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Status of concentrator collector and high-efficiency concentrator cell development

Description: Photovoltaic concentrator collectors are an attractive option for utility-scale photovoltaic power plants. This paper reviews the current status of photovoltaic concentrator collector and cell development. Included in the review is a discussion of the economic motivation for concentrators, a summary of recent concentrator collector and cell development, and a description of a major new program to accelerate development and commercial introduction of concentrator collectors. 21 refs., 1 fig., 3 tabs.
Date: January 1, 1990
Creator: Gee, J.M.
Partner: UNT Libraries Government Documents Department

High-efficiency one-sun photovoltaic module demonstration using solar-grade CZ silicon. Final report

Description: This work was performed jointly by Sandia National Laboratories (Albuquerque, NM) and Siemens Solar Industries (Camarillo, CA) under a Cooperative Research and Development Agreement (CRADA 1248). The work covers the period May 1994 to March 1996. The purpose of the work was to explore the performance potential of commercial, photovoltaic-grade Czochralski (Cz) silicon, and to demonstrate this potential through fabrication of high-efficiency cells and a module. Fabrication of the module was omitted in order to pursue further development of advanced device structures. The work included investigation of response of the material to various fabrication processes, development of advanced cell structures using the commercial material, and investigation of the stability of Cz silicon solar cells. Some important achievements of this work include the following: post-diffusion oxidations were found to be a possible source of material contamination; bulk lifetimes around 75 pts were achieved; efficiencies of 17.6% and 15.7% were achieved for large-area cells using advanced cell structures (back-surface fields and emitter wrap-through); and preliminary investigations into photodegradation in Cz silicon solar cells found that oxygen thermal donors might be involved. Efficiencies around 20% should be possible with commercial, photovoltaic-grade silicon using properly optimized processes and device structures.
Date: October 1, 1996
Creator: Gee, J. M.
Partner: UNT Libraries Government Documents Department

Phosphorus diffusions for gettering-induced improvement of lifetime in various silicon materials

Description: Solar-grade silicon frequently contains large quantities of defects and impurities that can significantly degrade the excess-carrier lifetime through introduction of recombination sites. The impurities frequently include metals as well as high concentrations of high carbon and/or oxygen. Defects and impurities can also degrade the electrical properties of solar cells fabricated in solar-grade silicon by causing shunt currents or excess junction current. Fabrication of acceptable solar cells from such materials requires processes that are tolerant of, or that can even improve impure and defective material. Phosphorus diffusion is a well-known technique for gettering of impurities in silicon. The effect of phosphorus diffusion on the excess-carrier lifetime in various silicon materials was investigated. The optimum phosphorus diffusion schedule and enhancement of lifetime was found to be material specific, with substantial (5-fold) increases found for some materials. Possible reasons for the variability of phosphorus gettering with different materials is discussed. 11 refs., 6 figs., 3 tabs.
Date: January 1, 1991
Creator: Gee, J.M.
Partner: UNT Libraries Government Documents Department

Phosphorus and aluminum gettering - investigation of synergistic effects in single-crystal and multicrystalline silicon

Description: Synergistic effects from simultaneous phosphorus-diffusion/aluminium alloy gettering are investigated in three different crystalline- silicon substrates. The silicon materials, experimental design, characterization, and analysis are presented. Some evidence for synergism is observed in the finished cells on all three substrates types. These results are combined with complementary observations of the effects of oxidation on bulk properties of previously gettered substrates to suggest a high volume, low cost, process implementation which could give up to 9% relative increase in efficiency.
Date: June 1, 1996
Creator: Schubert, W.K. & Gee, J.M.
Partner: UNT Libraries Government Documents Department

A theoretical investigation of effective surface recombination velocity in AlGaAs/GaAs heteroface solar cells

Description: An AlGaAs window layer is used in high-efficiency GaAs solar cells to reduce carrier recombination at the front surface. Free surfaces of III-V semiconductors have a high density of surface states that serve as recombination sites and create a depletion region at the front surface. We have performed a theoretical investigation of front-surface recombination that includes the effect of a surface space-charge layer. It was found that the surface space-charge layer can have a profound effect on front-surface recombination for thin or lightly doped window layers. 15 refs., 5 figs., 1 tab.
Date: January 1, 1990
Creator: Gee, J.M. & Drummond, T.J.
Partner: UNT Libraries Government Documents Department

