Kimberly A. Sablon, U. S. Army Research Laboratory
Lan Fu, "Australian National University Research School of Physics and Engineering"
Zhiming Wang, University of Electronic Science and Technology of China
Sudersena Rao Tatavarti, "MicroLink Devices, Inc."
Symposium Support Army Research Laboratory:
Magnolia Solar, Inc.
U.S. Naval Research Laboratory
E3/H2: Joint Session: Dye-Sensitized Solar Cells II
Monday PM, November 26, 2012
Hynes, Level 3, Ballroom A
2:30 AM - *E3.01/H2.01
Light-harvesting with Nanoscale Assemblies Incorporating Nanocrystals and Photosynthetic Molecules
Alexander O Govorov 1
1Ohio Univ Athens USAShow Abstract
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 . 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.  J. Lee, A. O. Govorov, and N. A. Kotov, Nano Letters 5, 2063 (2005).  P. Hernandez-Martinez and A. O. Govorov, Phys. Rev. B 78, 035314 (2008).  J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials 6, 291 (2007).  A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007).  A. O. Govorov, Adv. Mater., 20, 4330 (2008).  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).  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).  I. Carmeli, L. Lieberman, L. Kraversky, Z. Fan, A. O. Govorov, G. Markovich, and S. Richter, Nano Letters, 10, 2069 (2010).
3:00 AM - E3.02/H2.02
High Efficiency All-solid-state Dye-sensitized Solar Cells
In Chung 1 Byunghong Lee 2 Robert P. H. Chang 2 Mercouri G. Kanatzidis 1
1Northwestern University Evanston USA2Northwestern University Evanston USAShow Abstract
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 CuIn1-xGaxSe2. 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.
3:15 AM - E3.03/H2.03
Spray Deposition of CdS and PbS Quantum Dots for Efficient Semiconductor Sensitized Solar Cells
Isabella Concina 1 2 Nafiseh Memarian 4 Gurpreet Sing Selopal 2 1 Marta Maria Natile 3 Alberto Vomiero 1 2 Giorgio Sberveglieri 2 1
1CNR-IDASC Sensor Lab amp; Brescia University Brescia Italy2Brescia University Brescia Italy3Padova University Padova Italy4Semnan University Semnan Islamic Republic of IranShow Abstract
Due to their unique features, semiconductor quantum dots (QDs) are presented as the ultimate frontier as sensitizers for photoelectrochemical solar cells ,. 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 ,,[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 . This study is focused on the application of spray deposition (SD)  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.  S. Rühle et al., Chem. Phys. Chem. 2010, 11, 2290  A. Shabaev et al. Nano Lett. 2006, 6, 2856  Y-L. Lee and Y-S. Lo Adv. Func. Mat. 2009, 19 604  H. Lee H et al., Nano Lett. 2009, 9, 4221  A. Bragaet al., J. Phys. Chem. Lett. 2011 2 454  S. Che et al., J. Aer. Sci. 1998, 29 271
3:30 AM - E3.04/H2.04
High Efficiency Inkjet Printed DSSCs
Christopher Woodbury 1 Thad Druffel 2 Sheila Bailey 3 Delaina Amos 1
1University of Louisville Louisville USA2Conn Center for Renewable Energy Louisville USA3NASA Glenn Research Center Cleveland USAShow Abstract
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.
3:45 AM - E3.05/H2.05
Back Contact Type Dye-sensitized Solar Cells with Cylinder Shape-high Efficiency Cell by Using Optical Wave Guide Effect and Their Optical Simulation
Jun Usagawa 1 Sho Noguchi 1 Jin Ohara 1 Yuehi Ogomi 1 Shyam S Pandey 1 Shuzi Hayase 1
1Kyushu Institute of Technology Wakamatsu-ku Kitakyushu JapanShow Abstract
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.
E4: Thin Film Solar Cells I
Monday PM, November 26, 2012
Hynes, Level 3, Ballroom A
4:30 AM - E4.01
Simultaneously Optimized Design of Optical, Electrical, and Microstructural Properties in Thin-film Solar Cells
Michael G. Deceglie 1 Vivian E. Ferry 2 3 A. Paul Alivisatos 2 3 Harry A. Atwater 1
1California Institute of Technology Pasadena USA2Lawrence Berkeley National Laboratory Berkeley USA3University of California Berkeley USAShow Abstract
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.
