresonator patterned silicon thin film devices were fabricated on SOI wafers using electron beam lithography and reactive ion etching techniques after removal of the Si substrate. Angular-resolved reflection-transmission measurements were performed using an integrating sphere set-up. Photocurrent spectral response measurements were made using a lateral Schottky and p-i-n photodiodes fabricated using photolithography techniques. Comparisons between predicted and measured optical absorption and spectral response will be reported.

High performance light trapping concepts are a precondition for the continuing success of silicon thin film photovoltaic technologies. The authors present optical simulations of highly efficient three-dimensional periodic light trapping textures for polycrystalline silicon thin film solar cells, that have recently been demonstrated experimentally on large areas (50sqcm) [1]. Realistic solar cell models were developed on the basis of transmission electron microscopic images of silicon layers on periodically textured solgel coated glass substrates. In the experimental deposition procedure, amorphous silicon is first deposited on the textured substrates by electron beam evaporation, which leads to growth of dome-like silicon structures. During a subsequent annealing step, distinct regions of an interconnected poly-crystalline grid, isolated single crystals and amorphous material are formed. The amorphous regions and isolated crystals can be selectively removed. For the simulation of the structures, a precise finite element model of the silicon volume was built where all of these regions were considered separately. Solar cells in the superstrate layout with a standard contacting scheme, employing a ZnO:Al front contact and a ZnO:Al/silver back contact, were simulated using the experimental geometric structure of the silicon absorber. The superstrate contribution to light trapping was estimated using an incoherent iterative coupling of the solar cell model and the superstrate layer. In case of the untreated silicon layer, simulations showed a superstrate contribution to total absorptance of about 10%, integrated for wavelengths above 600nm and normal incidence on the device. Silicon absorptance values greater than 40% of the incoming light were maintained up to wavelengths above 1000nm, with only 2.4 micrometer effective absorber height. A further enhancement of the red response of the cell was attained by substitution of the conventional conformal back reflector by a flat back reflector concept. To obtain a high quality opto-electrical device in solar cell production, additional structure modifications might be necessary. By removing the amorphous silicon regions and embedding the free-standing isolated silicon crystals in organic material, we can manufacture a three-dimensional crystalline silicon solar cell design. The complete removal of the isolated crystals and amorphous regions, in contrast, leads to a periodic micro-hole structure. For assessing the optical performance of these individually modified structures, corresponding geometrical models were simulated. Resulting cell currents were calculated for all simulated cell designs. [1] T. Sontheimer et al., phys. stat.sol. RRL 5, 376 (2011)

It has been estimated that photovoltaics must reach a cost of production of ~0.50 $/W and a power conversion efficiency greater than 20% to achieve widespread parity with existing grid energy. Although a number of technologies have exceeded this efficiency threshold (e.g. monocrystalline silicon, GaAs, III-V multijunction cells), none of them is definitely capable of simultaneously achieving these high efficiencies and low costs. Here we explore a novel architecture that combines the high efficiencies of multijunction devices with the low cost and high throughput potential of organic photovoltaics. An organic-inorganic hybrid tandem photovoltaic (HTPV) is composed of an organic cell on top of one of a variety of inorganic cells, and has the potential to improve moderately efficient (~15%) inorganic technologies, such as silicon and CIGS, to over 20%. The cost of the organic absorber layers has been estimated to be less than $10 m^-2, so that an organic top cell may be added to a variety of commercial inorganic cells with little addition cost. Furthermore, organic photovoltaics have a significant processing advantage over inorganic alternatives because they can be easily deposited by solution processing techniques at or near room temperature. This enables the addition of an organic top cell to a variety of inorganic bottom cells without concern for how the deposition of the top cell will damage the performance of the bottom. We have explored the design space for HTPV to determine how such a device can be best implemented, and have modeled HTPV devices in a number of configurations to predict their efficiencies. The best organics for this application are wide band gap absorbers that achieve very high open circuit voltages. We find that organic top cells are theoretically capable of improving both moderately efficient silicon and CIGS bottom cells to over 20%. Furthermore, the addition of an organic top cell relaxes some design constraints on the bottom cells by serving as a substitute for top-surface passivation in silicon cells and very thin CdS layers in CIGS. This is primarily because these inorganic photovoltaic technologies have poor carrier collection efficiencies for blue and UV photons, and in an HTPV device the organic top cell absorbs strongly at these wavelengths, mitigating this loss. We find that today's polymer bulk heterojunction technology is already capable of adding significant improvement to CIGS bottom cells; using today's best-performing organic cells, which can reach voltages of ~0.95 V, an organic top cell can improve a 15% commercial CIGS cell to ~16.5%. Lastly, we report on the highest experimental efficiency achieved to-date for an HTPV device and discuss the design challenges of moving efficiencies toward the predicted 20%. Our work demonstrates that HTPV has the potential to reach the low cost and high efficiency targets that will make photovoltaics competitive with non-renewable sources of energy.

We report DFT calculations that study hybrid solar cells consisting of inorganic GaAs nanowires and organic polymer semiconductors. Our system of interest consists of GaAs nanopillars with a top orientation of (111) and six (110) side walls covered by a thin layer of polymer film. There are major advantages to the use of such nanostructure based hybrid systems including a dramatic increase in light absorption as well as carrier transport. We will show how to improve the efficiency of the hybrid solar cell system by making changes such as the type of surface passivating agent used, including alkane thiols, sulfides, and aromatic oligothiols. We find that the choice of the passivating agent is important, as it affects both the binding of the polymer to the GaAs surface as well as the electronic properties of the solar cell. We find that the passivating agents have different coverage, binding sites and binding energies on the Ga or As terminated (111), (100), and (110) surfaces. We use state of the art hybrid functionals to calculate the band alignment of a number of possible organic/inorganic solar cell systems. These results will guide in the design and lead to a better understanding of such systems.

Novel optoelectronic and sensing materials are based on semiconductor structures with plurality of discrete and charged quantum dots. In these materials both harvesting of subband photons and kinetics of photocarriers are determined by the quantum dot charge. The equilibrium value of the dot charge is given by selective doping of the interdot space. The nonequilibrium component of the dot charge depends on the carrier generation rate, electron and hole capture times, and their dependence on the dot charge. Selective doping of interdot space and corresponding dot charging strongly increase harvesting of subband photons. Charged dots also allow for creating very special 3D potential profiles required by applications. In particular, dot charging may be employed to create potential barriers around single quantum dots or groups of quantum dots and separate by these barriers the dots from the conducting channels. Manipulations with potential barriers provide an effective tool for suppression of fast capture processes, which increases the photocarrier lifetime and reduce the recombination losses. This presentation includes modeling of optoelectronic processes, design of novel quantum dot nanomaterials, and their fabrication. Dark I-V characteristics, their temperature dependences, spectral characteristic of photoresponse, and photoluminescence measurements provide complex characterization, which allows one to understand basic photoelectron processes and their dependences on the dot charge. Main experimental results are summarized in Fig. 1, which shows IR photoresponse via transitions through QD levels of photovoltaic devices (left) and photodetectors (right) as a function of the dot charge. As seen, both the short circuit current of the solar cell and photocurrent of the detector strongly increase due to dot charging. Providing high coupling to IR radiation and long photocarrier lifetime, the nanomaterials with charged dots have a number of other attractive features, such as high scalability and enhanced radiation hardness.

Vertically [1] or in-plane [2] arranged semiconductor quantum dot (QD) arrays emerged recently as promising structures for the high efficient solar cell devices. In this work, we have employed a multiband k.p theory combined with the periodic boundary conditions to calculate the electronic band structures, optical and dynamic processes of InAs/GaAs QD arrays and compare them to experimental results. For typical InAs/GaAs QD arrays with vertical coupling, we have estimated energy gaps between the valence band (VB) and intermediate band (IB) of 1.2 eV and between IB and conduction band (CB) of 0.124 eV. The predicted efficiency of such IBSC in the radiative limit is 39%. Our predictions suggest that the most promising design for an IB material that will exhibit its own quasi-Fermi level is to employ small InAs/GaAs QDs (~6-10 nm QD lateral size) [3]. With appropriate design of the QD array structural parameters: (1) the regions of pure zero DOS between IB and the rest of the CB states, that is desirable for “photon sorting” and increased efficiency of the cell are identified, and (2) the strong optically allowed excitation between IB and CB exists [3]. Analysis of various radiative and nonradiative processes indicates that most detrimental effect on transport properties originate from non-radiative Auger electron cooling process (2 ps) between IB and CB, that is 3 orders of magnitude faster that any other relaxation process in the IBSC [4]. We have shown that with appropriate band structure engineering, it is possible to place the intraband Auger electron cooling decay in the ns range [5]. Such an optimized design requires a VB confinementless QD structure. We have modeled series of InAs/GaAs QD arrays by varying in-plane QD densities through tuning the QD base length and the inter-dot distance. We found that at areal density of 8.3x10^11 cm^-2, an IB with band width of 50meV is formed at around 200meV below the CB edge of barrier material. Optical studies revealed that under this condition [8], the absorptions from IB to CB for both transverse electric(TE) and magnetic(TM) modes are comparable, which differ from the vertical coupling case where the TM absorption is suppressed. Enhanced absorption between IB-CB boosts the light harvest and attributions of the polarization insensitivity will be presented. The authors wish to thank NEDO Japan for funding this work. [1] A.Luque, A.Marti, Phys.Rev.Lett. 78, 5014 (1997) [2] Y.Shoji, K.Narahara, H.Tanaka, T.Kita, K. Akimoto and Y. Okada, J.Appl.Phys. 111, 074305 (2012) [3] S.Tomic, T.S. Jones and N.M. Harrison, Appl.Phys.Lett. 93, 263105 (2008) [4] S.Tomic, Phys.Rev.B 82, 195321 (2010) [5] S.Tomic, in Next Generation of Photovoltaics (Springer, Heidelberg 2012) [6] S.Tomic, A.Luque, A.Marti and L.Antolin, Appl.Phys.Lett. 99, 053504 (2011) [7] Y. Shoji, K. Akimoto and Y. Okada, 38th IEEE Photovoltaic Specialists Conf. (Texas, June 2012) 176 [8]S.Tomic,T.Sogabe,Y.Okada, in preparation

Many novel concepts for high-efficiency photovoltaic devices are based on the tunability of the optical and electronic properties of semiconductor nanostructures such as quantum wells, wires or dots [1-3]. For a proper inclusion of the quantum effects governing the optoelectronic characteristics of the nanostructures, like confinement and tunneling, into the description of the photovoltaic operation, a quantum kinetic theory of photovoltaic devices based on low dimensional absorbers and/or conductors was introduced, which treats dissipative quantum transport and quantum optics on equal footing [4-6]. In this paper, the theoretical framework, which is based on the steady-state non-equilibrium Green's function (NEGF) formalism, is extended to include non-radiative loss mechanisms by a quantum-kinetic equivalent of Shockley-Read-Hall recombination. In the present NEGF approach, the rates of scattering between extended band states and localized defect states are formulated in terms of the associated single particle Green's functions and the scattering self-energies for the microscopic process of carrier relaxation, for which a multi-phonon picture is used. The general theory is implemented for a photovoltaic system with selectively contacted extended state absorbers and a localized deep defect state in the energy gap, as well as a realistic thin-film pin-diode solar cell device with defect-rich layers in the intrinsic region. In the former case, the theory is shown to reproduce the original semi-classical result under the assumption of a quasi-equilibrium occupation of band and defect states. For both devices, the recombination of photogenerated carriers is investigated under different conditions concerning external bias and internal fields. Acknowledgements: Financial support is provided by the European Union FP-7 Programme under Grant. No. 246200. [1] N. J. Ekins-Daukes, K. W. J. Barnham, J. P. Connolly, J. S. Roberts, J. C. Clark, G. Hill, and M. Mazzer, Appl. Phys. Lett., 75, 4195 (1999) [2] M. A. Green, J. Mater. Sci. Eng. B, 74, 118 (2000) [3] A. Martí, N. Loacute;pez, E. Antolín, E. Cánovas, C. Stanley, C. Farmer, L. Cuadra, and A. Luque, Thin Solid Films, 511-512, 638 (2006) [4] U. Aeberhard and R.H. Morf, Phys. Rev. B, 77, 125343 (2008) [5] U. Aeberhard, Phys. Rev. B, 84, 035454 (2011) [6] U. Aeberhard, J. Comput. Electron., 10, 394 (2011)

A high temperature ceramic selective emitter for thermophotovoltaic (TPV) electric generators is described with a spectral match to GaSb IR cells. While solar cells generate electricity quietly and are lightweight, traditional solar cells are used with sunlight and only generate electricity during the day. Workers at JX Crystals invented the GaSb IR cell as a booster cell to demonstrate a solar cell conversion efficiency of 35%. JX Crystals now makes these IR cells. In TPV, these cells can potentially be used with flame heated ceramic emitters to generate electricity quietly day and night. One of the most important requirements for TPV is a good spectral match between the ceramic IR emitted and the IR PV cells. The first problem is to find, demonstrate, and integrate a doped ceramic IR emitter with a spectral match to these GaSb cells. Attempts have been made to use rare earth oxide ceramic emitters but the spectral emission lines are too narrow to transfer significant power efficiently to the cells. Recently, nickel oxide and cobalt oxide doped MgO-based ceramics have been shown experimentally and theoretically to have spectral selectivity but no attempts have been made to integrate these ceramic IR emitters into a fully operational TPV generator. Herein, we describe an appropriate ceramic emitter and a plan to integrate it with cells and a burner to demonstrate an operational TPV generator. Fuel fired TPV generators have 4 very interesting features. First, they have very high power densities and this makes the PV cells affordable. For example, with an emitter temperature at 1200 C, the cell electric power density can be over 1 W/cm2, 100 times higher than a traditional solar cell operating in sunlight. Second, they are very light weight. For example compared to a Li-ion battery, the TPV power system proposed here is lighter, has much higher specific energy, operates longer, and is very easily refueled. Third, these generators are quiet because the burn is continuous, and finally, fourth, a large number of hydrocarbon fuels can be used. The light weight and quiet features make these units interesting to the military for lighter batteries for soldiers or for power and propulsion systems for unmanned aerial vehicles (UAVs). The high power density, quiet, and fuel adaptability features make these units suitable for combined heat and power in home and industrial applications.

Advanced optical coatings comprised of nanostructured transparent conductive oxide (TCO) materials can enhance photovoltaic device performance by minimizing reflection losses and increasing the optical path length within thin-film solar cells. In this work, oblique-angle deposition is used to fabricate indium tin oxide (ITO) optical coatings with a porous, columnar nanostructure. Nanostructured ITO coatings are fabricated with a range of deposition angles, enabling the porosity and the refractive index to be tuned. Nanostructured ITO layers with a reduced refractive index have been incorporated into an omnidirectional reflector (ODR) structure capable of achieving high internal reflectivity over a broad spectrum of wavelengths and a wide range of angles. Such conductive, high-performance ODR structures on the back surface of a thin-film solar cell can potentially increase both the current and voltage output by scattering unabsorbed and emitted photons back into the active region of the device. The current output from thin-film solar cells can also be increased by incorporating nanostructured TCO layers onto the front surface to minimize reflection losses. Antireflection structures with a step-graded refractive index design have been fabricated using nanostructured ITO materials, and increased transmittance has been demonstrated over a wide range of wavelengths of interest for photovoltaic applications.