Crystalline-silicon photovoltaics: Necessary and sufficient

Description: Photovoltaic (PV) energy systems have always been dominated by crystalline-silicon (c-Si) technology, and recent developments persuasively suggest that c-Si will continue to be the dominant technology well into the next century. The authors explain why c-Si technology is fairing much better than previously expected, and discuss the impact of improvements currently under development. They use a ground-up, engineering-based approach to predict the expected evolution of this type of PV system, and argue that c-Si PV will be in a position to compete for the US residential power market starting in about the year 2010. This market alone will provide the opportunity for PV to supply several percent of the electrical energy used in the United States. Crystalline-silicon technology is therefore not just necessary for building a near-term PV industry; it also offers a low-risk approach to meeting long-term goals for PV energy systems.
Date: January 1, 1995
Creator: Basore, P. A. & Gee, J. M.
Partner: UNT Libraries Government Documents Department

Effect of oxidations on phosphorus-diffused crystalline-silicon substrates

Description: Phosphorus diffusions are used in the fabrication process for nearly all crystalline-silicon (c-Si) photovoltaic solar cells to form the emitter of the solar cell. These phosphorous diffusions are also well known to have beneficial gettering benefits, i.e., deleterious metallic impurities are gettered from the bulk of the c-Si substrate into the phosphorous doped layer. In this study, we examined the effect of oxidations performed after the phosphorus diffusion. We were particularly interested in using the oxidation to passivate the surface of the phosphorus diffusion. Post-diffusion oxidations or moderate temperature steps in oxidizing ambients are also commonly found in commercial fabrication sequences of c-Si solar cells. we found that the bulk lifetime was degraded in Czochralski (Cz) silicon due to the post-diffusion oxidation unless there was a gettering agent present during the oxidation. Possible explanations for these results are presented at the end of the paper.
Date: September 1, 1996
Creator: Gee, J.M.; King, R.R.; Reiss, J.H. & Mitchell, K.W.
Partner: UNT Libraries Government Documents Department

The crystalline-silicon photovoltaic R&D project at NREL and SNL

Description: This paper summarizes the U.S. Department of Energy R&D program in crystalline-silicon photovoltaic technology, which is jointly managed by Sandia National Laboratories and National Renewable Energy Laboratory. This program features a balance of basic an d applied R&D, and of university, industry, and national laboratory R&D. The goal of the crystalline-silicon R&D program is to accelerate the commercial growth of crystalline-silicon photovoltaic technology, and four strategic objectives were identified to address this program goal. Technical progress towards meeting these objectives is reviewed.
Date: December 31, 1996
Creator: Gee, J.M. & Ciszek, T.F.
Partner: UNT Libraries Government Documents Department

Advanced Silicon Space Solar Cells Using Nanotechnology

Description: Application of nanotechnology and advanced optical structures offer new possibilities for improved radiation tolerance in silicon solar cells. We describe the application of subwavelength diffractive structures to enhance optical absorption near the surface, and thereby improve the radiation tolerance.
Date: March 31, 1999
Creator: Gee, J.M.; Ruby, D.S. & Zaidi, S.H.
Partner: UNT Libraries Government Documents Department

The effect of gettering on areal inhomogeneities in large-area multicrystalline-silicon solar cells

Description: Multicrystalline-silicon (mc-Si) materials and cells feature large areal variations in material and junction quality. The regions with poor device quality have been predicted to have more recombination current at forward bias than a simple area-weighted average due to the parallel interconnection of the good and bad regions by the front junction. The authors have examined the effect of gettering on areal inhomogeneities in large-area mc-Si cells. Cells with large areal inhomogeneities were found to have increased non-ideal recombination current, which is in line with theoretical predictions. Phosphorus-diffusion and aluminum-alloy gettering of mc-Si was found to reduce the areal inhomogeneities and improve large-area mc-Si device performance.
Date: October 1, 1997
Creator: Gee, J.M. & Sopori, B.L.
Partner: UNT Libraries Government Documents Department