4:45 AM - E4.02
The Mechanism Created by the Cadmium Chloride Treatment to Improve the Efficiency of Thin Film CdTe Photovoltaics
John Michael Walls 1 Ali Abbas 1 Geoff West 1 BiancaMaria Maniscalco 1 Piotr Kaminski 1 Kurt Barth 2 Walajabad Sampath 2
1Loughborough University Loughborough United Kingdom2Colorado State University Fort Collins USAShow Abstract
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
5:00 AM - E4.03
Realistic Simulation of Polycrystalline CIGS Absorbers and Experimental Verification
Carlo Maragliano 1 Marco Stefancich 3 2 Stefano Rampino 2 Lorenzo Colace 2
1University of ``Roma Tre" Rome Italy2CNR-IMEM Parma Italy3Masdar Institute of Science and Technology Abu Dhabi United Arab EmiratesShow Abstract
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.
5:15 AM - E4.04
Comparative Study of CuIn1-xGaxS2 Thin-films Absorbers Fabricated in Copper-rich and Copper-poor Regimes
Neelkanth G. Dhere 1 Ashwani Kaul 1 Eric Schneller 1 Narendra Shiradkar 1
1University of Central Florida Cocoa USAShow Abstract
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.
5:30 AM - *E4.05
The US Department of Energy SunShot Initiative
Elaine Ulrich 1
1US Dept. of Energy Washington USAShow Abstract
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.
E1: Photovoltaic Technologies I
Monday AM, November 26, 2012
Hynes, Level 3, Ballroom A
9:30 AM - E1.01
Dislocation Density Reduction with the Application of Mechanical Stress
Douglas M Powell 1 Hyunjoo Choi 1 Sergio Castellanos 1 Tonio Buonassisi 1
1Massachusetts Institute of Technology Cambridge USAShow Abstract
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 . 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.  D. Franke et al., “Silicon ingot casting: process development by numerical simulations,” Solar Energy Materials & Solar Cells 72, 83-92 (2002).  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.  M.I. Bertoni et al., “Stress-enhanced dislocation density reduction in multicrystalline silicon,” Physica Status Solidi, RRL, 5, 28-30 (2011).
9:45 AM - E1.02
Transformative Methods for Solar Cell Contact Formation by N-gettering
Jon-Paul Maria 1 David Henry Hook 1 James LeBeau 1 Ian Cooper 2 Ajeet Rohatgi 2 Brian Laughlin 3
1NC State University Raleigh USA2Georgia Institute of Technology Atlanta USA3DuPont Microcircuit Materials Durham USAShow Abstract
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.
10:00 AM - *E1.03
Harvesting More of the Solar Spectrum with Quantum Dot-based Photovoltaics: Material Constraints and Opportunities
Matthew Doty 1 William Reid 1 Tobin Driscoll 2 Chelsea R. Haughn 1 Laura R. Vanderhoef 1 Joshua M. O. Zide 1
1University of Delaware Newark USA2University of Delaware Newark USAShow Abstract
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.
10:30 AM - E1.04
Iron Kinetics Simulation and Experimentation Demonstrating Potential for Novel Industrial Processing of Silicon Solar Cells
David P Fenning 1 Jasmin Hofstetter 1 Ashley E Morishige 1 Annika Zuschlag 2 Giso Hahn 2 Tonio Buonassisi 1
1Massachusetts Institute of Technology Cambridge USA2University of Konstanz Konstanz GermanyShow Abstract
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.
10:45 AM - E1.05
Comparison of Multi-crystalline Silicon PV Modulesrsquo; Performance under Augmented Solar Irradiation
Yang Hu 1 Donald Huckle 1 Daniel Dryden 1 Dave Hollinshead 2 Mark Schuetz 2 Roger French 1
1Case Western Reserve University Cleveland USA2Replex Plastics Mount Vernon USAShow Abstract
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.
E2: Next Generation Solar Cells I
Monday AM, November 26, 2012
Hynes, Level 3, Ballroom A
11:30 AM - *E2.01
Photonic Structures for High Efficiency Full Spectrum Photovoltaics
Harry A. Atwater 1
1California Institute of Technology Pasadena USAShow Abstract
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.
12:00 PM - E2.02
Exfoliated ~25mu;m Si Foil for Solar Cells with Improved Light-trapping
Sayan Saha 1 Dabraj Sarkar 2 Mohamed Hilali 1 Emmanuel Onyegam 1 Rajesh Rao 3 Ryan Smith 3 Dewei Xu 3 Leo Mathew 3 Dharmesh Jawarani 3 Ujjwal Das 4 Jerry Fossum 2 Sanjay Banerjee 1
1University of Texas - Austin Austin USA2University of Florida Gainesville USA3AstroWatt, Inc. Austin USA4University of Delaware Newark USAShow Abstract
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%.
