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The development of solar photovoltaic energy in Brazil
Solar Cells, 26 ( 1 9 8 9 ) 13 - 23
13
THE DEVELOPMENT OF SOLAR PHOTOVOLTAIC ENERGY IN BRAZIL N E E L K A N T H G. D H E R E *
Solar Energy Research Institute, 161 7 Cole Boulevard, Golden, CO 80401 (U.S.A.)
Summary Several groups in Brazil have been engaged in research and development of solar cells and solar cell materials. One company manufactures singlecrystal silicon solar cells, modules and systems, starting with imported pure silicon. It grows single-crystal silicon ingots and carries out cutting, wafer polishing, diffusion, contacting and module fabrication. The equipment manufactured includes water pumps and refrigerators. Emphasis on postgraduate education and applied research since the early 1970s has resulted in a critical mass of trained manpower and modern well equipped laboratories. Funding of specific photovoltaic projects in the years 1979 - 1986 helped to crystallize the research efforts, and to create groups dedicated to solar cells and to give those groups valuable experience. The funding has, however, been erratic and has lacked continuity. A new programme, PRO-SOLAR, has been modelled on the lines of an earlier, highly successful ethanol programme (PRO-ALCOOL). PRO-SOLAR has been developed by the Brazilian Ministry of Mining and Energy and is intended to promote the development and utilization of solar energy. Since other ministries are also participating and there is a broad base of industries, universities, research centres, associations and funding agencies, it is anticipated that significant results will be produced. It is suggested that economic competitiveness should be the goal of these efforts and hence, together with the demonstration projects that are being envisaged, concentrated efforts should be made towards the reduction of cost of solar cells and modules. Research and development leading to economically promising amorphous and polycrystalline thin film solar cells and equipment will require the participation of major industries.
1. Introduction
Funding of specific solar photovoltaic projects was initiated in Brazil towards the end of the 1970s and lasted for five to seven years. Prior to the * I n s t i t u t o Militar de E n g e n h a r i a , R i o de J a n e i r o , Brazil. 0379-6787/89/$3.50
© Elsevier S e q u o i a / P r i n t e d in T h e N e t h e r l a n d s
14 general awareness of the impending energy crisis, several groups were working on semiconductor devices and materials for applications in microelectronics. The specific funding allowed some groups to concentrate on photovoltaic activity, train personnel and gain valuable experience. Manufacture of solar cells, modules, systems and appliances was also begun. After the termination of solar funding, these groups have been striving to maintain some activity in research and development. A few groups are concentrating largely on the study of materials that are useful for solar cells. The industrial activity has continued but is being maintained at a low key. The emphasis on industrialization since the second world war, and the growth in postgraduate education, research and development since 1970, have resulted in a strong and varied industrial base, a reasonably large number of trained personnel, and fairly well equipped research and development institutions in Brazil. Projections show that the existing electricity generation capacity in Brazil will be insufficient in the 1990s. Brazil has an enormous land mass and a tropical climate. Large geographic regions have not yet been electrified. Most of these have a high level of insolation t h r o u g h o u t the year, some having amongst the highest levels in the world. Mean daily insolation in the north and northeast of Brazil ranges from a minimum of 184 W m -2 (Z~ Doca, Maranh~o) to a maximum of 260 W m -2 (Petrolina, Pernambuco). This, together with experience in research, development and production of solar cells, could permit Brazil to plan to harness solar energy on a large scale in the 1990s, and might also help to correct socially lopsided development. Recently, the Ministry of Mining and Energy created a group for the development of the solar energy programme (PRO-SOLAR) [1]. It is expected that the programme will provide sustained funding at a reasonable level so that this goal can be achieved. This paper describes the research, development and industrialization that is occurring in Brazil with respect to solar photovoltaic energy, and presents information on the PRO-SOLAR programme.
2. Education, research and development Since the early 1970s, postgraduate education has been considerably expanded and greater prominence has been given to applied physical sciences. Earlier, Brazilian physicists concentrated on pure and mostly theoretical physics. The lack of practical training at undergraduate levels is felt, even today, by students who do not come from the industrialized regions. Substantial funding was provided to laboratories working in microelectronics and materials. Modern well equipped laboratories could, therefore, be established. This permitted the creation of a critical mass of trained personnel, and research activity in areas related to microelectronics. When it first became clear that there was an impending energy crisis, the government chose to believe that Brazil was an "island of prosperity" in a world that elsewhere was troubled by this crisis. Later, a well structured and
15 comprehensive programme (PRO-ALCOOL) for the development of hydrated ethanol to be used as a substitute for gasoline was launched. This programme has been very successful. Approximately 2.5 million cars run on ethanol while all other cars utilize a 2 0 % - 22% mixture of ethanol in gasoline. An ambitious nuclear programme was also started, although it later turned o u t to be economically disastrous. A modest solar photovoltaic programme was initiated in 1979 - 1980. It helped to crystallize the research work and create groups dedicated to the development of solar cells. The activities of the principal groups are described below.
