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Advances and Opportunities in the Amorphous Silicon Research Field SERl/TP-211-2339 UC Category: 63 DE84004526 Advances and Opportunities in the Amorphous Silicon Research Field E. Sabisky W. Wallace A. Mikhail H. Mahan S. Tsuo May 1984 Presented at the 17th IEEE PV Specialists Conference Orlando, Florida 1-5 May 1984 Prepared under Task No. 3432.10 FTP No. 462 Solar Energy Research Institute. A Division of Midwest Research Institute 1617 Cole Boulevard Golden, Colorado 80401 Prepared for the U.S. Department of Energy Contract No. DE-AC02-83CH10093 Printed in the United States of America Available from: National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Price: Microfiche A01 Printed Copy A02 NOTICE This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. SERI/TP-211-2339 ADVANCES AND OPPORTUN1'11ES IN THE AMORPHOUS SILICON RESEARCH FIELD E. Sabisky, W. Wallace, A. Mikhail, H. Mahan, s. Tsuo Solar Energy Research Institute 1617 Cole Boulevard Golden, Colorado 80401 ABSTRACT In July 1982, not too many years after the first PY cell was made, RCA announced the first demonstration of The amorphous materials and thin-film solar cells an amorphous silicon solar cell having a conversion effi­ program was initiated by the U.S. Department. of Energy ciency of 10% (1), the efficiency figure established by gov­ in 1978 and then transferred to the Solar Energy Research ernment and industry experts as a crucial goal if thin-film Institute. The aim of the present DOE/SERI program is to solar cells were to be viable for large-scale electric power achieve 5-year DOE research goals by addressing photo­ generation applications. Of the seven research teams that voltaic research in single-junction amorphous thin films as have reported conversion efficiencies of at least 10%, well as the most promising option in high-efficiency, mul­ Komatsu (4) of Japan has announced the highest to date: tijunction solar cells. Multiyear subcontract .awards ini­ 10.7%. tiated in 1983 were designed to demonstrate a stable, small-area, p-i-n solar cell of at least 12% (AMl) effi­ Based on continuing improvements in photovoltaic ciency, a stable sub�odule of at least 8% (AMl) efficiency conversion efficiency and scientific understanding during (total area, 1000 cm ), and a proof-of-concept multijunc­ the past decade, the U.S. Department of Energy (DOE) tion amorphous silicon alloy thin-film solar cell that will considers amorphous silicon cell technology to be a strong lead to achieving an 18% efficiency goal in 1988. contender for photovoltaic power systems that will be capable of competing economically with conventionally produced electric power in the 1990s and beyond. To meet INTRODUCl'ION the technological requirements for PY electric power applications, DOE, through the Solar Energy Research Progress in the research, development, and commer­ Institute (SERI), developed a multiyear program plan cialization of amorphous silicon for solar cells has sur­ based on government/industry partnerships. These prised even its strongest proponents. Since 1974, when government/industry cost-shared programs are supported the first amorphous silicon solar cell was made, the 10% by a core government-sponsored research program that conversion efficiency milestone has been reached by one consists of researchers at universities, at DOE Research team (1) in 1982; now, seven research teams have reported Centers (SERI, JPL), at other government laboratories, conversion efficiencies of 10% and higher. Solar batteries and also by scientists in the private sector. Objectives of using amorphous silicon solar cells for consumer products the plan are to expand the state of the art in single­ were first introduced in 1980 and are now being produced junction amorphous silicon solar cells to achieve higher at a rate of 7 million units per month. The consumer conversion efficiencies over larger areas and to expand market accounted for about 3 MWp of amorphous silicon the data base on amorphous silicon alloy materials for use photovoltaic (PY) devices, 15% of all PY cells marketed in in potentially higher efficiency, multijunction solar cells. 1983 (2). The plan represents a balanced, phased approach by addressing PY research in single-junction amorphous thin Research into the properties of amorphous silicon films as well as the most promising options in high­ began in laboratories in the United States and abroad from efficiency, multijunction solar cells. Although the degree 196 8 to 1974. Scientists demonstrated experimentally of uncertainty associated with options in these multi­ that, contrary to general opinion, amorphous materials, junction concepts remains high, the multiple approach of particularly hydrogenated amorp.'lous silicon, can be grown single-junction thin films and multijunction concepts that have the optoelectronic properties necessary for use makes the probability of total