The cost potential of thin film technologies has always been regarded positive compared to traditional silicon solar cells. At Solliance we developed a cost of ownership calculation tool for future factories of both CIGS (S2S) and OPV (R2R). The calculation gives a more detailed insight in the cost buildup of the product and production process. This allows you to identify possibilities and strategic choices for design or equipment. It helps you also in identifying the sensitivity of your material costs. In this presentation, the cost breakdown of a CIGS factory will be discussed with a focus on the determination of the main cost drivers. The influence of the material utilization and price sensitivity of indium, copper and gallium are studied and put in perspective to the whole solar panel. Finally the strength of the model is shown in comparing different processes illustrated by two different options for the selenization process.

A conductive and black surface is usually based on carbon. We present a scalable and cost-effective way to fabricate silver nano-network embedded in silicon to form conductive black silicon surface (CBSS), by using In₂O₃/SiO_x bilayer lift-off metallization and catalytic etching, rather than the conventional nanofabrication techniques such as e-beam or nanoimprint lithography. The fabricated CBSS has high light absorption up to 97% (without ARC) in the range of 400-1000 nm and low sheet resistance close to 6 Omega;/square. Our process starts with deposition of an In/ SiO_x bilayer on a flat silicon wafer, where the thickness of the In film is smaller than its percolation threshold such that an In₂O₃ island film can be achieved after oxidation. The In₂O₃ island film serves as the mask for deposition of silver on the silicon wafer after undercut is formed by removing SiO_x in the gaps. The pre-blended solution of HF and hydrogen peroxide is then introduced as the last step to remove the In₂O₃/SiO_x bilayer and etch away the silicon beneath silver network to form a silver nano-network embedded silicon surface. Our simulation suggests that the high absorption of CBSS stems from the two kinds of surface microstructures generated in the chemical etching: the islands/grooves structure and the porous surface of the Si islands. The low sheet resistance comes from good connection of the silver nano-network. The CBSS might find applications where both high light absorption and high electrical conductivity are required simultaneously, such as solar energy devices.

Surfaces exhibiting both self-cleaning and antireflection properties would be very beneficial in many applications including solar cells, optical lenses, and light emitting diodes since they can improve the device performance by eliminating the reflection loses and also offer low cost maintenance due to the self-cleaning property. However, the challenge in preparing such multifunctional surfaces is to balance high surface roughness requirement of superhydrophobic coatings with the smooth surface requirement of antireflection coatings. Therefore, to prepare antireflective and superhydrophobic surfaces, roughness must be optimized such that it must be small enough to avoid light scattering and high enough to provide superhydrophobicity. Although several groups have prepared surfaces combining the superhydrophobicity and high light transmission, these works are far away from to fulfil all the requirements of an ideal self-cleaning antireflection surface for practical outdoor applications (e.g. solar cells) which should exhibit high water contact and low sliding angle, broad band antireflection in visible and near infrared (NIR) region at all incidence angles (omnidirectional), mechanical and thermal stability, in addition to ease of fabrication in large areas. In the present work, we report the preparation of large-area bioinspired multifunctional coatings from nanostructured organically modified silica colloids. Coatings mimic the self-cleaning property of superhydrophobic lotus leaves and omnidirectional broad band antireflectivity of moth compound eyes simultaneously. In order to demonstrate the wide applicability of these multifunctional coatings in photovoltaics, we prepared a coated solar cell surface and we observed an efficiency improvement of about 20% compared to uncoated cell. Also, owing to self-cleaning property, these coatings will prevent the dust accumulation on the solar cell surfaces which can significantly reduce the device performances in course of time. Furthermore, the coatings are mechanically robust thank to their organic-inorganic hybrid nature. We did not observed any destructive effect of excessive water dripping on coating surface. Moreover, superhydrophobicity of the coatings are thermally stable up to 500 °C. Clearly, such robust multifunctional coatings are sought for several applications including solar cells and other photovoltaic devices, optical lenses and windows. We believe that our novel coating is suitable for stepping out of the laboratory to practical outdoor applications.

Triple-junction solar cells have recently achieved efficiencies of 43.5% by combining materials with different bandgaps (E_g) to efficiently collect three different portions of the solar spectrum. To surpass 50% efficiency, additional junctions are required to split the spectrum further, necessitating the development of a solar cell material with E_g=2.0-2.2 eV for the top junction. In_yGa_1-yP (y=0.27-0.40) possesses a direct-gap in the appropriate E_g range, but is lattice-mismatched to conventional substrates such as GaAs and GaP. We thus employ a metamorphic GaAsP graded buffer to overcome the lattice mismatch with minimal threading dislocations (TDs). The materials aspects of growing tensile GaAsP on GaAs versus compressive GaAsP on GaP differ widely, requiring different metamorphic growth strategies. Previously, we demonstrated 2.07 eV metamorphic In_0.36Ga_0.64P solar cells on GaAs, but wide-E_g InGaP solar cells on GaP have yet to be achieved. Growth of wide-E_g InGaP on a GaP substrate would allow for thinner graded buffer layers and better integration into an inverted metamorphic multijunction scheme, but this requires better understanding of compressively graded GaAsP on GaP. In this work, we present molecular beam epitaxy growth of metamorphic GaAsP solar cells and describe recent growth adjustments that enabled us to dramatically improve the quality of metamorphic GaAsP on GaP. As a result, wide-E_g InGaP solar cells on GaP may now be attainable. The two major differences between tensile and compressive growth of metamorphic GaAsP are the formation of faceted trenches (FTs) in the tensile case and an elevated TD density (TDD) in the compressive case. To minimize FT density in tensile GaAsP, a very low grading rate (<0.2 %-mu;m^-1), or equivalently, a very thick graded buffer, is necessary; in order to reach the lattice constant of metamorphic 2.2 eV In_0.31Ga_0.69P on GaAs, a graded buffer >10 mu;m thick would be necessary to suppress FT formation. Here, we reduced the TDD of GaAsP on GaP from ~1×10⁷ to ~4×10⁶ cm^-2 while maintaining a thinner graded buffer than on GaAs. GaAsP on GaAs possesses a lower TDD of ~9×10⁵ cm^-2, but the prospect of a 2× thinner graded buffer on GaP remains attractive. Fabrication of nearly identical ~1.8 eV GaAs_0.66P_0.34 solar cells on both GaAs and GaP, allowed us to compare electrical properties of the graded buffer material. We found very similar current collection and fill factor for the two devices, but the cell on GaP suffered a 40 mV loss in open-circuit voltage (V_oc) compared with the cell on GaAs, yielding 1.24 and 1.28 V respectively. We expect the lower V_oc for compressive GaAsP is due to the increased TDD. Further improvements in graded buffer design should enable improved dislocation glide and lower TDD. Our recent reduction in TDD for compressively graded GaAsP now encourages the growth of wide-E_g InGaP on these templates, and progress toward InGaP solar cells with E_gge;2.07 eV on GaP will be presented.

Multi-junction III-V solar cells are based on a triple-junction design that employs a 1eV bottom junction grown on the GaAs substrate with a GaAs middle junction and a lattice-matched InGaP top junction. There are two possible approaches implementing the triple-junction design. The first approach is to utilize lattice-matched dilute nitride materials such as InGaAsN(Sb) and the second approach is to utilize lattice-mismatched InGaAs employing a metamorphic buffer layer (MBL). Both approaches have a potential to achieve high performance triple-junction solar cells. A record efficiency of 43.5% was achieved from multi-junction solar cells using the first approach [1] and the solar cells using the second approach yielded an efficiency of 41.1% [2]. We studied carrier dynamics in MOVPE-grown bulk dilute nitride materials nominally lattice matched to GaAs substrates: In(0.05-0.07)GaAsN(0.01-0.02)Sb(0.02-0.06) layers (Eg= ~1.0 - 1.2eV at RT), where carrier lifetime measurements are crucial in optimizing material growth and p-i-n field aided carrier extraction device design. The dilute nitride layers were clad by GaAs forming a double heterostructure (DH). The incorporation of N in InGaAsN led to degradation in photoluminescence (PL) efficiency, but the addition of Sb in InGaAsNSb improved the PL efficiency possibly due to the surfactant effect of Sb. Two-step post-growth thermal annealing processes were optimized to obtain maximum PL efficiencies. We employed time-resolved PL (TR-PL) techniques to measure carrier lifetimes from both as-grown and thermally annealed samples. Short carrier lifetimes of <30psec were obtained from as-grown InGaAsN and InGaAsNSb DH samples, but post-growth annealing yielded improvements in carrier lifetimes of both InGaAsN and InGaAsNSb DH samples. One InGaAsNSb DH sample showed a lifetime of ~ 200ps. We also studied MOVPE-grown bulk InGaAs layers (Eg= ~1.0 - 1.1 eV at RT) grown on step-graded MBLs on GaAs substrates. Chemical-mechanical polishing (CMP) was employed to remove a portion of InGaAs MBLs, followed by MOVPE regrowth of the DH on top of the MBL. Carrier lifetimes were measured from InGaAs samples with and without the CMP process and a high resolution TEM was employed to study defects in various structures. All samples showed comparable faster components of 304 - 481 ps, but the samples with the CMP process showed a significantly improved slower component of 9.6 ns compared to 0.9 - 2.1 ns of the samples without the CMP process. [1] M. Wiemer, V. Sabnis, and H. Yuen, Proceedings of SPIE 8108, 810804-1 (2011). [2] J. F. Geisz, D. J. Friedman, J. S. Ward, A. Duda, W. J. Olavarria, T. E. Moriarty, J. T. Kiehl, M. J. Romero, A. G. Norman, and K. M. Jones, J. Appl. Phys. 93, 123505 (2008).

Photovoltaic power generation can roughly be divided into four categories, silicon, thin film, organic, and III/V devices, based on the material comprising the photovoltaic devices. The first solar cell was made from silicon, and silicon still provides approximately 87% of the yearly installed photovoltaic power. There are three main types of thin film cells, amorphous silicon, cadmium telluride, and copper indium gallium diselenide. Many companies are working on these technologies, and cadmium telluride cells have achieved commercial success, providing about 11% of yearly photovoltaic installations. Organic solar cells are newcomers, but have achieved efficiencies of nearly 10%. III/V devices are typically high efficiency multi-junction devices based on the GaInP2/GaAs/Ge lattice matched triple junction architecture. We broadly review each of these technologies, and then discuss high efficiency multi-junction cells in particular. A mechanically stacked multi-junction device was first proposed about 1955. A number of approaches were proposed in ensuing years to develop a monolithic device, but it was not until 1978 that a practical AlGaAs/GaAs device was proposed that utilized a tunnel diode interconnect between the junctions. Developments over the next few years included replacing the oxygen sensitive AlGaAs with GaInP2 to make a GaInP2/GaAs dual junction device. The dual junction device evolved into a triple junction device through growth on a germanium substrate and development of an n/p Ge junction through diffusion of group V elements into the p-Ge substrate. The GaInP2/GaAs/Ge triple junction lattice matched device has been commercially successful for use in satellite power generation. The GaInP2/GaAs/Ge device has achieved an average performance of about 40% under 500x terrestrial concentration (AOD spectrum), and about 29.5% under the 1 sun space spectrum (AM0). However, there is a continual desire for higher efficiency cells, for both the terrestrial concentrator and the space power generation applications. A number of approaches have been studied to achieve higher efficiency cells. These include novel materials, mechanical stacks, and metamorphic approaches. We review each of these approaches, discussing pros and cons. Finally, we will focus on the inverted metamorphic multi-junction (IMM) solar cell, as it has achieved the highest efficiency for space applications (NASA confirmed 33.9% for 1 sun, AM0), and nearly the highest efficiency for terrestrial high concentration applications (NREL confirmed 42.4% for 325x, ASTM G173 direct).

Multijunction solar cells are currently one of the most promising high efficiency solar energy technologies. In order to grow high quality epitaxial layers in a monolithic fashion, the lattices of each layer must match. This stringent requirement places severe limitations on which materials can be used. An innovative approach for an all lattice-matched optimized design is presented with 5.807 #8491; lattice constant, together with a detailed analysis of its performance by means of full device modeling. The simulations suggest that a (1.93 eV) In_0.37Al_0.63As / (1.39 eV) In_0.38Ga_0.62As_0.57P_0.43 / (0.94 eV) In_0.38Ga_0.62As 3-junction solar cell can achieve theoretical efficiencies >51% under 100-suns illumination (with V_oc = 3.34 V). The effect of concentration into the device performance is analysed taking into account Auger and other recombination processes, which were incorporated to the modeling. As a key proof of concept, an equivalent 3-junction solar cell lattice-matched to InP is fabricated and tested, as well as each individual subcell current-matched at 12.0 mA/cm². The individual single-junction InAlAs, InGaAsP, and InGaAs subcells show efficiencies equal to 5.6, 8.0, and 9.4%, respectively, under AM 1.5g 1-sun illumination. The independently-connected single junction solar cells were tested in a spectrum splitting configuration, showing similar performance to the monolithic tandem device, with open circuit voltage equal to 1.8 V, demonstrating the very low resistance of the tunnel junctions used. The structural and electronic properties of each individual subcell will be discussed in details. These results represent an important step towards the development of ultra-high efficiency solar cells for concentrator systems.