2.1. Solar Cells Group, Microelectronics Laboratory, University of S~o Paulo This group began functioning in 1974, with a project that involved growing silicon single-crystal ingots by the Czochralski method. Research and development efforts aimed at finding cheaper techniques for the fabrication of single-crystal solar cells were begun in 1981. Methods for texturing silicon single-crystal surfaces and also for the deposition of transparent, conducting and antireflecting coatings of SnO2 were studied. The process of simultaneous deposition and diffusion of phosphorous to obtain a p - n junction, using a liquid source and a conventional diffusion furnace, was optimized. Processes of selective electroless nickel deposition, using photolithographic techniques, followed b y an S n - P b solder dip, were developed. This lowered the cost of contacting, since the use of vacuum and precious metals was avoided. Single-crystal silicon solar cells with photovoltaic conversion efficiencies of more than 12% (AM 1.5) were prepared. Single-crystal silicon cells fabricated at this laboratory are being utilized in the construction of an experimental space solar cell panel. Work on polycrystalline silicon solar cells was also initiated. Lately the group has concentrated its activities on the development of hydrogenated amorphous silicon (a-Si:H) solar cells [ 2 ]. 2.2. Solar Cells and Microelectronics Laboratory, Materials Science Department, Military Institute o f Engineering, Rio de Janeiro This group began work on semiconducting thin films and hybrid microelectronics in 1971. Preparation techniques, structure, morphology and semiconducting properties of thin films of CdTe, CdS, CdSe, InP and SnO2 were studied [ 3 - 6 ]. The group perfected methods of obtaining highly pure and reliable single-layer and multilayer coatings of contact materials, and methods of preparing resistive and dielectric coatings. A project for the development of CdS-Cu2S thin film solar cells b y a wet m e t h o d was initiated in 1981. An encapsulated 5 cm × 5 cm solar cell with efficiency greater than 5% and a 1 W panel was developed. The degradation of cell efficiency over a period of one year was studied [7]. The installation of an a-Si:H plasma deposition system has been a t t e m p t e d b u t so far has n o t been successful because of inadequate funding. Si-SnO2 solar cells have also been fabricated. After the initial evidence of excessive degradation of CdS-Cu2S solar cells,
16 the emphasis was shifted to CdS-CuInSe2 solar cells. Stoichiometric CulnSe2 thin films with chalcopyrite structure were prepared by co-evaporation of constituent elements, and the effect of small deviations from stoichiometry on the structure and semiconducting properties was studied [8]. CdSCuInSe2 solar cells with conversion efficiencies greater than 7% (AM 1.5) were prepared. This work is continuing. Plasma-assisted growth of CdTe, indium tin oxide thin films for solar cells is also being studied.
2.3. Photovoltaic Conversion Laboratory, Institute o f Physics, University o f Campinas This group began its activities in 1980. A few members of the group had previously worked on chemically deposited CdS thin films for solar cells. The group has worked on the preparation and semiconducting properties of plasma-deposited a-Si:H, a-SiN:H, a-SiC:H, a-GeN:H and a-GeSn:H thin films, and transparent and conducting thin films of SnO2 deposited by spray pyrolysis [9 - 11]. The effect of plasma conditions on doping and hydrogen incorporation was also studied. Solar cells with p - i - n junctions were fabricated and their photovoltaic properties were analyzed. Theoretical studies were carried o u t on the effect of different irradiation conditions on the efficiency of cascade cells [12]. A project on polycrystalline silicon solar cells was also undertaken. Since the termination of specific funding, the group has been working on the materials properties of elemental and alloy amorphous semiconductors.
2.4. Sensors and Materials Laboratory, Institute o f Space Research, S~o Josd dos Campos The Institute of Space Research began its solar cell project for the Brazilian satellite programme in the mid 1970s. The early emphasis was on the testing of space-worthy cells and modules. Later, projects were undertaken in collaboration with other laboratories for the fabrication of single-crystal silicon and GaAs solar cells. The laboratory has been analysing the flux m e t h o d for the study of charge transport in multilayer solar cells [ 13]. Generalized expressions have been derived for electric current density. These have provided some new contributions to open-circuit voltage. The effects of simultaneous radiation and thermal annealing on damage to solar cells in space has also been studied [14]. Two exponential electrotechnical models have been analyzed semi-empirically. An experimental space solar cell panel is being constructed using single-crystal solar cells fabricated at the Microelectronics Laboratory. It will be used in the first Brazilian satellite, to study the degradation of solar cells in space. Simulation and modelling of solar panels are being carried out so that the panels can be used to provide energy for the satellite.
17
2.5. Materials and Interfaces Laboratory, Department of Metallurgy and Materials Science, Coordination of Post-Graduate Programme in Engineering, Federal University of Rio de Janeiro This group began working in 1970, concentrating on the study of surfaces and interfaces. The first equipment for Auger electron spectroscopy was installed in 1975 and was utilized for the study of segregation in steels and interdiffusion in thin films [15]. A project on a-Si:H thin film solar cells was undertaken in 1982. The group has studied metal-a-Si:H Schottky barriers, interdiffusion and segregation in metallic thin films and at metalsemiconductor interfaces, hydrogen diffusion, antireflection and protective coatings, formation of silicides, failure of electronic devices, and electrical and optical properties of thin films. Schottky barrier and p - i - n junction a-Si:H solar cells have been prepared and characterized. An a-Si:H temperature sensor has also been developed. More recently, the group has been concentrating on the properties of a-Si:H thin films and related materials [16].