Optimization of textured-dielectric coatings for crystalline-silicon solar cells

Description: The authors report on the optimization of textured-dielectric coatings for reflectance control in crystalline-silicon (c-Si) photovoltaic modules. Textured-dielectric coatings reduce encapsulated-cell reflectance by promoting optical confinement in the module encapsulation; i.e., the textured-dielectric coating randomizes the direction of rays reflected from the dielectric and from the c-Si cell so that many of these reflected rays experience total internal reflection at the glass-air interface. Some important results of this work include the following: the authors demonstrated textured-dielectric coatings (ZnO) deposited by a high-throughput low-cost deposition process; they identified factors important for achieving necessary texture dimensions; they achieved solar-weighted extrinsic reflectances as low as 6% for encapsulated c-Si wafers with optimized textured-ZnO coatings; and they demonstrated improvements in encapsulated cell performance of up to 0.5% absolute compared to encapsulated planar cells with single-layer antireflection coatings.
Date: July 1, 1996
Creator: Gee, J.M.; Gordon, R. & Liang, H.
Partner: UNT Libraries Government Documents Department

Back surface cell structures for reducing recombination in CZ silicon solar cells

Description: Mass-produced terrestrial CZ silicon solar cells are currently entering the domain in which bulk diffusion length is comparable to the cell thickness, so that recombination at the back surface can have a significant effect on device performance. Three manufacturable processes that address the problem of back recombination are examined here: boron diffusion from a deposited doped SiO{sub 2}, layer; Al-alloyed layers using screen-printed paste; and use of a collecting n* layer on the back interdigitated with the positive electrode. 104-cm{sup 2} cells fabricated at Siemens Solar Industries using these back surface structures are characterized by current-voltage, spectral response, photoconductivity decay, and SIMS measurements.
Date: December 31, 1994
Creator: King, R. R.; Mitchell, K. W. & Gee, J. M.
Partner: UNT Libraries Government Documents Department

Back-Contact Crystalline-Silicon Solar Cells and Modules

Description: This paper summarizes recent progress in the development of back-contact crystalline-silicon (c-Si) solar cells and modules at Sandia National Laboratories. Back-contact cells have potentially improved efficiencies through the elimination of grid obscuration and allow for significant simplifications in the module assembly process. Optimization of the process sequence has improved the efficiency of our back-contact cell (emitter wrap through) from around 12% to near 17% in the past 12 months. In addition, recent theoretical work has elucidated the device physics of emitter wrap-through cells. Finally, improvements in the assembly processing back-contact cells are described.
Date: March 10, 1999
Creator: Bode, M.D.; Garrett, S.E.; Gee, J.M.; Jimeno, J.C. & Smith, D.D.
Partner: UNT Libraries Government Documents Department

High-efficiency cell structures and processes applied to photovoltaic-grade Czochralski silicon

Description: The authors performed a detailed study to examine the limiting performance available using photovoltaic-grade Cz silicon. Photovoltaic-grade silicon refers to silicon produced by the photovoltaic industry, which may differ from the silicon used in the semiconductor device industry in impurity and defect concentrations.The study included optimization of fabrication processes, development of advanced device structures, and detailed model calculations to project future performance improvements. Process and device optimization resulted in demonstration of 75-{micro}s bulk lifetimes and 17.6%-efficient large-area cells using photovoltaic-grade Cz silicon. Detailed calculations based on the material and device evaluation of the present work project efficiencies of 20% for photovoltaic-grade Cz silicon with properly optimized processing and device structures.
Date: December 1, 1996
Creator: Gee, J.M.; King, R.R. & Mitchell, K.W.
Partner: UNT Libraries Government Documents Department