Theoretical calculations show that dual-junction solar cells with top and bottom bandgaps (E_g) of 1.4-1.7 eV and 0.7-1.1 eV, respectively, can achieve efficiencies of 40-47% under 500× concentration. GaAsP and SiGe alloys lend themselves well to this application because they possess bandgaps in the desired range and can be grown lattice-matched to one another on Si substrates. To date, however, there has been relatively little work regarding GaAsP solar cells on SiGe/Si. This work investigates the microstructure and device performance of single-junction GaAs_0.86P_0.14 cells grown on Si_0.14Ge_0.86/Si virtual substrates as a step towards high-efficiency, low-cost dual-junction cells. Growth started with the deposition of low threading dislocation density (TDD~10⁵ cm^-2) p-type Si_1-xGe_x graded buffers (x=0-0.86) on 4° offcut Si by low-pressure chemical vapor deposition. A 10 nm Ge cap was included to facilitate native oxide removal and double-step formation in the subsequent III-V growth. Next, metalorganic chemical vapor deposition was used to grow a lattice-matched 100 nm p-type In_0.42Ga_0.58P nucleation layer, which also serves as a back surface field (BSF). Finally, the lattice-matched GaAs_0.86P_0.14 device layer (E_g=1.54 eV) was grown, consisting of a 1.5 µm p-type base and a 0.1 µm n-type emitter. Solar cells were fabricated using standard techniques. Cross sectional transmission electron microscopy (XTEM) showed that anti-phase domains (APDs) in the In_0.42Ga_0.58P BSF self-annihilate within 100 nm, suggesting that the nucleation layer prevents APDs from entering the device layer. The XTEM exhibited threading dislocations within the GaAs_0.86P_0.14 active region, implying a TDD>10⁷ cm^-2. Planar-view electron beam-induced current imaging (EBIC), which can measure defect densities over large areas, confirmed a TDD of ~1.5×10⁷ cm^-2. EBIC also revealed a low density of micron-scale dark loops surrounding light areas, indicating that some of the APDs reach the surface. These APDs are found in parallel lines, perpendicular to the offcut direction, and cover ~0.91% of the surface area. Despite the high TDD and a low concentration of APDs that reach the surface, the open circuit voltage (V_OC) of the cells is 1.05 V, which is ~92% of the theoretical maximum. The short circuit current density and fill factor of the cells are 8.3 mA/cm² and 0.68 respectively. These values can be improved by implementing a window layer, anti-reflection coating and better metal contacts. The high-V_OC GaAs_0.86P_0.14 cells on Si_0.14Ge_0.86/Si achieved here are very promising because theoretical efficiencies of 44% are possible for this combination of alloy compositions. Future work will concentrate on reducing TDD, improving current collection, and implementing a tunnel junction to cascade the GaAs_0.86P_0.14 top cell with a lower Si_0.14Ge_0.86 bottom cell. D.J. Friedman. CURR OPIN SOLID STATE MATER SCI, 14, 2010, 131-138

CdTe thin-film solar cells have made significant progress in the past 40 years. After some early work on CdTe/Cu_xTe solar cells, the CdS/CdTe superstrate structure was adopted in the early 1970s. With the use of O₂ in processing, CdCl₂ heat treatment, and Cu-based contacts, the performance improved to near-16% efficiency in the first 20 years. Development on the industrial front in the last 20 years can be attributed to the ease of manufacturing CdTe solar cells in the superstrate structure and development of high-deposition-rate techniques such as close-spaced sublimation and vapor-transport deposition. The performance has progressed slowly during this time to 17.3% efficiency. Further progress with the present approach seems unlikely due to a narrow window of processing parameters and the balance of various impurities used in fabrication. In this paper, we will present a short overview of superstrate-structure CdTe device development and discuss the key factors for performance improvement. In recent years, the substrate structure has attracted increasing attention because of potential benefits it may offer. This structure enables greater access to and control of the CdS/CdTe junction interface region than is obtainable in the superstrate configuration. The substrate structure may also have manufacturing and deployment benefits such as compatibility with flexible substrates and roll-to-roll processing. Work on substrate-structure CdTe devices has been rather limited due to relatively poor performance, with efficiencies stalled between 6% and 8% due to low open-circuit voltage (V_oc) and fill factor (FF). We will describe our recent work on substrate-structure CdTe devices that has led to V_oc of over 860 mV and efficiency approaching 11%. We have found key parameters for improved performance, notably the use of oxygen during the CdS deposition and CdCl₂ heat treatment, the use of Cu in the back contact, and a medium-temperature heat treatment of the device structure. Preliminary analysis using drive-level capacitance profiling shows an increase in carrier concentration when incorporating O₂ into the CdS layer. In addition, time-resolved photoluminescence measurements show an increase in minority-carrier lifetime with O₂ incorporation into CdS. Temperature-dependent V_oc analysis shows a significant effect of O₂ during processing on the dominant recombination mechanism. Device performance improves from under 3% to over 8%, with V_oc reaching over 860 mV after the devices are annealed at 250°C. We expect a significant change in the defect density responsible for the improvement in device performance. We will compare the properties of substrate-structure devices of different performance levels with properties reported for superstrate devices to develop an understanding of the mechanisms affecting device performance.

For semiconductors with non-shallow doping, there is no real distinction between defects and impurities. Both act as beneficial “dopant” or bad “trap”. Unlike Si with intentional shallow doping and no intentional traps, the common assumption Et = Ei, is not valid for thin film CdTe. Here, traps can be effective for wide range of energy levels above EF .To identify the real role of traps and dopants that limit the efficiency, a series of samples were investigated in thin film n+-CdS/p-CdTe solar cell, made with evaporated Cu as a primary back contact. It is well known that process temperatures and deep level defects are highly related. These deep level impurities have been investigated by using Thermally Stimulated Current (TSC) and Thermally Stimulated Capacitance (TSCAP) techniques. Temperature dependent C-V measurements indicate a high density of defects adjacent to the junction. The measurement results of our solar cells made at NJIT show that while modest amounts of Cu enhance cell performance by improving the back contact to CdTe, high temperature (greater than ~100°C) process steps degrade device quality and reduce efficiency. This is probably because of the well-established Cu diffusion from the back contact into CdTe. Hence, measurements were performed below room temperature (T = 77K to 300K). The distinct deep level traps observed from the low-temperature to room temperature measurements, are due to the ionization of impurity centers located in the depletion region of p-CdTe/n-CdS junction. For our n-CdS/p-CdTe thin film solar cells, certain double acceptor Cd vacancy levels and non-shallow acceptor Cu substitute of Cd level were observed. These levels are identical to the observed trap levels by other characterization techniques.

There has been much research and development devoted to the In(x)Ga(1-x)N system of alloy semiconductors for photovoltaic applications in the last decade, as well as for photocatalytic applications[1], given the potential of the system to harvest solar energy throughout the 0.7-3.4 eV range. In addition, there are many ongoing efforts to improve the efficiency of longer wavelength nitride light emitting diodes. However, there are still challenges in realizing these applications, including, but not limited to, phase separation in the intermediate alloy range and difficulties in scalability due to the rising cost and restrictions on the availability of indium. Here we report the synthesis of a new direct band gap semiconductor, ZnSnN2, that, as the low-band-gap member of the Zn(Si,Ge,Sn)N2 alloy family of semiconductors, enables us to now consider addressing some of these challenges with an alternative earth-abundant materials system with direct band gaps that span the energy range from 1.7 to 4.5 eV.[2] We describe our plasma-assisted vapor-liquid solid growth technique, used to produce polycrystalline material with average crystallite sizes of 100 nm. We describe the growth conditions that produce ZnSnN2 while avoiding production of the competing zinc and tin nitrides. We report on the structure and lattice parameters of ZnSnN2, as determined by x-ray diffraction spectroscopy, and the band gap, measured by photoluminescence excitation spectroscopy to be 1.7 ±0.1 eV. [1] T. Nakano, M. Hamada, S. Fuke, Fabrication and performance of photocatalytic GaN powders, Advanced Materials Research 222, 142 (2011). [2] A. Punya, W.R.L. Lambrecht, M. van Schilfgaarde, Quasiparticle band structure of Zn-IV-N2 compounds, Phys. Rev. B 84, 165202, 2011.

In this work, we report new phase of silicon thin-film on glass. We developed these films by decomposing high purity silane (SiH <4>) gas as well as hydrogen (H <2>) by heated catalyzer, tungsten wire, maintained lower than 1500°C. This technique is called hot wire chemical vapor deposition (hot-wire CVD) method. This technique recently became popular in depositing advanced polycrystalline silicon thin-film on glass as well as for fast hydrogen passivation of crystalline silicon (c-Si) (1). The silane and hydrogen molecules are catalytically decomposed into Si and 6H atoms. The decomposition efficiency in hot-wire CVD process is high (> 50%). The large abundance of silicon and hydrogen atoms within the chamber are condensed on glass substrate in a special environment, where photon density is enhanced by multiple reflection process. The substrate temperature is maintained at 250°C and chamber pressure is below 1 Torr. The silicon films on glass have been characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction and Raman Spectroscopy. We observed various characteristics of quasicrystalline structure of silicon on glass substrate. X-ray diffraction pattern shows nearly epi-Si growth on glass substrate having (111) orientation as well as small (220) and (311) diffraction peaks. Epitaxial-like growth on glass for undoped silicon film is a remarkable event in formation of solid from condensation of atoms and species from gas phase. SEM micrograph shows polycrystalline nature of the thin-film having no grain boundary (GB). AFM topography shows formation of fivefold symmetry and sixfold symmetry, i.e. quasicrystalline silicon structure on glass substrate. 1. Electronic properties and device applications of hot-wire CVD polycrystalline silicon films - A. R. Middya, J. Guillet, R. Brenot, J. Perrin, J. E. Bouree, C. Longeaud and J. P. Kleider, Mater. Res. Soc. Proc., Vol. 467 (1997) p. 271. (Invited)

The residual stresses, their gradients, and the mechanical strength of individual layers in 2-micron thick inorganic thin film photovoltaics were quantified. The thin film photovoltaics were consisted mainly of a top electrode, a silicon p-n junction diode, a Transparent Conductive Oxide (TCO) layer, and a thick aluminum substrate. The residual stresses were calculated by an analysis of straight and telephone cord type delaminations of the Si monolayer and the Si-TCO bi-layer, respectively. The residual stress in the Si monolayer was measured to be (-466±110) and in the Si-TCO bi-layer was (-676±95) MPa. Using curvature measurements on freestanding layers, the residual stress gradient in the Si layer alone was found to be 136±10 MPa/mu;m, while the residual stress gradient in the Si-TCO bi-layer was 451±32 MPa/mu;m. Finally, mono- and bi-layer strips were subjected to uniaxial tension with a microscale tension apparatus to determine the mechanical strength of each layer. The tensile strength of the Si mono-layer was 423±75.5 MPa, but the strength of the Si-TCO bi-layer was only 91±26 MPa. Our experiments showed that the particular TCO layer has a negative effect on the mechanical durability of the overall photovoltaic film. More importantly, this study concluded that the compressive residual stresses in the Si-TCO bilayer dramatically improve on the fracture resistance and mechanical durability of the photovoltaic thin films.

Quaternary dilute-nitride alloys, such as InGaAsN lattice-matched to GaAs, have been extensively studied for solar cell applications, although the resulting device performance has been significantly limited due to short minority carrier diffusion lengths, narrow depletion widths, and high background carbon incorporation. High background carbon concentrations generally observed in an MOVPE-grown InGaAsN material have been correlated with poor minority carrier diffusion lengths. Recently, high efficiency (43.5%) multi-junction solar cells employing MBE-grown InGaAsNSb materials have been reported. However, there are few reports on MOVPE-grown bulk InGaAsSbN for solar cell application. In the MOVPE growth of dilute-nitride materials, the background carbon concentration is dependent on the gas-phase growth conditions and selection of metalorganic sources. We have found that the background hole concentration of InGaAsN is an order of magnitude lower than that of InGaAsSbN for as-grown material with similar growth conditions. However, the PL intensity of InGaAsSbN is generally significantly higher than that of InGaAsN. The role of Sb in these materials is still under study. Sb incorporation has resulted in improved photoluminescence (PL) intensity and carrier lifetime of MOVPE-grown dilute-nitride materials. Here, we report on the impact of MOVPE growth conditions on the properties of InGaAs(Sb)N bulk layers on GaAs substrates and present results from dilute-nitride-antimonide-based solar cells. Bulk, nominally lattice-matched InGaAs(Sb)N has been grown on (100) GaAs substrates with band gap energies of 1.0 - 1.3 eV. Variable temperature PL measurements reveal deep level luminescence as well as band-to-band transitions. XRD and SIMS measurements indicate that compositional non-uniformities may be present in these films, although further detailed structural analysis is needed. Carrier lifetimes of 65 and 202 psec were measured from double heterostructures at RT using TR-PL techniques. Fabricated single-junction heterostructure solar cells without anti-reflection coating demonstrate a peak efficiency of 4.58% under AM 1.5 direct illumination. Acknowledgement: The work at UW-Madison is supported by the National Science Foundation through the Materials Research Science and Engineering Center (MRSEC) at the University of Wisconsin, grant DMR-1121288

Positive results from a promising and novel contender in the race for a flexible and transparent conductor with application to flexible displays and photovoltaics is presented. Bending vs. conductivity data from graphene on indium tin oxide, ITO, on flexible substrate polyethyleneterephthalate, PET, is compared with results from graphene on PET and ITO on PET. The data demonstrate the hybrid transparent conductor, graphene on ITO, largely maintains the high conductivity of ITO under bending. Samples of graphene on ITO on PET before bending show sheet resistance equal to that of the ITO before graphene transfer (60 Ohms per square). At a bending radius of ~4.5 mm sheet resistance increased to ~35 and ~7 times the initial value for ITO on PET and graphene on ITO on PET respectively. Graphene was prepared by chemical vapor deposition on copper catalyst and transferred to ITO and PET by wet transfer. We propose graphene bridges microcracks formed in ITO as the brittle ITO is bent, the “crack healing hypothesis.” Support is given by scanning electron microscopy images. Two point bend test data consists of sample curvature data from in situ optical methods. Conductivity data were taken by two probe electrical measurement. Data is presented in conductivity vs. bending curves showing the hybrid transparent conductor captures the high conductivity of ITO and the exceptional resilience of graphene&’s conductivity under bending.

Development of high quality III-V epitaxial films on inexpensive flexible substrates can be a game-changing enabler toward significantly reducing the cost and increasing the efficiency of thin film solar cells, as it offers the possibility of combining the unsurpassed performance of GaAs based multi-junction technologies (1 sun efficiency >36%) with a conventional roll to roll processing standard of thin film industry as afforded by polycrystalline metallic foil technology. Previously we have shown the possibility of fabrication of single crystalline GaAs epilayers on Ceria-coated (A. Mehrotra et al, 38th IEEEPVSC) and Ge/Ceria-coated (A. Freundlich et al, 35th IEEEPVSC) flexible polycrystalline thin metal foils substrates. While such epilayers have demonstrated minimal residual stresses (as extracted from PL analysis), absence of micro-cracks and antiphase disorder, they however exhibit high dislocation densities in excess of 10^8cm-2 which exceed by nearly two order of magnitude defect densities considered as acceptable for the demonstration of conventional high efficiency devices. Here we theoretically and experimentally demonstrate that through a careful design optimization, and despite these dislocation densities, ultra thin (< micron) dual junction solar cells with practical efficiencies in excess of 25% could be achieved. The study has also attempted to avoid the use of conventional indium-bearing III-V alloys and has focused on III-V semiconductor alloys made with more earth-abundant elements (i.e. Al, Sb, Nhellip;). We have evaluated as a function of dislocation densities, the design parameters for devices that comprise a 1.7 eV top AlGaAs solar cell and a 1.25 eV bottom GaAs/GaAs(N)Sb cells. The experimental validation of modeling data was undertaken by the fabrication the proposed thin film subcells on intentionally dislocated buffers. As shown here, and in-line with our modeling results, even for defect densities in excess of 10^8cm-2, we were able to fabricate sets of top and bottom cells with open circuits voltages of in excess of 1 and 0.8 volts respectively. A direct extrapolation of these preliminary experimental results already indicate the potential for fabricating thin-film III-V devices on metal foils with 1 sun efficiencies in excess of 16%.