12:15 PM - E2.03
Enhanced Light Trapping by Zinc Oxide Nanocone Array for High-efficiency Thin-film Silicon Solar Cells
Jonathan Pradana Mailoa 1 2 Yun Seog Lee 1 3 Inna Kozinsky 1
1Robert Bosch Research and Technology Center Palo Alto USA2Massachusetts Institute of Technology Cambridge USA3Massachusetts Institute of Technology Cambridge USAShow Abstract
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 . 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.  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).
12:30 PM - E2.04
Angle-insensitive Broadband Absorption Enhancement in Nanostructured Crystalline Silicon Solar Cells for Photovoltaic Applications
Ragip A Pala 1 Koray Aydin 2 Serkan Butun 2 Durmus Karatay 1 Ryan Briggs 1 Harry A Atwater 1
1California Institute of Technology Pasadena USA2Northwestern University Evanston USAShow Abstract
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 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.
12:45 PM - E2.05
Optical Properties and Limits of a Large-area Periodic Nanophotonic Light Trapping Design for Polycrystalline Silicon Thin Film Solar Cells
Daniel Lockau 1 2 Tobias Sontheimer 1 Veit Preidel 1 Christiane Becker 1 Frank Schmidt 2 Bernd Rech 1
1Helmholtz-Zentrum Berlin famp;#252;r Materialien und Energie Berlin Germany2Zuse-Institute Berlin Berlin GermanyShow Abstract
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) . 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.  T. Sontheimer et al., phys. stat.sol. RRL 5, 376 (2011)
Kimberly A. Sablon, U. S. Army Research Laboratory
Lan Fu, "Australian National University Research School of Physics and Engineering"
Zhiming Wang, University of Electronic Science and Technology of China
Sudersena Rao Tatavarti, "MicroLink Devices, Inc."
Symposium Support Army Research Laboratory
Magnolia Solar, Inc.
U.S. Naval Research Laboratory
E8: Next Generation Solar Cells II
Tuesday PM, November 27, 2012
Hynes, Level 3, Ballroom A
2:30 AM - E8.01
Modeling Low-cost Hybrid Tandem Photovoltaics with Power Conversion Efficiencies Exceeding 20%
Zach M Beiley 1 Andrea Bowring 1 Michael D McGehee 1
1Stanford University Stanford USAShow Abstract
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.
2:45 AM - E8.02
Computational Study of Hybrid Organic/Inorganic Solar Cells
Ted Yu 1 Ramesh Babu Laghumavarapu 2 Diana Huffaker 2 Christian Ratsch 1
1University of California, Los Angeles Los Angeles USA2University of California, Los Angeles Los Angeles USAShow Abstract
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.
3:00 AM - *E8.03
Nanomaterials with Charged Quantum Dots for Conversion and Sensing Applications
Vladimir Mitin 1 Kimberly Sablon 2 John Little 2 Andrei Sergeev 3 Nizami Vagidov 3 4
1University at Buffalo Buffalo USA2U.S. Army Research Laboratory Adelphi USA3University at Buffalo Buffalo USA4Optoelectronic Nanodevices LLC Amherst USAShow Abstract
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.
3:30 AM - E8.04
Modelling of Vertical and In-plane Quantum Dots Arrays for High Efficiency Solar Cells
Stanko Tomic 1 Tomah Sogabe 2 Yoshitaka Okada 2
1University of Salford Manchester United Kingdom2University of Tokyo Tokyo JapanShow Abstract
Vertically  or in-plane  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) . 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 . 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 . We have shown that with appropriate band structure engineering, it is possible to place the intraband Auger electron cooling decay in the ns range . 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 , 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.  A.Luque, A.Marti, Phys.Rev.Lett. 78, 5014 (1997)  Y.Shoji, K.Narahara, H.Tanaka, T.Kita, K. Akimoto and Y. Okada, J.Appl.Phys. 111, 074305 (2012)  S.Tomic, T.S. Jones and N.M. Harrison, Appl.Phys.Lett. 93, 263105 (2008)  S.Tomic, Phys.Rev.B 82, 195321 (2010)  S.Tomic, in Next Generation of Photovoltaics (Springer, Heidelberg 2012)  S.Tomic, A.Luque, A.Marti and L.Antolin, Appl.Phys.Lett. 99, 053504 (2011)  Y. Shoji, K. Akimoto and Y. Okada, 38th IEEE Photovoltaic Specialists Conf. (Texas, June 2012) 176 S.Tomic,T.Sogabe,Y.Okada, in preparation
3:45 AM - E8.05
Quantum-kinetic Theory of Defect-mediated Recombination in Nanostructure-based Photovoltaic Devices
Urs Aeberhard 1
1Forschungszentrum Juelich Juelich GermanyShow Abstract
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.  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)  M. A. Green, J. Mater. Sci. Eng. B, 74, 118 (2000)  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)  U. Aeberhard and R.H. Morf, Phys. Rev. B, 77, 125343 (2008)  U. Aeberhard, Phys. Rev. B, 84, 035454 (2011)  U. Aeberhard, J. Comput. Electron., 10, 394 (2011)
E9: Photovoltaic Technologies II
Tuesday PM, November 27, 2012
Hynes, Level 3, Ballroom A
4:30 AM - *E9.01
Ceramic IR Emitter with Spectral Match to GaSb PV Cells for TPV
Lewis Fraas 1 Kuanrong Qui 2
1JX Crystals Inc Issaquah USA2CANMET Energy Technology Centre-Ottawa, Natural Resources Canada Ottawa CanadaShow Abstract
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.