Boron-doped back-surface fields using an aluminum-alloy process

Description: Boron-doped back-surface fields (BSF`s) have potentially superior performance compared to aluminum-doped BSF`s due to the higher solid solubility of boron compared to aluminum. However, conventional boron diffusions require a long, high temperature step that is both costly and incompatible with many photovoltaic-grade crystalline-silicon materials. We examined a process that uses a relatively low-temperature aluminum-alloy process to obtain a boron-doped BSF by doping the aluminum with boron. In agreement with theoretical expectations, we found that thicker aluminum layers and higher boron doping levels improved the performance of aluminum-alloyed BSF`s.
Date: October 1, 1997
Creator: Gee, J.M.; Bode, M.D. & Silva, B.L.
Partner: UNT Libraries Government Documents Department

Simplified module assembly using back-contact crystalline-silicon solar cells

Description: The authors are developing new module concepts that encapsulate and electrically connect all the crystalline-silicon (c-Si) photovoltaic (PV) cells in a module in a single step. The new assembly process (1) uses back-contact c-Si cells, (2) uses a module backplane that has both the electrical circuit, encapsulant, and backsheet in a single piece, and (3) uses a single-step process for assembly of these components into a module. This new process reduces module assembly cost by using planar processes that are easy to automate, by reducing the number of steps, and by eliminating low-throughput (e.g., individual cell tabbing, cell stringing, etc.) steps. The authors refer to this process as monolithic module assembly since it translates many of the advantages of monolithic module construction of thin-film PV modules to wafered c-Si PV modules. Preliminary development of the new module assembly process, and some estimations of the cost potential of the new process, are presented.
Date: November 1, 1997
Creator: Gee, J.M.; Garrett, S.E. & Morgan, W.P.
Partner: UNT Libraries Government Documents Department

The effect of encapsulation on the reflectance of photovoltaic modules using textured multicrystalline-silicon solar cells

Description: Texturing multicrystalline-silicon cells is a promising technique for reducing reflectance losses. We investigated two methods for texturing multicrystalline-silicon solar cells - anisotropic chemical etch and mechanical dicing saw. Our work emphasized reducing reflectance in the encapsulated module by using optical confinement in the module. We found that optical confinement in the module is very important in the optimization of texture geometries.
Date: January 1, 1995
Creator: Gee, J. M.; Schubert, W. K.; Tardy, H. L.; Hund, T. D. & Robison, G.
Partner: UNT Libraries Government Documents Department

Reflectance control for multicrystalline-silicon photovoltaic modules using textured-dielectric coatings

Description: The authors describe a new approach for controlling the reflectance of photovoltaic modules with planar-surface solar cells. The new approach uses an optically thick, dielectric coating with a large refractive index and a textured surface; this dielectric coating is deposited on the planar-surface solar cell. The textured-dielectric coating works optically with the module encapsulation to promote optical confinement of rays inside the module encapsulation structure, which reduces the net reflectance of the photovoltaic module. The advantage of this approach is that deposition of a textured-dielectric film may be less costly and less intrusive on the cell manufacturing process than texturing multicrystalline-silicon substrates. The authors present detailed optical models and experimental confirmation of the new approach.
Date: January 1, 1995
Creator: Gee, J. M.; Tardy, H. L.; Hund, T. D.; Gordon, R. & Liang, H.
Partner: UNT Libraries Government Documents Department

InGaAsN Solar Cells with 1.0eV Bandgap, Lattice Matched to GaAs

Description: The design, growth by metal-organic chemical vapor deposition, and processing of an In{sub 0.07}Ga{sub 0.93}As{sub 0.98}N{sub 0.02} solar Al, with 1.0 ev bandgap, lattice matched to GaAs is described. The hole diffusion length in annealed, n-type InGaAsN is 0.6-0.8 pm, and solar cell internal quantum efficiencies > 70% arc obwined. Optical studies indicate that defects or impurities, from InGAsN doping and nitrogen incorporation, limit solar cell performance.
Date: November 24, 1998
Creator: Allerman, A.A.; Banas, J.J.; Gee, J.M.; Hammons, B.E.; Jones, E.D. & Kurtz, S.R.
Partner: UNT Libraries Government Documents Department