In this work, we present nanostructured core-shell solar cells based on patterned GaAs nanopillars grown by MOCVD¹. The patterns are lithographically defined and center-to-center pitch, hole size and tiling pattern can be precisely determined a-priori at nanometer resolution. The inherently catalyst-free approach eliminates any metal (i.e.) diffusion into the nanopillars that could reduce the carrier lifetime. In the first part, hybrid core-shell solar cells comprised of n-doped GaAs nanopillar cores and PEDOT conducting shells will be presented². PEDOT is electrodeposited to ensure high controllability at low cost and high optical absorption coefficient whereas the semiconducting nanostructures are responsible for high mobility required in carrier extraction. The properties of the polymer are tuned in-situ by incorporating different anionic dopants in the backbone, allowing for the lowering of the HOMO level (|ΔE| ~ 0.28 eV) that leads to an increase in open-circuit voltage (V_OC) and short-circuit current density (J_SC). A systematic tuning of the device properties results in a J_SC of 13.6 mAcm^-2, V_OC of 0.63 V, peak external quantum efficiency (EQE) of 58.5 %, leading to a power conversion efficiency (PCE) of 4.11 %. In the second part, high-bandgap in-situ passivation³ is applied to core-shell GaAs nanopillar junctions. Nanopillar devices are subject to surface states partly caused by atomic dangling bonds at the crystal surface. Lattice-matched InGaP shells are adopted as a high-bandgap material to lower carrier recombination at the surface. The shell growth occurs in-situ subsequent to the core-shell structure. Figures of merit such as rectification ratios in the order of of ~10, ideality factors of n ~ 1.58 and dark currents of ~ 30 nA at -1 V are indicative of high-quality p-n junctions. Under AM 1.5G conditions, V_OC of 0.44 V, J_SC of 24.3 mAcm^-2, fill factors of 62 % are measured, leading to PCE of 6.63 %. High photocurrent densities are also confirmed by EQE values above 70 % across the whole spectral regime of interest. Finite difference time domain (FDTD) simulations analyze both radial and axial built-in electric fields with respect to different carrier doping concentrations. [1] J. Shapiro et al, Appl. Phys. Lett. 97, 243102 (2010) [2] G. Mariani et al, Nano Lett. DOI: 10.1021/nl301251q (2012) [3] A. Lin at al, Nanotechn. 23, 105701 (2012)

Semiconductor nanowire arrays, such as p-n junction core-shell nanowires, can potentially provide efficient charge correction due to their radial junction and natural anti-reflective structure with enhanced light absorption. This could be extremely beneficial in the development of next-generation solar cells. We fabricated a photovoltaic device using a core-shell p-n junction GaAs and InP nanowire array on p-type GaAs and InP (111) A substrates, respectively. The structure of the SiO2 mask pattern and the growth conditions were the same as those already reported1,2). Spin coating was used after nanowire growth to fill the space between the nanowires with a transparent electrical insulator resin. A transparent indium tin oxide (ITO) film electrode was then sputtered onto the nanowire array. A comb-shaped Ag electrode was also formed on the transparent ITO electrode. The backside electrode of the substrate was formed by alloying Au-Zn. The chip was triangular and had 6-mm sides while the active nanowire area consisted of 0.8 × 0.8 mm2 segments. The photovoltaic performance was measured under Air Mass 1.5 Global illumination, where the radiation intensity was calibrated using a reference cell module just before the measurement. The InP cell exhibited 0.365 V of open circuit voltage (VOC), 17.3 mA/cm2 of short circuit current density (JSC), and a fill factor (FF) of 0.567 for a 3.59% overall efficiency. Next, the InP wire surfaces were passivated with a wider band gap material, AlInP, and the solar cell efficiency was increased to over 6% due to the decrease in the surface recombination of generation carriers. We also succeeded in fabricating a GaAs/InGaP cell that showed over 3% conversion efficiency. Finally, we demonstrate a ‘flexible nanowire array&’ without the substrates. It is easy to remove the wire part after all the device processes are completed due to the interface stress between the wires and substrate. This flexible device is advantageous in that it requires two or three orders fewer semiconducting materials than silicon solar cells. For huge solar cell power stations, the number of semiconductor materials needed is an important issue. Furthermore, after removing the wire parts, the substrate can be reproducibly used. If we can use same substrate several times, this is a cost effective process. I will explain this in more details at the conference. 1)H. Goto et al. Applied Physics Express 2 035004-1-3 (2009). 2)T. Fukui et al., AMBIO 41 suppl. 2 119 (2012).

Thin-film solar cells incorporating quantum dot active layers have recently emerged as a notable third-generation photovoltaic (PV) technology, largely due to the strong absorption, tunable infrared bandgap, and ambient-atmosphere stability of colloidal lead sulfide quantum dots (PbS QDs). Photoactive PbS QDs can be solution-deposited on a transparent zinc oxide (ZnO) film to form a depleted np-heterojunction device. However, this planar architecture incurs a fundamental trade-off between light absorption and carrier collection. To absorb most incident light, we need a ~1-µm-thick QD film, but to collect the majority of photocarriers, we need absorption to occur within a minority carrier diffusion length (~100 nm) of the ~150-nm-thick depletion region. By introducing 1-D nanostructures, we can decouple these parallel requirements and optimize for each independently. A vertical, QD-infiltrated array of hydrothermally-grown ZnO nanowires orthogonalizes the mechanistic length scales of absorption and collection: Light is fully absorbed as it traverses the thick QD film in the axial direction, while photocarriers are efficiently collected as they drift to nearby PbS/ZnO interfaces in the radial direction. Our research demonstrates that moving from a planar ZnO film to a nanowire array can significantly improve QDPV performance, increasing both short-circuit current density and overall power conversion efficiency by over 40% (to 21mA/cm² and 5%, respectively). We control and optimize the nanowire areal density using a thin polymer interlayer and confirm the near-complete infiltration of PbS QDs into the ZnO nanowire array via cross-sectional scanning electron microscopy and energy-dispersive x-ray spectroscopy. Our work on scalable bottom-up processing of ZnO nanowire-based QD solar cells suggests that 1-D nanostructures may be the key to enhancing the efficiency and hence the economic viability of quantum dot photovoltaics.

Silicon nanowires (Si NWs) have recently been investigated for the development of next-generation solar cells owing to strong broadband optical absorption. However, a high surface-to-volume ratio of nanowires intrinsically increases a recombination loss for lowering their conversion efficiency (CE) compared to a conventional planar counterpart. Two main issues to be resolved are difficulty in forming a robust front electrode between top-ends of NWs and metal, and a remarkable increase in surface recombination by unavoidable heavy doping of a thick NW emitter. Here, we demonstrate a facile approach to simultaneously integrate lightly-doped (doping level le; 2×10¹⁹ atom/cm³) antireflective NWs with a self-aligned ohmic contacts (heavily-doped, doping level ge;1×10²¹ atom/cm³). Metal-assisted chemical etching (MACE) forms the lightly-doped NWs at the low-doped bottom side of a planar n-doped emitter while etching-off their heavily-doped top-side; meanwhile, heavily n-doped regions underneath metal grids have retained to act as the self-aligned ohmic contacts. Phosphorus doping has been performed using a spin-on-doping technique to form p-n junction. The front metal grids using Ti and Au were simply prepared using a mask evaporation technique. After the metallization, antireflective Si NWs were prepared using MACE on the emitter regions exposed between metal grids. The internal quantum efficiency (IQE) was measured in the wavelength range of 400-1100 nm using a xenon light source and a monochromator. A remarkable enhancement of ~40 % in blue responses of IQE points out that the selective emitters efficiently collect the charge carriers photogenerated in NWs. As a result, adding a self-aligned selective emitter increased the cell CE up to 12.8 % in comparison with a conventional NW counterpart that shows the CE of 8.75 %.

Controlled patterning of large grained multi-crystalline silicon (Si) thin films of only ~6µm thickness on glass could be realized using nanosphere lithography (NSL) in combination with reactive ion etching (RIE). Thereby, silicon nanowires (SiNWs) formed perpendicular to the film surface, independently of the crystal orientation of the underlying grains and with entirely pre-determined geometrical properties such as lengths and diameters. The SiNW-patterned thin films show properties, which ideally qualify them as absorber material for photovoltaics (PV). X-ray photoelectron spectroscopy (XPS) studies showed an electronic grade surface quality of the SiNWs. Integrating sphere measurements could prove that patterning of Si thin films with SiNWs permits an enhancement of the spectral absorption of visible light of up to 50% and results in resonant absorption peaks depending on the geometry of the highly ordered SiNW arrays. In the SiNWs, radial and axial p-n junctions were formed by phosphorous diffusion and charge separation in the junctions of single wires could be proven by electron beam induced current (EBIC) measurements. Based on this, the absorptivity of single SiNWs, as the constituting unit of the resonantly absorbing arrays, could be further investigated. In the EBIC setup under the SEM, the external quantum efficiency of the now contacted single wires could be determined, by irradiating them with monochromatic light through an optical fiber. The measured illuminated current from single wires showed resonances for certain spectral lines, which could be correlated to resonances in the spectral absorption cross-section from first principle Mie calculations on cylindrical Si structures of the same diameter. These findings pave the way for tailoring the absorption in third generation thin film PV devices. Together with the high structural and electrical quality of the newly created absorber materials they open up a new research field of glass-based SiNW thin film PV.

Solution-processed colloidal quantum dot (CQD) solar cells have seen rapidly increasing efficiencies in recent years. Improved performance has resulted not only from rational engineering of the electronic properties of quantum dots film, the active layer of the cells, but also from development of novel device architectures. Specially, depleted bulk heterojunction (DBH) solar cells, adapted from organic photovoltaics, have been successfully integrated with CQDs with porous/ordered transparent n-type semiconductor serving as the electron acceptor. In this work, we demonstrate a high performance, nanostructured DBH photovoltaic device based on bottom-up self-assembled ZnO nanowires and PbS CQDs. The porous and open nanowire structure allows for conformal coating by PbS CQDs, leading to an interpenetrating DBH structure. Through the combination of interfacial modification and a well designed DBH structure, a record efficiency of over 7% has been achieved under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW cm-2. The results suggest that effective charge separation and proper phase separation are key factors in optimizing photovoltaic performance.

For decades solar-cell efficiencies have remained below the thermodynamic limits. However, new approaches to light management that systematically minimize thermodynamic losses will enable ultrahigh efficiencies previously considered impossible. Nanophotonic design can help overcome many of the fundamental limits that are encountered in solar cells. By fabricating carefully engineered nanostructures within the solar cell, it becomes possible to reduce the effect of non-radiative recombination by enhancing the radiative recombination rate, to enhance the amount of light trapping over a large spectral bandwidth, and even to exceed the Shockley-Queisser limit by reducing the angular emission cone of light. In this way, a single-junction solar cell with an efficiency of 42% becomes possible. Moreover, using a parallel multi-junction design in combination with nano- and micro-photonic spectral splitting structures, efficiencies beyond 70% are within reach. This presentation will review the various nanophotonic design principles to achieve these ultimate-efficiency solar cells, that are all based on dielectric light scattering structures. Refs: 1. Plasmonics for improved photovoltaic devices, H.A. Atwater and A. Polman, Nature Mater. 9, 205 (2010) 2. Photonic design principles for ultrahigh-efficiency photovoltaics, A. Polman and H.A. Atwater, Nature Mater. 11, 174 (2012)

Colloidal quantum dots (CQDs) combine facile solution processing with bandgap tuning quantum effects, and are promising materials for inexpensive and highly efficient solar cells and other optoelectronic devices. The current approach to fabricate CQD photovoltaic devices employing a p-n junction has relied on combining a conductive, bandgap tunable p-type PbS CQD solid with another fixed bandgap n-type material, e.g. titanium dioxide (TiO2). Though efficient for a very narrow range of CQD bandgaps centered about 1.3 eV, this scheme falls short when bandgaps optimal for multiple junction implementations are considered, due to lack of bandgap control in the bulk semiconductor. Here we report a novel approach to creating n- and p-type materials within the same CQD material system, which are stable, chemically compatible, high performance in a photovoltaic context, and quantum confined on each side of the junction. We achieve a broad doping spectrum, where a heavily doped (>10e18 cm-3) p-type CQD layer is interfaced with a lighter doped (10e16-10e17 cm-3) n-type CQD layer, to yield a compatible and functional rectifying junction. Such quantum-to-quantum junctions can span a range of bandgaps from 0.6 eV to 1.6 eV, which are highly desirable for multiple junction solar implementations that would otherwise be unattainable with current nanocrystal-to-bulk p-n interfaces. In the case of the 0.6 eV bandgap device in particular, 80% of the theoretical open circuit voltage limit was achieved, whereas a functional device could not be constructed using the conventional CQD-to-Bulk scheme with this same choice of bandgap. Under simulated AM1.5G illumination at 25°C, an optimal single quantum junction device with a 1.3 eV bandgap, showed a short-circuit current density Jsc of 20.9 mA/cm2, an open-circuit voltage Voc of 0.54 V, and a fill factor of 49.3%, resulting in a solar power conversion efficiency of 5.5%, already comparable with present state-of-the-art devices. Furthermore, these devices exhibit excellent thermal stability at 90°C and excellent performance retention over a span of 60 hours. Control over the majority carrier within the same CQD material system, and the successful combination of these materials into a single device, offers a new engineering avenue for quantum dot optoelectronic devices.

An intermediate-band (IB) solar cell is an attractive candidate to exceed the Shockley-Queisser limit [1]. To realize highly efficient IB solar cells, long lifetime of photo-generated carriers in the IB is essential. Semiconductor quantum dots (QDs) in which quantum confined levels serve as IBs can be utilized for the light-absorbing materials in IB solar cells. Especially, InAs QDs imbedded in GaAsSb barriers are promising because the type II band alignment suppress the radiative recombination of electrons confined at InAs QDs and itinerant holes in GaAsSb layers, leading to long carrier lifetime at IBs. We report that the intrinsic radiative recombination lifetime tau;_rad in the type II InAs/GaAsSb QDs under weak excitation is significantly longer than previously reported values. In addition, for further longer tau;_rad, we propose a concept to insert thin GaAs walls between the InAs QDs and GaAsSb barriers. We prepared the QD samples of various Sb compositions (x = 0sim;0.18) by molecular beam epitaxy (MBE) using the Stranski-Krastanov growth mode. The peak energy of the steady state photoluminescence (PL) spectra is lowered with increasing Sb composition, indicating the shift of the valence band maximum of the GaAsSb layers. The dependence of PL decay time tau;_PL on both the Sb composition and excitation intensity (38sim;460 mW/cm²) was systematically investigated with a time-correlated single-photon counting method using a 100 fs-Ti:sapphire laser operating at 800 nm. All the PL decay curves exhibited non-exponential profiles, and tau;_PL determined by dI(t)/dt×I(t)^-1, where I(t) is the time evolution of the PL intensity, was strongly dependent on the excitation intensity. The 18% Sb sample exhibited the longest tau;_PL being over 100 ns at an excitation intensity of 38 mW/cm², which is longer than twice the previously reported values. These properties were well explained by solving rate equations of the carrier density with neglecting nonradiative process, in which tau;_rad is inversely proportional to carrier density. We calculated the value of tau;_rad in InAs/GaAs_1-xSb_x QDs relative to that in type I InAs/GaAs QDs based on an effective mass approximation and found that the observed extremely long tau;_PL corresponds to tau;_rad [2]. We fabricated the wall-inserted type II QD samples consisting of InAs/GaAs/GaAs_0.82Sb_0.18 by MBE. In these structures electrons at the IB in the InAs QDs and holes in the valence band of the GaAsSb layers are farther separated compared with those in conventional type II QDs. The inserted 2 nm thick GaAs wall remarkably extended the lifetime as long as 220 ns for electrons at the IB [3]. [1] A. Luque and A. Martil., Adv. Mater.22, 160 (2010) [2] K. Nishikawa et al., J. Appl. Phys.111, 044325 (2012) [3] K. Nishikawa et al., Appl. Phys. Lett.100 , 113105 (2012)

In a typical single junction solar cell, over half of the sun&’s incident photon energy is not converted due both to transmission loss and carrier thermalization. Quantum dot (QD) superlattices have been proposed as a means to harness the lower energy photons normally lost to transmission, extending the absorption range of solar cell and thus increasing the short-circuit current. Two specific applications for this effect have recently emerged, namely, bandgap engineering of multi-junction solar cells and as a miniband in the intermediate band solar cell concept. However, the complexity of the QD growth makes realization of enhanced devices a delicate process. The development of successful QD growth techniques requires the correlation of device properties with specific growth parameters. A vital task in this cycle is the detailed electronic, optical, and thermal characterization of quantum dots and their effects on device performance. Ten, twenty, and forty-layer InAs/GaAs quantum dot (QD) embedded superlattice solar cells were compared to a baseline GaAs p-i-n solar cell. Proper strain balancing and a reduction of InAs coverage value in the superlattice region of the QD embedded devices enabled the systematic increase in short circuit current density with QD layers (0.02 mA/cm²/QD layer) with minimal open circuit voltage loss (~50 mV). The improvement in voltage was found to be due to a reduced non-radiative recombination resulting from a reduced density of larger defective QDs and effective strain management. The forty layer device exceeded the baseline GaAs cell by 0.5% absolute efficiency improving efficiency relative to the baseline by 3.6%. In addition, InAs quantum dots have been incorporated into the middle junction of an InGaP/(In)GaAs/Ge triple junction solar cell (TJSC) on four inch wafers, in aims of band gap engineering a high efficiency solar cell. Results of QD growth on 4” diameter Ge templates gave QD densities near 1×10¹¹ cm^-3 and QD height between 2-5 nm. Arrays of 10 layers of InAs QDs have been grown between the base and emitter in the middle cell of a full triple junction solar cell. Integrated current of the (In)GaAs junction with 10 layers of strain balanced InAs QD layers shows a gain of 0.27 mA/cm² beyond the band edge. One sun AM0 current-voltage measurements of QD TJSC show an efficiency of 26.9% with a V_oc of 2.57 V.

