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 Nanophoto