The effect of oxidations on phosphorus-diffused crystalline-silicon substrates

Description: The authors examined the effect of oxidation on phosphorus-diffused crystalline-silicon p-type substrates. Oxidations subsequent to the phosphorus diffusion are of interest for passivating surfaces, and are commonly found in both high-efficiency laboratory-cell and commercial-cell fabrication sequences. The authors found a degradation of the bulk lifetime due to the oxidation in a variety of crystalline-silicon substrates that were diffused in various laboratories. The degradation was avoided if there was aluminum present on the back surface of the wafer during the oxidation. The study suggests that impurities gettered during the phosphorus diffusion can be released back into the bulk during a subsequent oxidation, and that the aluminum suppressed the bulk lifetime degradation by reabsorbing these released impurities.
Date: August 1, 1997
Creator: Gee, J.M.; King, R.R.; Reiss, J.H.; Mitchell, K.W. & Narayanan, S.
Partner: UNT Libraries Government Documents Department

Novel InGaAsN pn Junction for High-Efficiency Multiple-Junction Solar Cells

Description: We report the application of a novel material, InGaAsN, with bandgap energy of 1.05 eV as a junction in an InGaP/GaAs/InGaAsN/Ge 4-junction design. Results of the growth and structural, optical, and electrical properties were demonstrated, showing the promising perspective of this material for ultra high efficiency solar cells. Photovoltaic properties of an as-grown pn diode structure and improvement through post growth annealing were also discussed.
Date: March 26, 1999
Creator: Allerman, A.A.; Chang, P.C.; Gee, J.M.; Hammons, B.E.; Hou, H.Q.; Jones, E.D. et al.
Partner: UNT Libraries Government Documents Department

Processing experiments for development of high-efficiency silicon solar cells

Description: Fabrication of high-efficiency silicon solar cells requires processing technology capable of maintaining long bulk carrier lifetime and low surface recombination. Development of long-lifetime processing techniques using experimental designs based on statistical methods is described. The first three experiments investigated pre-oxidation cleans, phosphorus gettering, and a comparison of different phosphorus diffusion sources. Optimal processing parameters were found to depend on type of silicon material. 2 refs., 2 figs., 2 tabs.
Date: January 1, 1990
Creator: Gee, J.M.; Basore, P.A.; King, D.L.; McBrayer, J.D.; Ruby, D.S.; Buck, M.E. et al.
Partner: UNT Libraries Government Documents Department

Process development for high-efficiency silicon solar cells

Description: Fabrication of high-efficiency silicon solar cells in an industrial environment requires a different optimization than in a laboratory environment. Strategies are presented for process development of high-efficiency silicon solar cells, with a goal of simplifying technology transfer into an industrial setting. The strategies emphasize the use of statistical experimental design for process optimization, and the use of baseline processes and cells for process monitoring and quality control. 8 refs.
Date: December 31, 1991
Creator: Gee, J. M.; Basore, P. A.; Buck, M. E.; Ruby, D. S.; Schubert, W. K.; Silva, B. L. et al.
Partner: UNT Libraries Government Documents Department

A simple single-photomask process for fabrication of high-efficiency multicrystalline silicon solar cells

Description: We have developed a simplified process sequence for fabrication of high-efficiency multicrystalline silicon (mc-Si) solar cells. Photolithography is required only to define the evaporated metal gridlines. We use this fast turn-around, high-yield baseline process to evaluate different mc-Si materials and new processing procedures. The process uses a one-step emitter diffusion/drive-in and an aluminum-alloyed back surface field to provide a well-passivated cell with excellent blue and red response. Laser-scribed cell-isolation grooves are used to define both moderate-area (4 cm{sup 2}) and large-area (42 cm{sup 2}) cells. We have been able to achieve minority-carrier diffusion lengths well over 300 {mu}m in 2-{Omega}cm mc-Si material and have achieved efficiencies of 16.2% in 4-cm{sup 2} cells. This was accomplished without hydrogenation to passivate bulk defects or texturing to reduce reflectance. By using the same fabrication process for both small-area and large-area cells, we intend to determine and minimize the effect of areal inhomogeneities on large-area mc-Si cell performance.
Date: July 1, 1994
Creator: Schubert, W. K.; Ruby, D. S.; Basore, P. A.; Gee, J. M.; Buck, M. E. & Tardy, H. L.
Partner: UNT Libraries Government Documents Department