2.6. Other groups Four groups working on photoelectrochemical solar cells are located at the Institute of Physics at the University of Campinas, the Institute of Physics and Chemistry at the University of S~o Paulo at S~o Carlos, the University of Rio Grande de Sul in Porto Alegre, and the University of Cear~ in Fortaleza. A small b u t dedicated group is working on methods for reducing the fabrication costs of single-crystal silicon solar cells at the Department of Electrical Engineering at the University of Campinas. Purification of materials, especially to obtain solar and electronic grade silicon, has been investigated at the Faculty of Engineering of the University of Campinas, and the Chemistry Department of the Military Institute of Engineering in Rio de Janeiro. 3. Industrialization The industrial production of solar cells in Brazil began with the fabrication of solar cell modules and systems utilizing imported solar cells. The applications were for telecommunications networks and for remote installations. A Brazilian company, Heliodin~mica, later started the production of 100 mm diameter single-crystal silicon ingots and solar cells. The process, developed locally, consists of the growth of p-type silicon single-crystal ingots, crystal cutting and wafer polishing, p - n junction formation by diffusion of phosphorus using POC13, and formation of an n - p - p + structure with diffusion from an aluminum paste for generating the back surface field. Front contact fingers are obtained by photolithography of electroless nickel and P b - S n solder dip. Back contacts utilize either the electroless nickel or aluminum paste and P b - S n solder dip. Characterization of the solar cells is carried out by I - V measurements in the dark and under simulated sunlight, and b y measurements of spectral response, junction depth,
18 diffusion length and minority carrier lifetime. Solar panels are tested at different temperatures. The factory has the capacity to produce 600 kWp per year. Since the full capacity is not being utilized, the company sells silicon wafers locally and exports them. It also manufactures water pumps, refrigerators and solar cell systems. Heliodin~mica has undertaken several development projects funded by Brazilian agencies. The early projects consisted of training technical personnel, crystal growing and cutting, wafer polishing, chemical treatment, diffusion, metallization, characterization and testing of solar cells, soldering, encapsulation, mounting and testing solar cell modules. Later projects have involved specular finish polishing of silicon wafers, clearing and packing to obtain wafers readily usable in the fabrication of integrated circuits, and development of a process for the purification of metallurgical grade silicon and to obtain oriented grain polycrystalline material to be used in the fabrication of solar cells. 4. Earlier funding The principal funding agencies for research and development projects in the photovoltaic area have been the Bank of National Development (BNDE), the National Fund for the Development of Science and Technology (FNDCT), the National Fund for Technical Development (FUNTEC/ BNDES), the Financing Agency for Studies and Projects (FINEP), the National Research Council (CNPq), the Bank of Brazil (FIPEC), the Research Foundation of S~o Paulo State (FAPESP), and the Electrical Utility Company of S~o Paulo State (CESP). The Organization of American States (OAS) also provided grants from 1981 to 1984. FINEP launched a photovoltaic programme in 1980 and provided specific grants to the principal institutions cited above. Unfortunately the grants dried up in 1986. CNPq has been funding small projects in solar photovoltaic energy. CNPq and the Ministry of Education (CAPES) provide scholarships for postgraduate studies in Brazil and abroad. Both FINEP and CNPq coordinated grants from OAS for photovoltaic energy. There has been considerable investment in the training of scientific personnel and the installation of modern well equipped laboratories. The scientific funding, however, has been erratic and has lacked continuity. This has resulted in a waste of manpower and resources. The research funds have usually been allocated for periods of two years, and over this period their values have been known to diminish dramatically because of high inflation rates. Lately, the funds have been allocated in an inflation adjustable unit (OTN). Over the years, the solar energy research and development programmes in Brazil have not provided effective and consistent support. Budgets have not been fixed for research, development or demonstration projects. Some investment for industrialization has come from private sources. As will be seen in the following section, this picture is about to change.
19 5. PRO-SOLAR In June 1987, the Brazilian Ministry of Mining and Energy created a group to study the actual state of the art and to formulate a national plan for the development of solar energy. The group is composed of representatives of the Ministry of Mining and Energy, the scientific community, industry, FINEP, Companhia Auxiliar de Empresas El~tricas Brasileiras (CAEEB), PetrSleo Brasileiro (PETROBRAS), Centrais El~tricas Brasileiras (ELECTROBRAS), Telecomunicaq6es Brasileiras (TELEBRAS), the Brazilian Association of Solar energy (ABEnS) and the Brazilian Association of Electrical Engineers (ABEE). The Directorate (Conselho Diretor) is composed of the Secretaries General of the Ministries of Science and Technology, Finance, Industry and Commerce, Mining and Energy, and Planning (Secretaria de Planejamento e Coordina~o da Presid~ncia da Republica). The Executive Council is composed of representatives of the planning and financing agencies, state and private companies, universities and research centres. The group has met several times during the last twelve months and has already developed a Directive Plan (Plano Diretor) for the scientific and technological development and utilization of solar energy. The plan calls for systematic action to create and maintain the financial mechanisms for the acquisition of solar energy systems, and for training, research and development, information dissemination and industrial production of solar equipment. It provides directives for specific actions together with and institutional arrangements for their coordination and execution and for the creation of the financial mechanisms. It also includes studies of the viability of substituting photovoltaic energy for conventional energy in water pumping with both submersed and ground-level pumps, in refrigeration, and in electrification of small rural properties. The initial efforts would be directed towards obtaining municipal, state and federal support, and towards assimilating existing activities within PRO-SOLAR. A solar energy documentation centre and information network would be created. Attempts would be made to enact necessary legislation and to designate technological centers for certification. There would be specific programmes for training at different levels in the development and utilization of solar energy, for funding of research and development, for mapping of regional and national solar energy potential, for the establishment of standards for solar architecture and civil construction, and for certifying materials and equipment. The utilization of solar energy systems will be stimulated through projects designed to reduce their cost, and through fiscal incentives and compensation and other benefits to users and producers. Municipalities, state companies and federal agencies would be encouraged to allocate specific funds and some funds would be made available by the Programme of Energy Mobilization (PME). Specific funds will be made available from the federal treasury through the federal budget, PME, and the National Development Fund, and will be centralized and will constitute the financial support for PRO-SOLAR. Additional funds for