program success very high. in th in-film solar cells. The first hydrogenated amorphous silicon PY cell was produced in 1974 by RCA scientists, and was the subject of a 1977 U.S. Patent (3). In July 1976, RESEARCH PROGRAM the U.S. Government issued a subcontract to RCA to sup­ port the research and development of amorphous silicon The current Research and Development Subcon­ for applications in photovoltaics. On November 4, 1978, tracts Program inc ludes 16 subcontractors: seven with the U.S. Congress passed the Solar Photovoltaic Energy industry, seven with universities, and two govemment Research, Development, and Demonstration Act. This laboratories. Researchers at the DOE Research Centers action, as well as those of the Japanese Government, sig­ (SERI and JPL) are also involved in amorphous silicon nificantly influenced research and development in solar research. The program is divided into two primary and energy by providing substantial increases in funding. five supporting research activities. The two primary 1 SERI/TP-211-2339 research activities are single-junction solar cells and Where II Goes ••• multijunction, stacked solar cells. The supporting research activities encompass material deposition rate, alternative material deposition methods, light-induced effects, device testing and reliability, and theory. The two primary research activities involve strong Large government/industry partnerships using multidisciplinary business teams whose objective is to improve PY device efficien­ Small cies. The requirements for such multidisciplinary 36% business research are based on the fact that various parts of thin­ 42% film PY devices are strongly interdependent and cannot be easily isolated. Th is is a consequence of the thin-film nature of the technology whereby surfaces/interfaces play a major role in the properties of the device. Research in single-junction solar cells will lead to meeting the near­ term DOE goals on thin-film PY modules and will also provide the basis for the longer-term development of multijunction, stacked solar cells. Multijunction, stacked solar cells have the potential to meet higher efficiency goals. Secondary research support activities relate to specific tasks that support the two primary activities and, in addition, provide the mechanism to investigate newer options. Universities Figure l shows budget allocations for current sub­ contracted programs. The principal materials being examined are hydrogenated amorphous silicon, amorphous Wh•lll Does. .. silicon-carbon and silicon-germanium alloys, micro­ crystalline phosphorus-silicon-hydrogen, microcrystalline Support boron-silicon-hydrogen, and passivators for reducing activities defects. Device structure is limited to p-i-n; other structures are used for diagnostic purposes. Thin-film 23% preparation approaches are currently restricted to glow discharge (de and ac) deposition in the two primary research activities. Glow discharge deposition using Single higher order silane gas is also being investigated for high junction growth rates. Chemical vapor deposition (CYD) using 57% higher order silane gases for film preparation is the only other preparation method currently supported. The opto­ electronic properties of amorphous materials are being studied by all methods, with an emphasis on minority carrier diffusion length. EFFICIENCY STATUS The historical trend of the efficiency of p-i-n solar Figure 1. Budget Allocations for the Amorphous cells for small and large areas is shown in Figure 2. The Materials Subcontracted Program, Fiscal Year most rapid progress in cell e ficiency has occurred since � 1984(October1, 1983 - September 30,1984) 1978. For small-area (-1 cm ) solar cells, the cell effi­ ciency was 4% in 1978, reached 10.1% in July 1982, and is presently reported to be at 10.7%. These achievements years along with calculated efficiencies using these were for so-called p-i-n type solar cell structures with the parameters and a range of actual experimental efficiency amorphous material grown by glow discharge deposition. values achieved in that year are reported. These data For larger area solar cells, the ti-n cell efficiency was have been one of the better barometers for predicting 2% in 1979 for an area of 5� cm , and it is now over 5% future trends. The calculated efficiency obtained in this for a total area of 350 cm • U.S. companies not under manner was 6.7% in 1979 and is now over 14%-a doubling government contract have report�d conversion efficien­ in efficiency. This can be compared with actual experi­ cies over 8% for an area of 100 cm�. mental values of 4.5% in 1979 and 10.7% in 1984-again, a doubling in efficiency. The s ort-circuit current density Table l gives performance data on the best reported � (Jsc> remained near 14 mA/cm for 1979, 1980, and.1981. single-junction p-i-n am�rphous silicon solar cells having an area of at least l cm .
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