To further enhance power conversion of semiconductor solar cells the use of sub-bandgap structures with an extended absorption beyond the host materials is employed. Among these, structures utilizing quantum confinement effect such as quantum dots (QDs) [1] have attracted considerable interest. Quantum dot solar cells (QDSCs) are considered as a promising candidate for the implementation of the intermediate band solar cell concept [2]. Benefited from the sub-bandgap absorption of near infrared radiation, extended photoresponse and increased short-circuit current density (Jsc) have been obtained for QDSC [3]. However, the current contribution from QDs to the whole device has been found insufficient to balance out the open-circuit voltage degradation for an improved overall efficiency [3]. To enhance the short circuit current from QD solar cells, the method of increasing the stacking number of multiple QD layers and thus the QD absorption has been investigated [4, 5]. However as the number of the QD layers was increased, degraded device performance has been observed in QDSCs, which has been partially ascribed to the difficulty in carrier extraction in QDSCs with large stacked QD layers [4, 5]. To further investigate the carrier collection properties of QDSCs and how the device performance can be improved, in this work we present a detailed study of electroluminescence (EL) on QDSCs containing 10, 15 and 20 layers of self-assembled In0.5Ga0.5As/GaAs quantum dots. The EL measurements were performed under different temperatures and carrier injection levels and the effects on the devices EL peak position, intensity and linewidth will be analyzed. Furthermore, the validity of reciprocity relation [6] between the solar cell external quantum efficiencies and their respective EL spectrum will be also evaluated to reveal the fundamental physical processes that influence the key device characteristics due to the incorporation of multiple layers of quantum dots. References [1]V. Aroutiounian, et al., J. Appl. Phys. 89 (4), 2268 (2001). [2]A. Luque, et al., Adv. Mater. 22 (2), 160 (2010). [3]H. F. Lu, et al., Appl. Phys. Lett. 98 (18) (2011). [4]A. Takata, et al., 35th IEEE Photovoltaic Specialists Conference, New York, 2010, p. 1877. [5]S. M. Hubbard, et al., 35th IEEE Photovoltaic Specialists Conference, New York, 2010, p. 1217. [6]U. Rau, Phys. Rev. B 76, 085303 (2007).

Due to its unique electronic properties the quantum dot (QD) has attracted much attention in intermediate band (IB) solar cell research.[1] The IB solar cell concept proposes that upconversion, the energetic addition of two low energy quanta resulting in one high energy quantum, increases the possible energy conversion efficiency. To realize efficient upconversion, quantum structures can be applied to generate states within the bandgap of the host semiconductor. Several reports proved the upconversion of photons into shorter wavelength in the infrared (IR) regime by using InAs embedded in GaAs.[2,3] While the structure and the mechanisms responsible for upconversion could not be identified in these reports, the QD has been extensively investigated for IB solar cells. Strictly speaking, the IB is not a prerequisite for efficiency increase through upconversion. Intermediate states which locally enhance the interaction between carriers are sufficient. Accordingly, we proposed that another quantum structure, what we call the quantum well island (QWI), is probably more suited for upconversion in solar cells than the quantum dot. The QWI is a very thin flat disk shaped structure with height of several monolayers and diameter on the order of several tens to hundreds of nanometers. We point out that the QWI&’s larger diameter enables efficient photon absorption, leading to enhanced multi carrier generation at low excitation power densities. Additionally, we reported the photon upconversion from the IR to the visible through InAs QWIs by our unique device structure.[4] It was experimentally verified that InAs QWIs enable efficient photon upconversion of infrared photons to visible photons, which is not possible for InAs quantum dots. This is due to the QWI&’s smaller height providing higher confined states. That an efficient upconversion process occurs in these flat quantum structures was shown with the correlation between the upconverted photoluminescence (PL) and the QWI PL spectra. Supported by theoretical calculations, we concluded that a momentum conserving Auger process between two excitons gives rise to an upconverted electron and hole. The free carriers generated by upconversion in the QWI structure can be used to improve solar cell efficiency by additional current through IR absorption, similar to the IB solar cell concept. We verified that our novel sample structure design is capable of photocurrent generation under illumination of the IR part of one sun, promising an improved energy conversion efficiency for solar cells. This work was supported by JST-CREST and by the Strategic Research Infrastructure Project, MEXT, Japan. [1]A. Luque and A. Marti, Phys. Rev. Lett. 78, 5014(1997). [2]C. Kammerer et al., Phys. Rev. Lett. 87, 207401(2001). [3]P. P. Paskov et al., Appl. Phys. Lett. 77, 812(2000). [4]D. M. Tex and I. Kamiya, Phys. Rev. B 83, 081309(R)(2011).

A significant impediment to the use of epitaxial lift off as a means to reduce the cost of solar energy conversion using very high efficiency III-V semiconductor solar cells is damage to the parent wafer incurred during the lift off process. In this work, we discuss the avoidance of wafer damage using a series of epitaxial protection layers[1, 2]. Using this approach, we demonstrate damage-free removal of high efficiency active layers and then multiple reuse of the parent wafer. Multiple cycles of ELO followed my wafer reuse can be an effective means for achieving very low cost (<$1/Wp) and extremely lightweight solar cells. An integral part of our process is the adhesive-free transfer of GaAs thin-film solar cells onto plastic substrates by combining epitaxial lift-off (ELO) with cold weld bonding. Combination of these technologies enables 1) the fabrication of light weight, flexible thin-film GaAs solar cells, 2) a transfer process that cold-weld bonds the GaAs epitaxial-cell onto an accepting plastic foil substrate, and 3) the multiple growths from a single parent substrate the requires no destructive mechanical polishing process. We show that p-n junction GaAs solar cells grown on original and reused wafers have power conversion efficiencies of ~23%, under simulated AM1.5G illumination. The ability to perform multiple growths from a single parent wafer promise to dramatically reduce production costs for high power conversion efficiency GaAs solar cells. The cost-effectiveness of this scheme will also be discussed. [1] K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010. [2] K. Lee, J. D. Zimmerman, X. Xiao, K. Sun, and S. R. Forrest, "Reuse of GaAs substrates for epitaxial lift-off by employing protection layers," J. Appl. Phys., vol. 111, p. 033527, 2012.

State-of-the-art III-V solar cells have shown repeated >40% efficiencies, and are under intensive development efforts in both the space power and terrestrial photovoltaic industries. Low bandgap semiconductors are of particular interest for use in high efficiency multi-junction solar cell device architectures. The InP substrate offers lattice matched materials closer to the ideal bandgaps for solar energy conversion removing the 1.42 eV middle junction constraint present in Ge-based triple junction devices. The InGaAs alloy, lattice matched to InP provides a 0.74 eV bandgap, which is slightly higher than the ideal bottom junction bandgap of 0.7eV. In order to tune this bandgap to match the ideal value, the use of InGaAs quantum wells (QW) in the i-region of a solar cell is proposed. Growth of lattice mismatched InGaAs Multiple QWs (MQW) will be investigated by along with strain-balanced barriers consisting of the AlInGaAs alloy system. Low bandgap semiconductors also face the challenge of high non-radiative recombination rates and therefore are sensitive to i-region thickness and background doping. For this reason, geometrical considerations such as QW location will be experimentally investigated for its effects on diode dark current density parameters. Photoluminesence and high-resolution x-ray diffraction will be used to examine the radiative recombination efficiency and lattice matching properties of the MQW. In addition, full 8-band k*p band structure and current density-voltage modeling will be used to aid in the evaluation of the performance of these solar cells.

Wire array solar cells benefit from enhanced coupling of light into the active area of the device, significantly decreased collection lengths due to radial charge separation and collection, and easier access to grain boundaries for passivation which may enable future deposition on non-wafer substrates. However, they can also suffer from a decreased open-circuit voltage due to an increase in the junction area and increased recombination due to fully-depleted absorber volumes and resulting series resistance. We report on an analysis of the junction operation of micro/nano-wire array based GaAs solar cells through temperature and light intensity dependent current-voltage analysis. Prototype devices were fabricated with varying wire diameters and lengths, and interwire-spacing (pitch), and compared to planar GaAs control parts. We see evidence of non-ideal recombination athways due to space-charge region and tunneling recombination mechanisms coupled with activation energies for recombination that are significantly less than the band gap of GaAs. These results indicate that passivation of surface states at the junction may be necessary for full realization of the benefits of wirearray solar cells.

For photovoltaics to reach grid parity, i.e. be cost competitive with conventional sources of energy, such as coal and nuclear power, there must be a simultaneous increase in solar cell efficiencies and an overall decrease in their manufacturing cost. Flat silicon panels have been leading the way forward with costs as low as $0.70/watt, and typical efficiencies in the range of 15-20%. Although silicon panel technology sets the current standard for solar power, further advancements will come at a slow pace as the technology has nearly reached maturity. In contrast, dramatic increases in efficiency have been achieved with multi-junction solar cells composed of III-V semiconductors, albeit using relatively high-cost manufacturing processes. Manufacturing efficiencies >40% have been achieved by triple junction solar cells composed of GaInP/InGaAs/Ge. Variations on this design, including the use of metamorphic layers and dilute nitride subcells, now hold the world record for solar cell efficiency of 43.5%. These third generation photovoltaic technologies are expected to reach the 50% efficiency benchmark within the next decade, assuming that current PV device efficiencies continue to increase by ~1%/year. Here we present numerical and experimental research from the University of Ottawa SUNLAB that explores the potential avenues for achieving a >50% efficiency solar cell and cost reduction strategies. This research includes: four junction solar cell designs utilizing dilute nitrides and quantum confinement structures; cost reduction of epitaxial growth via the growth of III-V semiconductors on porous Si substrates; numerical explorations of I-III-VI material systems for the bottom sub-cell design of a III-V:CIGS multijunction cell; the potential benefits of up/down conversion to reduce the effects of electron thermalization; and light guided high concentration photovoltaics systems that have dimensions similar to a flat silicon panel.

Despite recent encouraging results for MBE grown devices, the development of III-V multijunction devices incorporating 1.0-1.2 eV dilute nitride (i.e. GaInNSbAs) subcells has been often limited by poor minority carrier diffusion lengths (20-40 nm) encountered in bulk dilute nitrides (i.e. grown by MOVPE). These short carrier diffusion lengths have led to severe degradations of device open circuit voltages and have been a major hindrance toward the development of efficient multijunction photovoltaics. In this work we consider an alternate design, where dilute nitride quantum wells are inserted within the intrinsic region of a conventional GaAs p-i-n sub- cell. Realization of such single junction device is shown to significantly improve the open circuit voltages compared to bulk-counterparts. Furthermore it is shown that the somewhat unique properties of dilute nitrides (increased electron effective masses and negligible valence band offset) may be used to exploit aperiodic quantum well designs that favor faster carrier escapes leading to devices with near ideal carrier collection efficiencies. The implementation of such devices is shown to provide a pathway toward the realization of ultra-efficient multi-junction devices with projected practical efficiencies in excess of 50%.

Intermediate band solar cells have been proposed as a pathway to ultra-high efficiency cells that break the Shockley-Queisser limit by enabling the utilization of sub-band gap photons [1.] One possible method of fabricating an intermediate band material is to use impurities that introduce electronic levels within the band gap. Theory suggests that if these impurities are incorporated at sufficiently high concentration to form a band of delocalized states within the band gap, Shockley-Reed-Hall recombination will be suppressed; this idea is known as “lifetime recovery [2.]" In this study, we investigate a proposed intermediate band material, silicon hyper-doped with sulfur. This material system exhibits strong sub-band gap optical absorption [3] and metallic conductivity at sufficiently high sulfur concentrations [4,] which makes it a strong candidate for an impurity-band material. This talk will present FTIR optical data and analysis that illuminates the sub-band gap absorption mechanism and the nature of the metal-insulator transition in this material. Our data suggest that a gap between impurity band (IB) and conduction band (CB) exists for moderate sulfur concentrations, but this gap disappears for higher sulfur concentrations as the IB and CB overlap due to IB broadening. Additionally, low-temperature photoconductivity will be used to investigate the lifetime of carriers excited via sub-band gap photons to test whether lifetime recovery is possible in this material system. [1] A. Luque and A. Martí. Phys. Rev. Lett. 78, 5014 (1997) [2] A. Luque and A. Martí, Adv. Mater. 22, 160 (2010). [3] T. G. Kim, J. M. Warrender, and M. J. Aziz, Appl. Phys. Lett. 88, 241902 (2006) [4] M. T. Winkler, D. Recht, M. Sher, A. Said, E. Mazur, and M. Aziz, Phys. Rev. Lett. 106, 178701 (2011)

Quantum confinement and miniband formation in periodically aligned silicon nanocrystals (Si-NCs) embedded in silicon oxide, carbide or nitride provide the band gap engineering facilities required for the creation of novel Si based absorber materials. These materials can be used as high-band gap components in all-silicon multi-junction solar cell devices designed for achieving efficiencies beyond the Shockley-Queisser-Limit while relying exclusively on abundant and non-toxic materials. The conventional approach to realize Si quantum dot absorbers is the fabrication of superlattice structures, in a procedure consisting of deposition and annealing of alternating stoichiometric barrier and Si rich matrix layers. Due to the competing nature of carrier confinement requiring high band offset and carrier transport demanding low band offset between Si-NCs and the embedding material, the choice of different materials for matrix and barrier is indicated. We consider SiO_x (x<2) as a promising matrix, since the phase separation is thermodynamically favorable, the quantum confinement pronounced and the amorphous SiO₂ phase stable at high temperature. SiC as barrier material generates low band offset between Si and SiC and thus gives rise to enhanced tunnel probability as compared to SiO₂. We introduced the novel SiC/SiO_x hetero-superlattice (HSL) with near-stoichiometric SiC as vertical barrier layer and silicon rich SiO_x as lateral matrix layer for the quantum dot formation deposited using PECVD for low-cost production. With this technique we were able to optimize the desired structure showing formation of Si-NCs in the oxide layers. However, increase in the density of paramagnetic centers, in sub-band gap absorption and in dark conductivity as well as the decrease in photoluminescence intensity during the post deposition annealing indicate the formation of annealing induced defects. We partially passivated these defects by incorporating hydrogen into the annealed samples using various techniques. For still further optimization, understanding the nature of these defects in SiC and SiO_x materials and their multilayer systems is crucial. We analyzed HSL as well as single layers of SiC and SiO_x annealed at various temperatures and passivated by various techniques using a number of characterization methods. The majority of spin resonant and/or optically active defects is identified as Si and C dangling bonds as well as strained Si-C bonds in the SiC layers. The different evolution of the structural, optical and electrical properties of these material systems during annealing and passivation is traced back to the fundamental difference in their atomic structure. Based on these findings, we discuss the general feasibility of HSL as a novel Si quantum dot absorber material for photovoltaic applications.