20 PRO-SOLAR will come from a surcharge that will be added to the price of gasoline and other petroleum derivatives. Portions of the budgets of state companies directly involved with the utilization of solar energy will be reserved for specific PRO-SOLAR projects of concern to them, and also for training, technical exchange and other related uses. Special lines of credit from public and private banks will be made available to producers and users for the installation of solar equipment relevant to the national programme. Traditional sources of funds, such as FNDCT, FUNTEC/BNDES, FIPEC, state funding agencies, bilateral funds with state, federal or external agencies, and also transferred funds, donations and voluntary income tax deductions, will be utilized for PRO-SOLAR projects, installation of solar equipment and other specific applications. Fiscal incentives will be provided in the form of reductions or exemptions in production and transport of goods, and also in housing and urban land. Additionally, the electric utilities will allow the user to deduct from electricity bills the cost of solar installations. Among the various sources, the most important is that from the percentage added to the price of petroleum derivatives, which would maintain its value in spite of inflation. Based on informal information, it is estimated that it would be equivalent to over five million U.S. dollars per year. This would be divided between solar thermal and solar photovoltaic projects. PRO-SOLAR funds will be used specifically for research and technological development through pilot plant demonstrations, cost/benefit optimization, manpower training, sponsorship of meetings and other events, establishment of data banks, standardization and certification of solar equipment and systems, establishment of special lines of credit, tests, solarimetric data collection and national and international technical and scientific exchange. The following comparative cost analyses of photovoltaic systems and conventional energy sources have been carried out for diesel prices between 0.02 and 0.10 OTN 1-I (the OTN is a treasury unit which maintains approximate parity with hard currencies; 1 0 T N ~ U S $ 7) and interest rates of 6%, 8% and 10% p.a. It should be noted that diesel and kerosene prices are subsidized in Brazil, as in other developing countries, because of their impact on transportation and household costs. It should also be noted that in Brazil, savings accounts maintain their value in OTN and receive an interest of 0.5% per month. The systems chosen for analysis were (i) water pumping (with both ground-level and submersed pumps}, (ii)refrigeration (with or w i t h o u t other uses such as illumination, radio and television) and (iii) electrification of small rural properties. These systems were chosen on the basis of the needs of remote villages and communities not connected to electrical grids. Several remotely located regions in Brazil have very high insolation. 5.1. Water pumping Water for human and animal consumption and for irrigation is a basic necessity for the sustenance and development of communities in remote regions. Photovoltaic systems are modular and use energy more efficiently,
21 while the smallest available and reliable diesel pumps are found to be t o o large for many applications. Thus the photovoltaic option may prove to be more attractive at lower capacities. Ground-level pumps with a capacity of 40 000 1 per day at a water head of 12 m, have been considered for purposes of drinking, washing and irrigation, while submersed pumps for tube-wells, with a capacity of 27 000 1 per day at a water head of 30 m, have been considered for small rural communities, especially in the northeast of Brazil. Directly coupled diesel pumps have been considered for comparison with ground-level pumps, while diesel generators have been considered for comparison with submersed water pumps. Each photovoltaic system consists of a solar panel and a centrifugal d.c. motor. The costs of 444 Wp panels for a ground-level pump (1920 OTN) and 980 W~ panels for a submersed pump (4480 OTN) ( 4 . 3 2 - 4.57 OTN Wp-l) represent the largest fractions of the costs of photovoltaic systems. Photovoltaic systems are therefore cheaper to maintain (1% p. a.) than diesel systems (10% p. a.). It has been shown that the ratio of total installation and operational costs of conventional diesel systems to the corresponding costs of photovoltaic systems, over the solar panel lifetime of 20 years, would be in the range 0.56 - 1.25. For the ground level pump, the solar option would become competitive at diesel prices of over 0.08 OTN 1-1 and interest rates of below 8% p. a., while for the submersed pumps it would become competitive near diesel prices of 0.09 - 0.1 OTN 1-1 and interest rates of 6% - 7% p. a. The photovoltaic option may still be considered on the basis of maintenance, reliability, transport and ambient impact of the diesel systems.
5.2. Refrigerators The refrigerators considered are used to preserve vaccines in remotely located health clinics or to preserve f o o d in farm houses. The photovoltaic option would require 111 Wp solar pannels (480 OTN), a refrigerator (110 OTN) and batteries (74 OTN), compared with a kerosene refrigerator (60 OTN) which would need 1.25 1 of kerosene per day. The third o p t i o n - that of a diesel g e n e r a t o r - turns o u t to have excessive capacity, with running costs from 2.1 to 5.5 times greater than those of photovoltaic solar options. It may, therefore, be seriously considered only with electrification as described below. It is shown that the cost of running single isolated refrigerators on kerosene, with no other electrical requirements, would range from 0.33 to 0.8 that of the photovoltaic option. Here again, the photovoltaic option may be considered on the basis of reliability, transport and maintenance of o p t i m u m temperature range.
5.3. Electrification The electrification of small rural properties in remote regions has been considered for the following applications {daily consumption in the parentheses}: illumination (60 W h), radio communication (33 W h), television (45 W h), refrigeration (300 W h), pumping 1000 1 of water per day (200 W h), for a total of 668 W h per day. It would require 258 Wp
22 solar panels (1120 OTN), power regulation (42 OTN), 540 A h/12 V batteries (148 OTN) and other components. Even the smallest available diesel generator that is reliable would still be utilized below its capacity. In this case the installation and operational cost of the conventional diesel generator is 1.6 - 5.5 times that of the photovoltaic option.