5:00 AM - E9.02
Nanostructured Transparent Conductive Oxides for Photovoltaic Applications
Roger E Welser 1 2 Adam W Sood 1 2 3 Jaehee Cho 3 E. Fred Schubert 3 Jennifer L Harvey 4 Nibir K Dhar 5 Ashok K Sood 1 2
1Magnolia Solar, Inc. Albany USA2Magnolia Optical Technologies, Inc. Woburn USA3Rensselaer Polytechnic Institute Troy USA4NYSERDA Albany USA5DARPA/MTO Arlington USAShow Abstract
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.
5:15 AM - E9.03
Cost of Ownership Calculations for CIGS and OPV
Niels Van Loon 2 Jan Gilot 1 2 Ionut Barbu 2 Ronn Andriessen 1 Ando Kuypers 2
1Holst Centre Eindhoven Netherlands2TNO Eindhoven NetherlandsShow Abstract
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.
5:30 AM - E9.04
Silver Nano-network Embedded Conductive Black Silicon Surface
Tianyi Sun 1 Chuanfei Guo 1 Yang Wang 2 Krzysztof Kempa 1 Zhifeng Ren 1
1Boston College Chestnut Hill USA2South China Normal University Guangzhou ChinaShow Abstract
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/SiOx 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/ SiOx 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 SiOx in the gaps. The pre-blended solution of HF and hydrogen peroxide is then introduced as the last step to remove the In2O3/SiOx 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.
5:45 AM - E9.05
Large-area Multifunctional Nanoporous Coatings for Photovoltaics
Adem Yildirim 1 2 Mohammad Ghaffari 1 Tural Khudiyev 1 2 Ali Kemal Okyay 1 2 3 Mehmet Bayindir 1 2 4
1Bilkent University Ankara Turkey2Bilkent University Ankara Turkey3Bilkent University Ankara Turkey4Bilkent University Ankara TurkeyShow Abstract
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.
E6: High Efficiency Solar Cells I
Tuesday AM, November 27, 2012
Hynes, Level 3, Ballroom A
9:30 AM - E6.01
InGaP and GaAsP Solar Cells for Multi-junction Applications
Stephanie Tomasulo 1 Kevin Nay Yaung 1 Minjoo Larry Lee 1
1Yale University New Haven USAShow Abstract
Triple-junction solar cells have recently achieved efficiencies of 43.5% by combining materials with different bandgaps (Eg) 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 Eg=2.0-2.2 eV for the top junction. InyGa1-yP (y=0.27-0.40) possesses a direct-gap in the appropriate Eg 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 In0.36Ga0.64P solar cells on GaAs, but wide-Eg InGaP solar cells on GaP have yet to be achieved. Growth of wide-Eg 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-Eg 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 In0.31Ga0.69P 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 GaAs0.66P0.34 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 (Voc) compared with the cell on GaAs, yielding 1.24 and 1.28 V respectively. We expect the lower Voc 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-Eg InGaP on these templates, and progress toward InGaP solar cells with Egge;2.07 eV on GaP will be presented.