Thin film solar cells offer a cheap alternative to high efficiency 1st generation technologies. A major limitation to many thin film designs is reduced absorption due to the small optical path length. Photonic crystals have been considered as potential absorber layers due to their ability to control optical dispersion. Most studies on photonic crystal solar cells have focused on perfect lattices, and often without regard to how incoupling can be achieved into the guided modes of the layer. We identify limitations of using simple, perfectly periodic photonic crystal designs and show that higher photocurrents can be achieved by incorporating defects into the photonic crystal lattice, as well as varying the shape of the crystal&’s unit cells in the vertical direction. We first optimize simple, periodic lattices and identify general trends and design rules for designing periodic photonic crystal absorber layers. In particular, for optimized absorption over the solar spectrum, we find that a consistent ratio of radius to lattice constant is ~0.30-0.35 regardless of lattice type or slab thickness. Also, for an array of low index, non-absorbing holes in an absorbing slab, we find that an optimized square array is consistently better than a hexagonal lattice due to the larger filling fraction of absorbing material in a square lattice. For a 200nm Si photonic crystal slab, we can enhance absorption by as much as 200% compared to a planar slab with a 2-layer AR coating. This enhancement is greater than that which can be achieved with a non-absorbing, 2D photonic crystal layer on top of an absorbing planar slab. We show that introducing defects into otherwise perfect photonic crystal lattices can further increase absorption in two ways. First, by increasing the local density of optical states (LDOS) within the absorber region, as well as by increasing the incoupling to the photonic crystal slab. By incorporating even the simplest arrays of defects into an optimized periodic lattice, we find that an increase of ~0.5-2 mA/cm2 can be achieved. Additionally, we investigate the strategic placement of defects within a photonic crystal bandgap, so as to create an optical cavity with an increased LDOS at the resonant frequency. Other perturbations on the perfectly ordered lattice are also found to enhance light absorption, such as tapering the structure in the vertical direction, or by introducing multiplayers incorporating two or more radii or lattice constants. We also investigate the LDOS within the optimized photonic crystal designs and correlate absorption enhancements with increased LDOS. Lastly, we investigate the potential of the designs incorporating strong LDOS enhancements to exceed the ergodic light trapping limit.

We describe the use of dispersion engineered photonic materials to develop a new photovoltaic technology that can achieve much higher efficiencies than traditional devices through the modification of spontaneous emission. The limiting efficiency of photovoltaic energy conversion was determined by Shockley and Queisser using the theory of detailed balance, which described the balance between absorption and emission of photons. However, when the solar cell is formed from a photonic crystal or a similar material is placed on top of a solar cell, both the absorption and emission of photons is modified, a fact not considered in the original formalism. Here we show that photonic crystal structuring can lead to two efficiency improvements. First, if the active layer is made of a photonic crystal, suppression of the spontaneous emission at the semiconductor bandgap, as can be done with photonic crystals, will enable a drastic reduction in reverse saturation current. For high quality materials like GaAs, modest suppression of spontaneous emission can lead to a several percent efficiency improvement. In addition to detailed calculations, preliminary experiments with photonic crystals fabricated out of GaAs solar cells using focused ion beam sculpting will be presented. Second, the addition of a photonic crystal above a planar cell can lead to an effective modification of the energy bandgap of the material through an elevation of the quasi-Fermi level, which can subsequently change its maximum theoretical efficiency. Under ideal circumstances, this effect can improve the efficiency of a low bandgap solar cell device from less than 20% up to the Shockley-Queisser limit of 33.5%.

Conjugated polymer/fullerene-based bulk heterojunction (BHJ) organic photovoltaic (OPV) devices show promise to be lightweight, flexible, cost-effective alternatives to traditional inorganic solar cells. However, the efficiencies and lifetimes of BHJ-OPVs are unable to compete with current photovoltaic technologies, with a maximum recorded efficiency of 10% and typical lifetimes less than 1 year (cf. inorganic solar cells with maximum efficiency of 34.1% and lifetime of 25+ years) [1]. It has been shown that the lifetime of OPVs can be improved by using the device in an inverted configuration (ie. metal anode and transparent conducting oxide cathode). Similarly, it has also been proposed that the efficiency of OPVs can be improved by utilizing plasmonic nanostructures to enhance the optical electric field within the photoactive layer [2]. Here, we propose the use of plasmonic nanoantennas integrated into high work function metals as optically-active anodes in inverted OPV devices. We have calculated the performance parameters - the short-circuit current density (Jsc), the open-circuit voltage (Voc), and the fill factor - for inverted OPVs containing optically-active anodes and have compared the results to 1) planar inverted OPVs (which lack the nanostructures), 2) conventional OPVs containing optically-active cathodes, and 3) planar conventional OPVs. The optical electric field was calculated using finite-difference time domain (FDTD) simulations for both nanostructured and planar devices and verified analytically using transfer matrix (TM) methods for planar devices. We have fabricated an array of optically active electrodes to be used in the various device configurations by thermally evaporating metal through nanoporous alumina membranes. The optical properties of the bare nanostructures and the photoactive layer on the metal electrode (both planar and nanostructured) have been characterized using UV-VIS reflectance spectroscopy with an integrating sphere attachment and dark field imaging spectroscopy, and the nanostructures have been imaged using scanning electron microscopy (SEM). Preliminary calculations in devices containing the optically-active electrodes show an enhancement in the photocurrent of 1.5, resulting from an increased red-edge absorption in the photoactive layer, leading to an improvement in the overall efficiency of the device. [1] Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (version 39). Prog. Photovolt: Res. Appl. 2012, 20, 12-20. [2] Morfa, A. J.; Rowlen, K. L.; Reilly III, T. H.; Romero, M. J.; van de Lagemaat, J. Plasmon-Enhanced Solar Energy Conversion in Organic Bulk Heterojunction Photovoltaics. Appl. Phys. Lett. 2008, 92, 013504.

Highly flexible organic photovoltaic (OPV) cells are fabricated on zinc indium tin oxide (ZITO) as transparent conducting electrode. Amorphous ZITO films are deposited on Arylitetrade; substrate via pulsed laser deposition (PLD). Excellent optical transmittance for visible spectrum is achieved. The ZITO films demonstrate a resistivity of ~20Omega;/sq, comparable to the commercially available indium tin oxide (ITO) films. Only small degradation in conductivity is observed after controlled bending test. Using a novel high efficiency bithiophene imide (BTI) polymer, the resulting pristine OPV devices demonstrate similar power conversion efficiencies (PCE) to devices using commercially available ITO on glass as substrate. Furthermore, good device PCEs are maintained after bending test.

Silicon-organic heterojunction (SOH) solar cells represent a new class of photovoltaic device., in which light is absorbed in crystalline silicon, but without a p-n junction formation or high temperature (> 100°C) processing. A thin organic layer with a high LUMO serves to block majority carrier electron current from the silicon to the anode (for low dark current and high VOC). The work function of a transparent conductive anode provides the build-in field in the silicon to collect photo-generated minority carriers [1]. For low-cost production, screen-printing of silver contacts is typically used in silicon cells, but this requires a sintering process at around 300-600°C, which is too high to be compatible with organics. In this work we present (i) a transfer printing method to create patterned metallization on the device with maximum temperature of 80°C, and (ii) show experimentally that the blocking of majority carriers is so efficient that the remaining dark current consists predominately of minority carrier injection into the silicon, and (iii) demonstrate an efficient hybrid photovoltaic device with power conversion efficiency of 10.5% and fill factor reaching 85% under AM 1.5 illumination, (with all processing below 90°C). In this work three different approaches towards the anode structure, which consists of doped PEDOT as a transparent conductor followed by a fingered metallization to extract the photocurrent are used. First, a traditional electrode formed via shadow mask on PEDOT is used as the baseline device but is not scalable to large area. Second, Silver nanowire dispersions were also deposited on the devices to enhance lateral conductivity, improving fill factors greatly. Finally, a printed metallization scheme was developed in order to allow for future large-scale, low temperature processing of these devices. Devices with printed metal electrodes show minimal differences from those with thermal evaporation. To verify the effectiveness of the organic electron-blocking layer, the predominant mechanism of the resulting dark current mechanism was verified by a transient measurement of the stored minority carriers in the silicon. This showed that the current from majority carrier electrons was so effectively blocked that the remaining current was due to the injection of holes from the anode into the n-type silicon as minority carriers, independent of the type of anode. [1] S. Avasthi et al., Adv. Mater. Vol. 23 Iss. 48 p.5762-5766 (2010)

Remarkable progress happened in organic photovoltaic during the last years. Efficiencies beyond 10% were reported, as well as lifetimes beyond 10 years. With the first applications coming to market, the costs of OPV become more and more decisive. Large area coating and printing are certainly the technologies with the lowest production related costs. Large area printing appears to be more attractive for module production, but suffers from low viscous inks and the drawback of contact based printing principles. Large area coating is very attractive in terms of homogenous coating at high speed and is compatible to the low viscous inks. However, the drawback of coating is the very limited down-web resolution and the non- existing cross-web resolution. Thin-film photovoltaic modules require the sectioning into multiple cells which are connected in series. Three scribing patterns P1, P2 and P3 (Fig. 1) are typically required to form a monolithically interconnected module. In this contribution we discuss the combination of laser patterning and slot-die coating as a high speed, high precision production method for organic photovoltaic. A state of the art high power, high repetition rate femtosecond lasers with micro joule pulses is utilized for this study. The unique advantage of material processing with sub-picosecond lasers is efficient, fast and localized energy deposition, which leads to high ablation efficiency and accuracy in nearly all kinds of solid materials. The most sensitive materials in an organic solar cell stack are the semiconductor layer and the electrodes. We determined the ablation threshold of the single materials, and demonstrate that the individual ablation threshold for the single layers is sufficiently different to allow patterning with a single lasing wavelength and at highest speed. Functional modules are demonstrated for the normal as well as inverted architecture. We further demonstrate patterning with 10 micron resolution, and discuss whether today's electrode materials are sufficiently conductive for geometrical fill factors beyond 90%.

The quest for an efficient and inexpensive way of converting solar energy into electricity has drawn many researchers&’ attention to Organic Photovoltaic Cells (OPV) during recent years.¹ To date, the most successful solution processable OPV devices are achieved with Polymer-Fullerene blends, consisting of an electron-donating (p-type) conjugated polymer and an electron-accepting (n-type) fullerene, such as PCBM, as active layer composite. Power conversion efficiencies exceeding 9% were demonstrated recently.² However, for commercialization of this technology, further improvements not only concerning efficiency, but also lifetime and stability, are needed. Hence, new organic materials which can fulfill all necessary parameters for high performance OPV, such as light harvesting, charge transfer, charge transport and thermal, chemical and photostability are required.³ Perylene bisimides (PBI) and PBI related derivatives are a relevant class of n-type semiconductors due to their relatively high electron affinity and strong visible light absorption, combined with good photochemical and thermal stability and low material costs.⁴ Even though it was shown that PBIs can be better electron acceptors than PCBM with respect to charge photogeneration,⁵ their performance in solar cells is still significantly lower. This is mainly attributed to charge recombination in domains of strongly aggregated PBIs. To optimize these devices, structural order of PBI thin films and morphology in corresponding blends with p-type materials need to be controlled. Herein, three different N-substituted PBIs are compared. All three compounds comprise an unsymmetrical substitution pattern, which reduces aggregation in thin films significantly, compared to the symmetrical analogue. The nature of the substituents was varied from hydrophobic alkyl chains to hydrophilic ethylene oxide chains to allow for tuning self-assembly properties of the compounds. Aggregation and crystallinity in thin films were studied by polarized optical microscopy, UV/vis spectroscopy, X-ray diffraction (XRD) and Atomic force microscopy (AFM). A correlation of structure/morphology and charge transport properties, measured by the Space-Charge Limited Current method (SCLC), is given. XRD experiments give evidence for a highly ordered and oriented packing of the PBIs after an annealing step. Electron mobilities as high as 7×10^-3cm²V^-1s^-1 were measured. REFERENCES: ¹ J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A. J. Heeger, Science2007, (317), 222. ² http://www.polyera.com. ³ C. Li, H. Wonneberger; Adv. Mater.2012, (24), 613. ⁴ A. Wicklein, A. Lang, M. Muth and M. Thelakkat, J. Am. Chem. Soc.2009, (131), 14442. ⁵ S. Shoaee, Z. An, X. Zhang, S. Barlow, S.R. Marder, W. Duffy, M. Heeney, I. McCulloch, J. R. Durrant, Chem. Commun.2009, 5445.

Spectrum modification through bi-molecular upconversion, with a liquid phase upconverter, has been shown to enhance the external quantum efficiency (EQE) of partially transparent amorphous silicon solar cells under moderate concentration, Cheng et al. [1]. Sensitizer and emitter molecules work together to combine the energy of two below bandgap photons to produce a single photon with sufficient energy to be absorbed by the solar cell. The process exploits long lived triplet states and the spin conservation inherent in triplet-triplet annihilation to combine the energy of these below band-gap photon pairs. The result is prompt fluorescence from an emitter molecule, which produces a photon possessing significantly higher energy than either of the incident photons. The efficiency of bi-molecular upconversion depends strongly on the concentration of excited triplet states, as the rate of annihilation (k_Annihilationprop;[triplets]²) must be sufficiently rapid such that the rates of phosphorescence and non-radiative decay(k_Phos,NR prop;[triplets]) become insignificant, Cheng et al. [2]. The concentration of triplet states can be increased by providing more intense incident light or by concentrating the active molecules spatially. This spatial concentration has a limit when in solution as the molecules are able to aggregate or otherwise precipitate. This stalls the upconversion process by reducing the degree of mixing in the solution. The formation of a well mixed solid would reduce the average empty volume around active molecules by a factor on the order of 10³, increasing the potential density of excited states by a correspondingly significant amount. Therefore the fabrication of a well mixed thin film is a clear route to producing an efficient solid state upconverter. In this study, a number of thin films containing upconverting molecules were fabricated through spin casting, wire bar coating and drop casting. The ability the films to perform upconversion was recorded by observing time resolved phosphorescence and delayed fluorescence from each film under pulsed laser excitation. Initial experiments with drop casting and spin coating of a mixture of the upconverting molecules found that, on their own, the small molecules films did not produce any measurable upconverted light. It was found that the addition of a small amount, up to 10% by weight, of the optically and electrically inert polymer poly(methyl methacrylate) (PMMA) increased the number of upconverted photons such that up to 20% of photons emitted from the films were created through upconversion, the remaining 80% being produced by phosphorescence. Kinetic data from these experiments are presented and compared to a detailed rate model allowing insight into the changes in the energy transfer processes. Steps towards further efficiency gains are also discussed. [1] Cheng et al. Energy Environ. Sci., 2012, 5, 6953-6959 [2] Cheng et al. Phys. Chem. Chem. Phys., 2010, 12, 66-71

"Geometric photovoltaics" refers to the manner and extent to which charge carrier transport varies as a function of the geometrical configuration of the photovoltaic junction. Geometrical configurations of photovoltaic architectures can be broken down into conventional planar junctions, for which carrier dynamics are fundamentally one dimensional, and unconventional non-planar junctions, for which one of the fundamental aims is to separate optical and electronic length scales. To date, planar solar cell architectures still hold all efficiency records over non-planar counterparts. Moreover, while intellectually pleasing, it is not a trivial task to decouple electronic and photonic pathways, and a detailed understanding of the physics governing transport in non-planar photovoltaic junctions is still lacking. By aptly detailing the device physics for varying geometrical configurations of a photovoltaic junction, the geometry of the junction can be studied to ascertain which configurations maximally improve cell efficiency. Because of their often severely limited minority carrier recombination lifetimes, non-crystalline materials potentially benefit from non-planar photovoltaic architectures more so than solar cells fabricated from monocrystalline materials. Here, total current of the photovoltaic junction is derived as a function of its geometrical configuration and analytical expressions for device transport and efficiency are numerically calculated for an array of vertically aligned, nanocoaxial, a-Si solar cells. In addition to efficiency comparisons between coaxial and planar photovoltaic junctions, physical differences in the generation and recombination rates, based purely on geometrical considerations, are analyzed for non-planar architectures.