6. Final comments
PRO-SOLAR is based on careful study and strives to address the problem of the development and utilization of solar energy in Brazil. The participation of different ministries, together with the broad base of industries, universities, research centres, scientific associations and funding agencies, should assure the success of this endeavour. Experience with the earlier highly successful PRO-ALCOOL programme shows that when planned and executed properly such programmes bring great benefits, utilizing locally available resources and reducing the chronic dependence on external resources. PRO-ALCOOL was n o t without its economic cost. With the decrease in petroleum prices in the world market, the early hope of attaining self-sufficiency has not materialized, and PRO-ALCOOL constitutes a huge drain on the exchequer. Pursued properly, the photovoltaic option would turn o u t to be very beneficial and, in five to ten years, could even become economically competitive. For this to happen, the development efforts need to be directed towards reducing the high cost of solar photovoltaic panels and components in the Brazilian market. There must also be testing of the long-term p.erformance of the local solar panels and components. Demonstration projects are an essential ingredient of any new energy technology and should receive a proper share of the funds. However, the solar option should n o t be pursued only on the basis of universal availability and abundance. Efficiency and economic competitiveness within the next decade should be the goal of PRO-SOLAR. The efficiency of thin film solar cells and panels has been increasing continuously. More importantly, work with different materials is showing promising results [ 17 ]. Single-heterojunction CuInGaSe2 thin film solar cells have recently shown an active area, photovoltaic conversion efficiency of 14.1%. Multijunction a-Si:H:F solar cell efficiencies are greater than 12.4%. The efficiencies of four-terminal a-Si:H/CuInSe2 cascade solar cells have reached 14.6%. Stable thin film CdTe panels have been fabricated with aperture area of 938 cm: and module efficiency of 11.1%. It has therefore been predicted that the cost of solar cell panels could be reduced to U S $ 1 Wp-1. These promising results on several fronts suggest that the emphasis should be placed on research and development directed at economically promising amorphous and polycrystalline thin film solar.cells and modules, and that major industries should be encouraged to participate in the manufacture of solar equipment, all of which will lead to higher efficiency, lower costs and economic competitiveness.
23
Acknowledgments This work was supported by the U.S. Department of Energy under contract DE-AC02-83CH10093. It was also partially supported by the Brazilian Ministry of Education, through CAPES. The author is grateful to Dr. Jorge Cals Coelho, Coordinator of the group that prepared the programme PRO-SOLAR, for permission to utilize the information. The opinions expressed in this paper are those of the author and do not represent the opinions of SERI, DOE or the Brazilian Ministry of Mining and Energy.
References 1 Piano Diretor, Programa Nacional de Energia S o l a r - - P R O - S O L A R , Brazilian Ministry of Mining and Energy, Secretariat of Technology, Coordination of Energy Resources, 1988. 2 A. M. de Andrade, to be published. 3 N. G. Dhere, R. G. Pinheiro and N. R. Parikh, J. Vac. Sci. Technol., 11 (1974) 599. 4 N. G. Dhere, N. R. Parikh and A. Ferreira, Thin Solid Films, 44 (1977) 83. 5 N. G. Dhere and R. R. de Avillez, Proc. 7th Int. Vacuum Congress and 3rd Int. Conf. on Solid Surfaces, Vienna, 1977, Australian Vacuum Society, Vienna, p. 1931. 6 R. G. Dhere, L. R. O. Cruz and N. G. Dhere, Proc. Int. Conf. on Trends and New Applications o f Thin Films, Strazbourg, 1987, Soci6t6 Fran~aise du Vide, Paris, p. 526. 7 N. G. Dhere, I. G. Mattoso, H. R. Moutinho, C. L. Ferreira, L. R. O. Cruz and R. G. Dhere, Rev. Brazil. Apl. Vdc., 6 (1986) 245. 8 N. G. Dhere, M. C. Louren~o and L. L. Kazmerski, Sol. Cells, 16 (1986) 369. 9 F. Alvarez, Appl. Phys. Lett., 47 (1985) 960. 10 F. Alvarez and I. Chambouleyron, Sol. Energy Mater., 10 (1984) 151. 11 R. A. Barrio, A. S. Carriqo, F. C. Marques, J. Sajurjo and I. Chambouleyron, J. Phys. C, 1 (1989)69. 12 I. Chambouleyron, Sol. Cells, 12 (1984) 393. 13 R. Kishore, Rev. Brazil. Apl. Vdc., 7 (1987) 148. 14 R. Kishore, to be published. 15 W. Losch and H. Niehus, Rev. Brazil. Apl. Vdc., 5 (1985) 209. 16 H. Paes, C. Gatts and W. Losch, Rev. Brazil. Apl. Vdc., 8 (1988), in the press. 17 N. G. Dhere, Proc. 1st Iberian Vacuum Congress Braga, 1988, in Vacuum (1989), to be published.
13
THE DEVELOPMENT OF SOLAR PHOTOVOLTAIC ENERGY IN BRAZIL N E E L K A N T H G. D H E R E *
Solar Energy Research Institute, 161 7 Cole Boulevard, Golden, CO 80401 (U.S.A.)
Summary Several groups in Brazil have been engaged in research and development of solar cells and solar cell materials. One company manufactures singlecrystal silicon solar cells, modules and systems, starting with imported pure silicon. It grows single-crystal silicon ingots and carries out cutting, wafer polishing, diffusion, contacting and module fabrication. The equipment manufactured includes water pumps and refrigerators. Emphasis on postgraduate education and applied research since the early 1970s has resulted in a critical mass of trained manpower and modern well equipped laboratories. Funding of specific photovoltaic projects in the years 1979 - 1986 helped to crystallize the research efforts, and to create groups dedicated to solar cells and to give those groups valuable experience. The funding has, however, been erratic and has lacked continuity. A new programme, PRO-SOLAR, has been modelled on the lines of an earlier, highly successful ethanol programme (PRO-ALCOOL). PRO-SOLAR has been developed by the Brazilian Ministry of Mining and Energy and is intended to promote the development and utilization of solar energy. Since other ministries are also participating and there is a broad base of industries, universities, research centres, associations and funding agencies, it is anticipated that significant results will be produced. It is suggested that economic competitiveness should be the goal of these efforts and hence, together with the demonstration projects that are being envisaged, concentrated efforts should be made towards the reduction of cost of solar cells and modules. Research and development leading to economically promising amorphous and polycrystalline thin film solar cells and equipment will require the participation of major industries.