9:45 AM - E6.02
Carrier Dynamics and Defects in Bulk 1eV InGaAsNSb Materials and InGaAs Layers with MBL Grown by MOVPE for Multi-junction Solar Cells
Yongkun Sin 1 Stephen D LaLumondiere 1 Brendan Foran 1 William T Lotshaw 1 Steven C Moss 1 Tae Wan Kim 2 Steven Ruder 3 Luke J Mawst 2 Thomas F Kuech 3
1The Aerospace Corporation El Segundo USA2University of Wisconsin - Madison Madison USA3University of Wisconsin - Madison Madison USAShow Abstract
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  and the solar cells using the second approach yielded an efficiency of 41.1% . 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.  M. Wiemer, V. Sabnis, and H. Yuen, Proceedings of SPIE 8108, 810804-1 (2011).  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).
10:00 AM - *E6.03
High Efficiency Multi-junction Solar Cells: Past, Present, and Future
Paul Robert Sharps 1 Dan Aiken 1 Andreea Boca 1 Ben Cho 1 Dan Chumney 1 Art Cornfeld 1 Sang-Soo Je 1 Yong Lin 1 James McCarty 1 Fred Newman 1 Pravin Patel 1 John Spann 1 Mark Stan 1 Jeff Steinfeldt 1
1Emcore Corporation Albuquerque USAShow Abstract
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).
10:30 AM - E6.04
Towards an All Lattice-matched 3-junction Solar Cell with Efficiency >50%
Marina S. Leite 1 2 3 Robyn L. Woo 4 Jeremy N. Munday 1 5 William D. Hong 4 Shoghig Mesropian 4 Daniel C. Law 4 Harry A. Atwater 1
1CALTECH Pasadena USA2NIST Gaithersburg USA3University of Maryland College Park USA4Boeing-Spectrolab Inc. Sylmar USA5University of Maryland College Park USAShow Abstract
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) In0.37Al0.63As / (1.39 eV) In0.38Ga0.62As0.57P0.43 / (0.94 eV) In0.38Ga0.62As 3-junction solar cell can achieve theoretical efficiencies >51% under 100-suns illumination (with Voc = 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.
10:45 AM - E6.05
GaAsP Solar Cells on SiGe/Si Virtual Substrates for Dual-junction Applications
Joseph Faucher 1 Stephanie Tomasulo 1 Andrew Gerger 2 Anthony Lochtefeld 2 Chris Ebert 3 Minjoo Larry Lee 1
1Yale University New Haven USA2Amberwave, Inc. Salem USA3Veeco Instruments Inc. Somerset USAShow Abstract
Theoretical calculations show that dual-junction solar cells with top and bottom bandgaps (Eg) 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 GaAs0.86P0.14 cells grown on Si0.14Ge0.86/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 Si1-xGex 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 In0.42Ga0.58P nucleation layer, which also serves as a back surface field (BSF). Finally, the lattice-matched GaAs0.86P0.14 device layer (Eg=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 In0.42Ga0.58P 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 GaAs0.86P0.14 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 (VOC) 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-VOC GaAs0.86P0.14 cells on Si0.14Ge0.86/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 GaAs0.86P0.14 top cell with a lower Si0.14Ge0.86 bottom cell. D.J. Friedman. CURR OPIN SOLID STATE MATER SCI, 14, 2010, 131-138
E7: Thin Film Solar Cells II
Tuesday AM, November 27, 2012
Hynes, Level 3, Ballroom A
11:30 AM - *E7.01
Overview of Thin-film CdTe Solar Cells Technology and Recent Advances
Ramesh Dhere 1 Joel Duenow 1 Clay Dehart 1 Jian Li 1 Darius Kuciauskas 1 Timothy Gessert 1
1National Renewable Energy Laboratory Golden USAShow Abstract
CdTe thin-film solar cells have made significant progress in the past 40 years. After some early work on CdTe/CuxTe 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 (Voc) and fill factor (FF). We will describe our recent work on substrate-structure CdTe devices that has led to Voc 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 Voc analysis shows a significant effect of O2 during processing on the dominant recombination mechanism. Device performance improves from under 3% to over 8%, with Voc 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.
12:00 PM - E7.02
Temperature Dependence of Electrical Characterization in n+ - CdS/ p - CdTe Thin Film Solar Cells - Study of Deep Defects
Poonam Rani Kharangarh 1 George E Georgiou 1 Ken K Chin 1
1NJIT Newark USAShow Abstract
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.
12:15 PM - E7.03
Synthesis and Characterization of ZnSnN2 - A New Semiconductor with a Direct Band Gap of 1.7 eV
Paul C Quayle 1 Keliang He 1 Jie Shan 1 Kathleen Kash 1
1Case Western Reserve University Cleveland USAShow Abstract
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 d