Ternary InxGa1-xN (InGaN) alloys can be tailored of having optical band gaps between infrared (Eg,InNasymp;0.7 eV) and ultraviolet (Eg,GaN = 3.4 eV), thus covering nearly the entire solar spectrum. For this reason, InGaN epilayers have been investigated for use in photovoltaic solar cells for the past years. At present, almost all photovoltaic device structures reported have exhibited very low short circuit currents and thus very low solar conversion efficiency. This phenomenon has been attributed to point and extended defect chemistry in InGaN epilayers (e.g. vacancies, misfit dislocations, and v-defects), as well as to spinodal decomposition of the strained InGaN wurtzite lattice system. With increasing indium concentration in the InGaN epitaxial layers, these defects become more dominant and challenge the formation of indium-rich InGaN epilayers and heterostructures needed for photovoltaic solar cells with optical band gap values needed for multijunction photovoltaic device structures. In this work, the effect of point- and extended-defects on InGaN solar cell performance has been investigated. We will also report on the growth and characterization of InN and indium-rich InGaN epilayers that have been grown by novel growth concepts, including growth at superatmospheric reactor pressures, the use of compositionally graded transition layers, digital alloying, and enhanced precursor pyrolysis. Selected results will be presented and the influence of these approaches on the structural and optoelectronic properties of these epilayers will be discussed.

We describe recent progress and future prospects for enhancement of solar cells using nanophotonic structures. Materials costs are a large component of total module costs for wafer based solar cells. As a result, there is a trend to thin film solar cells, in which the active semiconductor layer is deposited on a cheap substrate such as glass. However, this leads to reduced absorption, particularly for silicon based solar cells because of the indirect band gap of silicon. Therefore there is a need to increase the optical thickness of thin film solar cells, by trapping light inside the cell. Traditionally this has been done by etching the surface of the substrate or active layer, but such etching processes also lead to enhanced surface re-combination and reduced quality of the deposited semiconductor. The use of nano-photonic structures deposited directly on a planar device is a promising new approach to increasing absorption in thin film solar cells. Two particularly promising approaches are the use of high index dielectric scattering structures such as TiO2 and metallic structures based on plasmonic resonances. Recently, we have achieved a doubling of the photocurrent in thin film polycrystalline silicon solar cells, through the use of a combination of plasmonic particles and dielectric scatterers [1]. Nanoimprinted TiO2 also has the potential to provide both light trapping and passivation for solar cells [2]. We have recently shown that the strong scattering by metal particles on semiconductors is due to a resonant mode at the interface of the particle and semiconductor [3]. As well as strong scattering, these modes also have low parasitic losses that have the potential to provide better light trapping than conventional pyramid based light trapping schemes [4]. By combining a periodic array of metal nanoparticles with a back reflector in a resonant arrangement, nearly 100% of incident light can be diffracted outside the escape cone [5]. References 1. A. Basch, F. J. Beck, T. Söderström, S. Varlamov, K. R. Catchpole, “Combined plasmonic and dielectric rear reflectors for enhanced photocurrent in solar cells”, Appl. Phys. Lett. 100, 243903 (2012). 2. J. Barbé, A. Thomson, E. Wang, K. McIntosh, K.R. Catchpole, “Nanoimprinted TiO2 Sol-gel Passivating Diffraction Gratings for Solar Cell Applications”, Progress in Photovoltaics, 20(2), 143 (2012). 3. F. J. Beck, E. Verhagen, S. Mokkapati, A. Polman, K. R. Catchpole. “Resonant surface plasmon polariton modes supported by discrete metal nanoparticles on high-index substrates”, Optics Express, Vol. 19, Iss. S2, pp. A146-A156 (2011). 4. Fiona J. Beck, Sudha Mokkapati, Kylie R. Catchpole, “Light trapping with plasmonic parti-cles: beyond the dipole model”, Opt. Exp., Vol. 19 Issue 25, 25230 (2011). 5. E. Wang, T. P. White, and K. R. Catchpole, “Resonant enhancement of dielectric and metal nanoparticle arrays for light trapping in solar cells”, Optics Express, available online (2012).

We have proposed a new scheme to overcome the efficiency limit of crystalline silicon solar cell.[1] This scheme is "stack-junction", which applied the concept of multi-junction to Si solar cell. However, unlike regular tandem cells, the stack-junction cell has four terminals. And no current matching is needed because top and bottom sub cells are electrically separated. In the stack-junction cell based on Si top sub cell, the optimum band gap of the bottom cell is 0.68 eV in order to fully utilize the whole solar spectrum. Germanium is selected as a material for bottom sub cell because the band gap of Ge is 0.66 eV and very closes to the optimum. Si top cell has interdigitated front contact (IFC) structure and Ge bottom cell has interdigitated back contact (IBC) structure. IFC Si junction was formed by boron Ion Implantation (IIP) on N type CZ wafer. Thermal oxide combined with PECVD SiNx ARC was used to passivate the Si surface. For local contacts, high dose IIPs were used and titanium silicidation were formed by one step RTA. IBC Ge cell process is similar to IFC Si cell fabrication. Ge solar cell is not as extensively investigated as silicon. The optimization of resistivity, surface passivation, junction formation and metal contact are needed in order to realize highly efficient IBC Ge cell. The optimum resistivity of substrate for IBC Ge cell is much different with the screen-printing (SP) cell which is generally used for multi-junction. According to our simulation results, the optimum resistivity of P-type Ge is 0.13 Omega;cm for IBC cell. On the other hand, the optimum resistivity of SP Ge cell is 0.028 Omega;cm that is much smaller than that of IBC Ge cell. For the junction formation, Arsenic that is relatively heavy atom is selected as IIP species because of the relatively heavy weight of germanium atom in comparison with silicon. The selection of passivation layer is also important. In general, oxides are the best passivation layer. However, in the case of Ge, the GeOx was excluded from candidates for passivation until now, because the GeOx is hygroscopic and soluble in water. In the present work, we have overcome the limitations through new processes. The surface recombination velocity of 200cm/s is achieved using this method. For local contacts, high dose IIPs were used and titanium germanide were formed by RTA. We have fabricated 20.9% top Si / 1.6% bottom Ge stack junction with 22.5% module efficiency. Simulations with experiments have been performed. We eventually hope to obtain 26.5% stack module efficiency, higher than the practical efficiency limit of the single Si junction. [1] D.Kim, et al. 38th PVSC proceedings, 2012

Photovoltaic building blocks may have potential application in the fields of BIPV (building integrated photovoltaics), electrical aircrafts, and so on. In this application, substrates of solar cells should have appropriate mechanical properties as building blocks, and at the same time, they should have designed optical properties for highly efficient photovoltaic power generation. For this purpose, we have proposed a honeycomb-structured substrate for three dimensional thin film Si solar cells, targeting higher strength per weight and low sensitivity to the angles of solar irradiation, which is a prototype photovoltaic building block based on thin film Si. We have demonstrated two kinds of honeycomb-structured substrates: one is based on anodized aluminum oxide, and the other is based on Si. The minimum feature sizes (20 ~ 30mu;m) of honeycomb-structures are much larger than visible wavelengths, which can emulate the solar performances for differently designed building blocks with larger feature sizes. From solar cell demonstrations using these two different substrates, we will show the process related issues in three dimensional thin film solar cells, and suggest some methods of substrates design for building blocks application. The performance of thin film Si solar cells on three dimensional substrates depends largely on the conformality of thin film deposition. We have compared plasma-enhanced chemical vapor deposition method with photo-chemical vapor deposition, and found that the photo-chemical vapor deposition exhibits larger step coverage, which implies higher versatility of photo-chemical vapor deposition in the fabrication of three dimensional thin film solar cells. The photocurrent generation characteristics of proposed three dimensional solar cells show reduced angular dependences compared to solar cells on planar substrates. Due to the effect of multiple reflections on honeycomb surfaces, three dimensional solar cells exhibited 10~20% enhancement of photocurrent generation at low angles of incidence larger than 40 degrees. In conclusion, honeycomb structure is of great importance in photovoltaic building block not only due to its mechanical benefits but also due to higher photovoltaic conversion efficiencies at low angles of solar irradiation. Moreover, short and mid-term strategies on the development of photovoltaic building blocks will also be introduced.

The presentation will review how multi-junction III-V cells can be optimized, using semiconductor bandgap engineering, to perform with conversion efficiencies in the 40% range when used in Concentrated PhotoVoltaic (CPV) applications operating at concentrations typically between a few hundreds and 1000 suns. Such devices and materials are placing interesting characterization and design requirements in order to meet the unique low cost, high material quality, low electrical resistance, and high thermal conductivity properties necessary for the high performance and the necessary reproducibility for such large scale applications. That III-V semiconductor volume application is expected to require industrial players to produce several millions of wafers to be grown by MOCVD and fabricated in cells sizes typically between a fraction of cm^2 up to 1 cm^2 depending on the system&’s optics and the concentration used. At that scale, CPV cells will likely be the largest market for compound semiconductor devices, and it is therefore important to understand the cost and performance drivers and the related design rules. The above characterization and design challenges therefore unlock opportunities for interesting and relevant research in i) the semiconductor epitaxy for nanostructrures and advanced semiconductor devices, ii) comprehensive studies of the optoelectronic properties of such semiconductor nanostructures, and iii) advanced material and device characterization techniques and reliability studies.

High density and high capacity storage of solar energy is strongly demanded for the dissemination of sustainable renewable energy systems. Solar hydrogen generation has been regarded to be a promising candidate for such a purpose, but most of experimental energy conversion efficiency has been a few percent at most. We here combined a concentrator photovoltaic (CPV) module and a polymer electrolyte electrochemical cell (PEEC) and evidenced the solar-to-hydrogen energy conversion efficiency exceeding 12% using a conventional GaInP/(In)GaAs/Ge 3-junction cell under 10 suns concentration. The optimal design of a multi-junction cell will be discussed which will enable the solar-to-hydrogen energy conversion efficiency as high as 30%. In our setup, the CPV cell and the PEEC were connected directly without any load adjustment. The merit of using CPV with a multi-junction cell as a source of electrochemical potential is two folds: (1) a single cell yields the potential large enough to promote electrochemical splitting of pure water, and (2) a large current density under sunlight concentration enables almost unity quantum efficiency from electrons to hydrogen by minimizing the effect of parasitic reactions. A series connection of silicon photovoltaic cells never allowed us to obtain highest conversion efficiency because the series connection results in a reduced current density per light-receiving area. The criteria for the highest solar-to-hydrogen energy conversion efficiency is the voltage matching between a tandem cell and an electrochemical cell: if we prefer the simplest configuration of 1 to 1 connection between a PV and an electrochemical cell, the operation voltage of the PV cell should be 1.4-1.5 V, which is a typical operation voltage for an efficient water splitting cell, and the current from the PV should be maximized. This is quite different from the design strategy for conventional multi-junction cells. We would propose that a (In)AlGaAs/Ge 2-junction cell is the most promising for the combination with a water-splitting cell. The cell outputs at the operation condition are estimated to be 1.5 V and 7.2 A/cm2 under 300 suns that lead to the solar-to-hydrogen energy conversion efficiency of 30% with an assumption of unity quantum efficiency at the electrochemical cell, which is realistic under large current density. This lattice-matched material system will be readily implemented with existing growth technology and the high-efficiency solar hydrogen generation will be affordable in the quite near future.

High efficiency solar to electric energy conversion necessitates efficient conversion across the full solar spectrum from 280 nm to 4µm. We are investigating a three-channel system for such conversion combining photovoltaic (PV) and solar thermal approaches. Incident solar spectrum is first concentrated and then split into two major parts: PV and thermal. The PV part of the spectrum is further split into several subbands, which are directed to bandgap appropriate solar cells either laterally aligned on the same Si substrate or on their individual Si substrate. Instead of using expensive Ge or III-V substrates, single crystalline thin film Ge is grown on top of a Si substrate through a two-step selective epitaxial growth. This Ge film is used as a virtual substrate for III-V epitaxy. Unlike vertically stacked multijunction solar cells, no current matching is required. At long and very short wavelengths where PV efficiency is low, solar radiation is directed to a thermal storage tank for electricity generation using heat engines. The PV waste heat from the solar cells due to thermalization of high energy photo-electrons will also be collected for additional electricity generation. The advantages of our approach lie in the high system efficiency with relatively low cost and the capability of generating electricity day and night through thermal energy storage. The solar concentrator is composed of a large array of heliostats that reflect light onto secondary mirrors, which in turn direct light down and onto the entrance of a compound parabolic concentrator. Radiation is then fed into a cavity beam homogenizer, which has several exits equipped with wavelength selective filters, allowing radiation to reach the PV cells and thermal receiver. The concentration factor can reach several hundred times. High temperature heat storage (600-900 °C) is achieved by using molten salt as a phase change material with added graphite flakes for high thermal conductivity. The high temperature heat storage tank allows utilization of supercritical CO2 Brayton cycle, which is more efficient than traditional steam Rankine cycle used in current solar thermal power plants. The PV waste heat can be used as a heat source for electricity generation through an organic Rankine cycle. The PV efficiency is determined by the concentration factor, solar cell temperature, junction materials chosen, and how the solar spectrum is split between the PV and thermal part and among the different bandgap solar cells. Detailed analysis will be carried out through optical and thermal simulations and MATLAB optimizations to decide the optimal values of these variables for the highest system efficiency including the PV, thermal and PV waste heat utilization for electricity generation. With optimized design and neglecting optical loss, system power conversion efficiency can reach more than 50%, including around 10% absolute contribution from thermal and PV waste heat utilization.