1. Introduction
Funding of specific solar photovoltaic projects was initiated in Brazil towards the end of the 1970s and lasted for five to seven years. Prior to the * I n s t i t u t o Militar de E n g e n h a r i a , R i o de J a n e i r o , Brazil. 0379-6787/89/$3.50
© Elsevier S e q u o i a / P r i n t e d in T h e N e t h e r l a n d s
14 general awareness of the impending energy crisis, several groups were working on semiconductor devices and materials for applications in microelectronics. The specific funding allowed some groups to concentrate on photovoltaic activity, train personnel and gain valuable experience. Manufacture of solar cells, modules, systems and appliances was also begun. After the termination of solar funding, these groups have been striving to maintain some activity in research and development. A few groups are concentrating largely on the study of materials that are useful for solar cells. The industrial activity has continued but is being maintained at a low key. The emphasis on industrialization since the second world war, and the growth in postgraduate education, research and development since 1970, have resulted in a strong and varied industrial base, a reasonably large number of trained personnel, and fairly well equipped research and development institutions in Brazil. Projections show that the existing electricity generation capacity in Brazil will be insufficient in the 1990s. Brazil has an enormous land mass and a tropical climate. Large geographic regions have not yet been electrified. Most of these have a high level of insolation t h r o u g h o u t the year, some having amongst the highest levels in the world. Mean daily insolation in the north and northeast of Brazil ranges from a minimum of 184 W m -2 (Z~ Doca, Maranh~o) to a maximum of 260 W m -2 (Petrolina, Pernambuco). This, together with experience in research, development and production of solar cells, could permit Brazil to plan to harness solar energy on a large scale in the 1990s, and might also help to correct socially lopsided development. Recently, the Ministry of Mining and Energy created a group for the development of the solar energy programme (PRO-SOLAR) [1]. It is expected that the programme will provide sustained funding at a reasonable level so that this goal can be achieved. This paper describes the research, development and industrialization that is occurring in Brazil with respect to solar photovoltaic energy, and presents information on the PRO-SOLAR programme.
2. Education, research and development Since the early 1970s, postgraduate education has been considerably expanded and greater prominence has been given to applied physical sciences. Earlier, Brazilian physicists concentrated on pure and mostly theoretical physics. The lack of practical training at undergraduate levels is felt, even today, by students who do not come from the industrialized regions. Substantial funding was provided to laboratories working in microelectronics and materials. Modern well equipped laboratories could, therefore, be established. This permitted the creation of a critical mass of trained personnel, and research activity in areas related to microelectronics. When it first became clear that there was an impending energy crisis, the government chose to believe that Brazil was an "island of prosperity" in a world that elsewhere was troubled by this crisis. Later, a well structured and
15 comprehensive programme (PRO-ALCOOL) for the development of hydrated ethanol to be used as a substitute for gasoline was launched. This programme has been very successful. Approximately 2.5 million cars run on ethanol while all other cars utilize a 2 0 % - 22% mixture of ethanol in gasoline. An ambitious nuclear programme was also started, although it later turned o u t to be economically disastrous. A modest solar photovoltaic programme was initiated in 1979 - 1980. It helped to crystallize the research work and create groups dedicated to the development of solar cells. The activities of the principal groups are described below.
2.1. Solar Cells Group, Microelectronics Laboratory, University of S~o Paulo This group began functioning in 1974, with a project that involved growing silicon single-crystal ingots by the Czochralski method. Research and development efforts aimed at finding cheaper techniques for the fabrication of single-crystal solar cells were begun in 1981. Methods for texturing silicon single-crystal surfaces and also for the deposition of transparent, conducting and antireflecting coatings of SnO2 were studied. The process of simultaneous deposition and diffusion of phosphorous to obtain a p - n junction, using a liquid source and a conventional diffusion furnace, was optimized. Processes of selective electroless nickel deposition, using photolithographic techniques, followed b y an S n - P b solder dip, were developed. This lowered the cost of contacting, since the use of vacuum and precious metals was avoided. Single-crystal silicon solar cells with photovoltaic conversion efficiencies of more than 12% (AM 1.5) were prepared. Single-crystal silicon cells fabricated at this laboratory are being utilized in the construction of an experimental space solar cell panel. Work on polycrystalline silicon solar cells was also initiated. Lately the group has concentrated its activities on the development of hydrogenated amorphous silicon (a-Si:H) solar cells [ 2 ]. 2.2. Solar Cells and Microelectronics Laboratory, Materials Science Department, Military Institute o f Engineering, Rio de Janeiro This group began work on semiconducting thin films and hybrid microelectronics in 1971. Preparation techniques, structure, morphology and semiconducting properties of thin films of CdTe, CdS, CdSe, InP and SnO2 were studied [ 3 - 6 ]. The group perfected methods of obtaining highly pure and reliable single-layer and multilayer coatings of contact materials, and methods of preparing resistive and dielectric coatings. A project for the development of CdS-Cu2S thin film solar cells b y a wet m e t h o d was initiated in 1981. An encapsulated 5 cm × 5 cm solar cell with efficiency greater than 5% and a 1 W panel was developed. The degradation of cell efficiency over a period of one year was studied [7]. The installation of an a-Si:H plasma deposition system has been a t t e m p t e d b u t so far has n o t been successful because of inadequate funding. Si-SnO2 solar cells have also been fabricated. After the initial evidence of excessive degradation of CdS-Cu2S solar cells,
16 the emphasis was shifted to CdS-CuInSe2 solar cells. Stoichiometric CulnSe2 thin films with chalcopyrite structure were prepared by co-evaporation of constituent elements, and the effect of small deviations from stoichiometry on the structure and semiconducting properties was studied [8]. CdSCuInSe2 solar cells with conversion efficiencies greater than 7% (AM 1.5) were prepared. This work is continuing. Plasma-assisted growth of CdTe, indium tin oxide thin films for solar cells is also being studied.
2.3. Photovoltaic Conversion Laboratory, Institute o f Physics, University o f Campinas This group began its activities in 1980. A few members of the group had previously worked on chemically deposited CdS thin films for solar cells. The group has worked on the preparation and semiconducting properties of plasma-deposited a-Si:H, a-SiN:H, a-SiC:H, a-GeN:H and a-GeSn:H thin films, and transparent and conducting thin films of SnO2 deposited by spray pyrolysis [9 - 11]. The effect of plasma conditions on doping and hydrogen incorporation was also studied. Solar cells with p - i - n junctions were fabricated and their photovoltaic properties were analyzed. Theoretical studies were carried o u t on the effect of different irradiation conditions on the efficiency of cascade cells [12]. A project on polycrystalline silicon solar cells was also undertaken. Since the termination of specific funding, the group has been working on the materials properties of elemental and alloy amorphous semiconductors.
2.4. Sensors and Materials Laboratory, Institute o f Space Research, S~o Josd dos Campos The Institute of Space Research began its solar cell project for the Brazilian satellite programme in the mid 1970s. The early emphasis was on the testing of space-worthy cells and modules. Later, projects were undertaken in collaboration with other laboratories for the fabrication of single-crystal silicon and GaAs solar cells. The laboratory has been analysing the flux m e t h o d for the study of charge transport in multilayer solar cells [ 13]. Generalized expressions have been derived for electric current density. These have provided some new contributions to open-circuit voltage. The effects of simultaneous radiation and thermal annealing on damage to solar cells in space has also been studied [14]. Two exponential electrotechnical models have been analyzed semi-empirically. An experimental space solar cell panel is being constructed using single-crystal solar cells fabricated at the Microelectronics Laboratory. It will be used in the first Brazilian satellite, to study the degradation of solar cells in space. Simulation and modelling of solar panels are being carried out so that the panels can be used to provide energy for the satellite.
17
2.5. Materials and Interfaces Laboratory, Department of Metallurgy and Materials Science, Coordination of Post-Graduate Programme in Engineering, Federal University of Rio de Janeiro This group began working in 1970, concentrating on the study of surfaces and interfaces. The first equipment for Auger electron spectroscopy was installed in 1975 and was utilized for the study of segregation in steels and interdiffusion in thin films [15]. A project on a-Si:H thin film solar cells was undertaken in 1982. The group has studied metal-a-Si:H Schottky barriers, interdiffusion and segregation in metallic thin films and at metalsemiconductor interfaces, hydrogen diffusion, antireflection and protective coatings, formation of silicides, failure of electronic devices, and electrical and optical properties of thin films. Schottky barrier and p - i - n junction a-Si:H solar cells have been prepared and characterized. An a-Si:H temperature sensor has also been developed. More recently, the group has been concentrating on the properties of a-Si:H thin films and related materials [16].
2.6. Other groups Four groups working on photoelectrochemical solar cells are located at the Institute of Physics at the University of Campinas, the Institute of Physics and Chemistry at the University of S~o Paulo at S~o Carlos, the University of Rio Grande de Sul in Porto Alegre, and the University of Cear~ in Fortaleza. A small b u t dedicated group is working on methods for reducing the fabrication costs of single-crystal silicon solar cells at the Department of Electrical Engineering at the University of Campinas. Purification of materials, especially to obtain solar and electronic grade silicon, has been investigated at the Faculty of Engineering of the University of Campinas, and the Chemistry Department of the Military Institute of Engineering in Rio de Janeiro. 3. Industrialization The industrial production of solar cells in Brazil began with the fabrication of solar cell modules and systems utilizing imported solar cells. The applications were for telecommunications networks and for remote installations. A Brazilian company, Heliodin~mica, later started the production of 100 mm diameter single-crystal silicon ingots and solar cells. The process, developed locally, consists of the growth of p-type silicon single-crystal ingots, crystal cutting and wafer polishing, p - n junction formation by diffusion of phosphorus using POC13, and formation of an n - p - p + structure with diffusion from an aluminum paste for generating the back surface field. Front contact fingers are obtained by photolithography of electroless nickel and P b - S n solder dip. Back contacts utilize either the electroless nickel or aluminum paste and P b - S n solder dip. Characterization of the solar cells is carried out by I - V measurements in the dark and under simulated sunlight, and b y measurements of spectral response, junction depth,
18 diffusion length and minority carrier lifetime. Solar panels are tested at different temperatures. The factory has the capacity to produce 600 kWp per year. Since the full capacity is not being utilized, the company sells silicon wafers locally and exports them. It also manufactures water pumps, refrigerators and solar cell systems. Heliodin~mica has undertaken several development projects funded by Brazilian agencies. The early projects consisted of training technical personnel, crystal growing and cutting, wafer polishing, chemical treatment, diffusion, metallization, characterization and testing of solar cells, soldering, encapsulation, mounting and testing solar cell modules. Later projects have involved specular finish polishing of silicon wafers, clearing and packing to obtain wafers readily usable in the fabrication of integrated circuits, and development of a process for the purification of metallurgical grade silicon and to obtain oriented grain polycrystalline material to be used in the fabrication of solar cells. 4. Earlier funding The principal funding agencies for research and development projects in the photovoltaic area have been the Bank of National Development (BNDE), the National Fund for the Development of Science and Technology (FNDCT), the National Fund for Technical Development (FUNTEC/ BNDES), the Financing Agency for Studies and Projects (FINEP), the National Research Council (CNPq), the Bank of Brazil (FIPEC), the Research Foundation of S~o Paulo State (FAPESP), and the Electrical Utility Company of S~o Paulo State (CESP). The Organization of American States (OAS) also provided grants from 1981 to 1984. FINEP launched a photovoltaic programme in 1980 and provided specific grants to the principal institutions cited above. Unfortunately the grants dried up in 1986. CNPq has been funding small projects in solar photovoltaic energy. CNPq and the Ministry of Education (CAPES) provide scholarships for postgraduate studies in Brazil and abroad. Both FINEP and CNPq coordinated grants from OAS for photovoltaic energy. There has been considerable investment in the training of scientific personnel and the installation of modern well equipped laboratories. The scientific funding, however, has been erratic and has lacked continuity. This has resulted in a waste of manpower and resources. The research funds have usually been allocated for periods of two years, and over this period their values have been known to diminish dramatically because of high inflation rates. Lately, the funds have been allocated in an inflation adjustable unit (OTN). Over the years, the solar energy research and development programmes in Brazil have not provided effective and consistent support. Budgets have not been fixed for research, development or demonstration projects. Some investment for industrialization has come from private sources. As will be seen in the following section, this picture is about to change.