As incentive programs for solar energy are gradually being phased out around the world, solar must quickly become a viable and competitive option to the mainstream, fossil-based power generation technologies. As such, it must begin to assume more of the characteristics of the traditional technologies in order to be compatible with utility generation business models. With this goal in mind, Solergy has realized a series of innovations that include optics, concentrator module design, and tracking to create a high performing, long-lasting high concentration photovoltaic (HCPV) system. Because of its truly upgradeable design, Solergy&’s HCPV system can actually increase its power output over its lifetime and then subsequently extend its life out to 40 years. Initial full-scale demonstration tracker has been operating in the field for several months and is routinely achieving >25% AC efficiency and generating nearly 10 kWhrs/kW on clear sunny days. This talk will discuss in more detail, both laboratory and field performance results, as well as the types of technological advances required to achieve such a robust high performance system. In particular, the talk will cover the world&’s only all-glass concentrating lens, high performance tracking, and the unique upgrade mechanism.

We present photoluminescence (PL) quenching and ultrafast interfacial charge transfer dynamics studies of poly (3-hexylthiophene-2,5-diyl) (P3HT) thin films doped with monodisperse TiO₂ nanoparticles. Significant PL quenching is obtained when the P3HT films contain up to 80% TiO₂ nanoparticles in weight due to charge transfer. Polaron absorption after the photoinduced charge separation in the presence of 80% TiO₂ is significantly enhanced with longer-lived lifetimes in contrast to neat P3HT films. The absorption of polarons created at the P3HT/TiO₂ interface is found to exhibit a linear dependence on the P3HT/TiO₂ interfacial area per unit volume. Surprisingly, PL is enhanced for films containing 20% TiO₂ as compared to neat P3HT films, which is attributed to disordering of polymer chain and increase in film roughness as shown by steady state PL and electrogenerated chemiluminescence (ECL) studies. ECL studies show a decrease in the ECL turn-on potential of P3HT/TiO₂ composites proportional to their TiO₂ content, corresponding to the ECL yield increases, indicating the formation of an effective interpenetrating network of TiO₂ and disordering of polymer packing which allows the ECL coreactant to transport through the film for efficient ECL generation.

Organic solar cells have recently passed the 10% efficiency barrier, which is a milestone in the road towards their commercialization.(1, 2) Like their fully organic counterparts, hybrid solar cells use a donor polymer to absorb light, while the organic acceptor (mostly fullerene) is typically replaced by a metal oxide. The latter can be synthesized in various nano-patterns prior to the deposition of the donor polymer, which finally results in a rigid heterojunction, thus giving hybrid solar cells the edge regarding morphological stability.(3, 4) Unfortunately, their efficiency is still falling behind on fully organic solar cells, despite the favorable properties of typical acceptor metal oxides as compared to fullerene derivatives.(5, 6) Several reasons have been postulated for this, with the poor compatibility between polymer and metal oxide being designated as the biggest culprit, both in terms of morphology and associated electronic properties.(7, 8) This contribution presents systematic strategies to improve the efficiency of hybrid solar cells. After substantiating the physical equivalence between organic and hybrid solar cells (and thus the rationale that they should be able to perform comparably), different aspects of the hybrid devices will be highlighted with a keen eye on optimization, both from a fundamental as well as an engineering point-of-view. Taking into account geometric and optical considerations together with charge transport properties and electronic structure, for both ingredients (metal oxide, polymer) several tactics for efficiency enhancement will be discussed that might assist in raising the efficiency of hybrid solar cells to the level of organic solar cells or beyond. (1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Prog. Photovoltaics Res. Appl. 2012, 20, 12 (2) Soslash;ndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Mater. Today 2012, 15, 36 (3) Olson, D. C.; Piris, J.; Collins, R. T.; Shaheen, S. E.; Ginley, D. S. Thin Solid Films 2006, 496, 26 (4) Djurisic, A. B.; Chen, X.; Leung, Y. H.; Man Ching Ng, A. J. Mater. Chem. 2012, 22, 6526 (5) Baeten, L.; Conings, B.; Boyen, H.-G.; D&’haen, J.; Hardy, A.; D'olieslaeger, M.; Manca, J. V.; Van Bael, M. K. Adv. Mater. 2011, 23, 2802 (6) Huang, J.; Yin, Z. G.; Zheng, Q. D. Energy Environ. Sci. 2011, 4, 3861 (7) Conings, B.; Baeten, L.; Boyen, H.-G.; Spoltore, D.; D&’haen, J.; Grieten, L.; Wagner, P.; Van Bael, M. K.; Manca, J. V. J. Phys. Chem. C 2011, 115, 16695 (8) Park, B.; Lee, J.-H.; Chang, M.; Reichmanis, E. J. Phys. Chem. C 2012, 116, 4252

The energy cost associated with separating tightly-bound excitons, photo-generated electron-hole pairs, and extracting free charges from highly disordered low mobility networks represent fundamental losses for the two most prominent low cost photovoltaic technologies: organic and dye-sensitized solar cells. Here, we present a low cost solution-processable solar cell based on a mesoporous superstructured absorber with intense visible-to-near-IR absorptivity. This meso-superstructured solar cell (MSSC) exhibits exceptionally few fundamental energy losses illustrated by generating open-circuit photovoltages of over 1.1 V, despite the relatively narrow absorber band gap of 1.55 eV. With a power conversion efficiency of 10.9% in a single junction device under simulated full sunlight, this solid-state photovoltaic concept is close to competitive with established thin-film solar technologies. It has imminent scope to deliver a nanotechnology solution for large scale solar power generation at the lowest cost.

The tunable energy levels and high carrier mobility of silicon quantum dots (SiQD) complement the strong visible range absorption of conjugated polymers and so offer a promising hybrid solar cell paradigm. However, the efficiencies of these composites are typically lower than those of their purely organic counterparts [1,2]. A lack of understanding as to why this is so, and how it can be remedied, has motivated our theoretical study of a representative SiQD/P3HT system. First principles computational tools were used to implement a new, non-adiabatic phonon assisted quantum tunneling model in order to quantify the charge separation and transport dynamics. The predicted charge separation rate is in good agreement with that measured experimentally [3], and it is much higher than the photoluminescence rate of the P3HT. This implies that excitons should be able to efficiently dissociate into free carriers. In addition, the charge hopping proceeds much more quickly than interfacial charge recombination, so a considerable fraction of free electrons and holes should be able to diffuse to the electrode based on the predicted carrier mobilities. In fact, the results indicate that P3HT/SiQD blends have the potential to out perform the widely used, purely organic, P3HT/PCBM architecture. We conclude that the low efficiency of current P3HT/SiQD solar cells originates from the high density of defects and a distribution of SiQD sizes. The influence of dangling bonds, oxidation, passivation and doping on the charge dynamics will be discussed to provide insights for improving the performance of these hybrid materials. This work is supported by the NSF through the Renewable Energy Materials Research Science and Engineering Center (REMRSEC). References [1] S. Niesar, et al., Green 1, 339, 2011. [2] C. Liu et al., Nano Lett. 9, 449, 2009. [3] D. Herrmann et al., J. Am. Chem. Soc. 133, 18220, 2011.

It has been known for over half a century that the efficiency of even ideal single-junction photovoltaic cells that absorb all the incoming solar radiation is limited to 31% (for one sun illumination) by intrinsic losses such as band edge thermalization and radiative recombination. Approaches to exceed this intrinsic thermodynamic limit, named after Shockley and Queisser, included using concentrated sunlight, engineering multiple-junction and intermediate-band solar cells, exploiting mechanisms of electronic up- and down-conversion of photons, thermophotovoltaic, and thermophotonic. The resulting limiting efficiency can e.g. reach 72% in the case of 36-junction solar cell under illumination by a 1000-times concentrated sunlight. We will present a fundamentally new approach to exceeding the Shockley-Queisser limit, which is based on the mechanism of thermal upconversion of photons. We will demonstrate that the limiting intrinsic efficiency of an ideal single-junction solar cell can be increased up to 72% under illumination by an unconcentrated sunlight. A more detailed analysis of non-ideal solar cells that allows for up to 20% of absorption/re-emission losses yields limiting efficiency values exceeding 45%. Finally, we will discuss practical device realizations of the proposed theoretical scheme. Acknowledgment: This work is supported as part of the ‘Solid State Solar-Thermal Energy Conversion Center (S3TEC)&’, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.

Single-walled carbon nanotubes (SWNTs) possess outstanding electrical and thermal conductivities and enormous tensile strength, which makes them a versatile material for many applications. Recently, hybrid photovoltaic based on Carbon nanotube (CNT)/n-type crystalline silicon (Si) has been reported [1]. A major benefit of such an approach is the combination of charge separation at the CNT/Si interface along with charge transport and collection, which eliminating metal wiring from the device surface that usually blocks a portion of incident light. It also avoids the high-temperature diffusion and element doping steps in traditional Si cell fabrication, leading to a reduced production cost. We present a comparative study on various SWNT/Si hybrid solar cells with SWNT TCEs prepared by three different preparation methods, Mayer rod coating [2], spray as well as a newly developed super-acid sliding cast method. Films by super-acid sliding method demonstrate optoelectronic performance, morphology (smoothness and alignment), mechanical properties that are advantageous to other methods. By applying simple/cost-effective post-treatments to as-fabricated cells including HF treatment and various p-type doping, we have greatly and uniformly improved the photovoltaic efficiency irrespective of SWNT preparations methods; A maximum efficiency of 10.5 % is achieved under AM 1.5G illumination. We systematically compare the photovoltaic performance of each solar cell before/after the treatments and between SWNT preparation methods, and discuss various factors contributing to the cell efficiencies. 1. A. Y. Cao, Y. Jia, J. Q. Wei, K. L. Wang, Q. K. Shu, X. C. Gui, Y. Q. Zhu, D. M. Zhuang, G. Zhang, B. B. Ma, L. D. Wang, W. J. Liu, Z. C. Wang, J. B. Luo and D. Wu, Adv. Mater., 20, 4594 (2008). 2. X. Li, F. Gittleson, M. Carmo, R. C. Sekol and A. D. Taylor, Acs Nano, 6, 1347 (2012).

Roll-to-roll process has been widely used in the printing industry at high process speeds and recently adopted by organic solar cell manufacturers to demonstrate flexible, large-area, low-cost organic solar modules. The so-called stripe geometry is widely used in roll-to-roll process in order to minimize the resistive power loss in large-area solar modules. However, it has its limitations since the area used by the cell-to-cell connections reduces the overall active area of the module, leading to a significant reduction of the power conversion efficiency in modules compared to small-area cells. Recently, we demonstrated improved performance for large-area organic solar cells and modules by integrating metal grids directly with transparent electrodes. However, we found that it was difficult to coat the solution-based organic materials uniformly on top of the substrate due to the thick metal grids. In this talk, we will report on a new approach to implement a transparent flexible substrate with an embedded thick metal grid. Because the thick metal grid was embedded in the flexible substrate, we were able to spin coat thin organic materials uniformly on top of the large-area substrate where thick metal grids were embedded. For the same reason, organic materials can be coated on this metal-grid-embedded flexible substrate using a roll-to-roll process. Furthermore, the metal lines can be made narrow, reducing the shadowing loss to only 5.5%, and thicker, resulting in lower resistive losses in large-area devices with higher currents. To make an embedded metal grid on an SU8 thin film, we deposited 200 nm copper on a glass substrate as a seed layer. On top of this seed layer, mold structures were created using SU8 followed by electroplating Cu with a thickness of up to 15 µm. Another layer of SU8 was applied on top of the entire surface. The fabricated SU8 film with embedded metal grids was peeled off from the glass substrate. After removing the seed layer, a high conductivity PEDOT:PSS (PH1000) with 5% DMSO was spin-coated followed by polyethylenimine ethoxylated (PEIE) spin-coating. The active layer of poly(3-hexylthiophene) (P3HT): Indene-C60 Bis-Adduct (ICBA) (1:1, weight ratio) was spin-coated followed by deposition of MoO3 and Ag under vacuum. The completed device had an active area of 9.3 cm2 and a structure of SU8/embedded-metal-grid/PEDOT:PSS(PH1000)/PEIE/P3HT:ICBA/MoO3/Ag. The device exhibited a fill factor of 0.6, a short-circuit current density of 5.8 mA/cm2, and an open-circuit voltage of 0.81 V, yielding a power conversion efficiency of 2.8% under 100 mW/cm2 air mass 1.5G illumination.

With the recent advancements in the field of thin film solar cells, numerous light-trapping paradigms have been developed to help mitigate the low absorptance rates inherent in having thin photovoltaic active layers. This report presents experimental and simulated data on a particular type of plasmonic concentrator designed for photovoltaic applications, known as a nano-hole array (NHA). When employed in a solar cell, thousands of shallow sub-wavelengths holes are milled on the surface of the back metal contact. These holes serve to collect the incident light and redirect it along the dielectric / metal interface in the form of surface plasmon polaritons (SPPs). Absorptance in the dielectric material can then be enhanced through two distinct mechanisms: (i) diffractive scattering at each hole location generates SPP modes that propagate parallel to the interface, thus increasing the scale of interaction length between light and matter; (ii) constructive interference can occur between SPP modes excited by neighboring holes, potentially leading to increased local electromagnetic fields. Despite preliminary research on nano-hole arrays for photovoltaic applications, a systematic understanding of the role of structural geometry and SPP interference effects in plasmonically-enhanced absorption paradigms is still lacking. To this end, a certain class of two-dimensional array will be investigated: quasi-periodic (QP). QP arrays lack the long-range translational symmetry typical of periodic arrays, but can have very high degrees of local order and rotational symmetry, which is completely absent in random arrays. Most importantly, QP arrays can pack many more holes into a plane than any periodic (e.g. square, triangular, etc.) array can. A 24-nm-thick layer of the organic bulk heterojunction P3HT:PCBM was deposited on silver substrates milled with various periodic, QP and random arrays of nano-holes; their optical properties were studied by illuminating the sample with broad-band, randomly polarized light. Absorption was seen to increase across the entire lambda; = 400 nm to 800 nm spectrum; the exact nature of the enhancement depends on the type of pattern used (identified by its order of local rotational symmetry) and the average hole-hole spacing. When expressed in terms of short-circuit current density (Jsc), it was observed that all the devices with NHA&’s milled on them had higher Jsc values that devices without an NHA, with the biggest improvement being a 56% relative increase. The purpose of this talk is to expound on these and other results which show the dependence of absorptance enhancement on rotational symmetry and hole-hole spacing. Additionally, the underlying physics of SPP&’s (generation, propagation, interference, and absorption) will be discussed; finally, current-voltage data from an operating solar cell will be presented.

The domain size of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) within the bulk heterojunction of organic photovoltaics (OPVs) is of great importance to the performance and lifetime of the device. This study focuses on the use of polymer-tethered fullerenes (PTFs) as an additive to reduce the initial domain size of PCBM as well as to mitigate the growth of these domains over time that typically results in decreased device efficiency. Films were prepared from a 1:1 mixture of PCBM:P3HT on silicon wafers to approximate a bulk heterojunction system. PTFs were added in concentrations ranging from 0 to 20 percent. The samples were then annealed at 150°C for varying times ranging from 5 to 60 mins and examined by SEM to determine the average PCBM domain sizes for each sample. As expected, the control sample containing no additive showed significant growth in the PCBM domains as the sample was annealed with a final average domain size of 290 square microns. However, addition of as little as 5% PTFs produced average PCBM domain sizes after 60 minutes of annealing of 28 square microns. Addition of 20% PTF resulted in an average PCBM domain size of 2 square microns. Based on these encouraging results, devices containing these architectures are being prepared.