19 5. PRO-SOLAR In June 1987, the Brazilian Ministry of Mining and Energy created a group to study the actual state of the art and to formulate a national plan for the development of solar energy. The group is composed of representatives of the Ministry of Mining and Energy, the scientific community, industry, FINEP, Companhia Auxiliar de Empresas El~tricas Brasileiras (CAEEB), PetrSleo Brasileiro (PETROBRAS), Centrais El~tricas Brasileiras (ELECTROBRAS), Telecomunicaq6es Brasileiras (TELEBRAS), the Brazilian Association of Solar energy (ABEnS) and the Brazilian Association of Electrical Engineers (ABEE). The Directorate (Conselho Diretor) is composed of the Secretaries General of the Ministries of Science and Technology, Finance, Industry and Commerce, Mining and Energy, and Planning (Secretaria de Planejamento e Coordina~o da Presid~ncia da Republica). The Executive Council is composed of representatives of the planning and financing agencies, state and private companies, universities and research centres. The group has met several times during the last twelve months and has already developed a Directive Plan (Plano Diretor) for the scientific and technological development and utilization of solar energy. The plan calls for systematic action to create and maintain the financial mechanisms for the acquisition of solar energy systems, and for training, research and development, information dissemination and industrial production of solar equipment. It provides directives for specific actions together with and institutional arrangements for their coordination and execution and for the creation of the financial mechanisms. It also includes studies of the viability of substituting photovoltaic energy for conventional energy in water pumping with both submersed and ground-level pumps, in refrigeration, and in electrification of small rural properties. The initial efforts would be directed towards obtaining municipal, state and federal support, and towards assimilating existing activities within PRO-SOLAR. A solar energy documentation centre and information network would be created. Attempts would be made to enact necessary legislation and to designate technological centers for certification. There would be specific programmes for training at different levels in the development and utilization of solar energy, for funding of research and development, for mapping of regional and national solar energy potential, for the establishment of standards for solar architecture and civil construction, and for certifying materials and equipment. The utilization of solar energy systems will be stimulated through projects designed to reduce their cost, and through fiscal incentives and compensation and other benefits to users and producers. Municipalities, state companies and federal agencies would be encouraged to allocate specific funds and some funds would be made available by the Programme of Energy Mobilization (PME). Specific funds will be made available from the federal treasury through the federal budget, PME, and the National Development Fund, and will be centralized and will constitute the financial support for PRO-SOLAR. Additional funds for
20 PRO-SOLAR will come from a surcharge that will be added to the price of gasoline and other petroleum derivatives. Portions of the budgets of state companies directly involved with the utilization of solar energy will be reserved for specific PRO-SOLAR projects of concern to them, and also for training, technical exchange and other related uses. Special lines of credit from public and private banks will be made available to producers and users for the installation of solar equipment relevant to the national programme. Traditional sources of funds, such as FNDCT, FUNTEC/BNDES, FIPEC, state funding agencies, bilateral funds with state, federal or external agencies, and also transferred funds, donations and voluntary income tax deductions, will be utilized for PRO-SOLAR projects, installation of solar equipment and other specific applications. Fiscal incentives will be provided in the form of reductions or exemptions in production and transport of goods, and also in housing and urban land. Additionally, the electric utilities will allow the user to deduct from electricity bills the cost of solar installations. Among the various sources, the most important is that from the percentage added to the price of petroleum derivatives, which would maintain its value in spite of inflation. Based on informal information, it is estimated that it would be equivalent to over five million U.S. dollars per year. This would be divided between solar thermal and solar photovoltaic projects. PRO-SOLAR funds will be used specifically for research and technological development through pilot plant demonstrations, cost/benefit optimization, manpower training, sponsorship of meetings and other events, establishment of data banks, standardization and certification of solar equipment and systems, establishment of special lines of credit, tests, solarimetric data collection and national and international technical and scientific exchange. The following comparative cost analyses of photovoltaic systems and conventional energy sources have been carried out for diesel prices between 0.02 and 0.10 OTN 1-I (the OTN is a treasury unit which maintains approximate parity with hard currencies; 1 0 T N ~ U S $ 7) and interest rates of 6%, 8% and 10% p.a. It should be noted that diesel and kerosene prices are subsidized in Brazil, as in other developing countries, because of their impact on transportation and household costs. It should also be noted that in Brazil, savings accounts maintain their value in OTN and receive an interest of 0.5% per month. The systems chosen for analysis were (i) water pumping (with both ground-level and submersed pumps}, (ii)refrigeration (with or w i t h o u t other uses such as illumination, radio and television) and (iii) electrification of small rural properties. These systems were chosen on the basis of the needs of remote villages and communities not connected to electrical grids. Several remotely located regions in Brazil have very high insolation. 5.1. Water pumping Water for human and animal consumption and for irrigation is a basic necessity for the sustenance and development of communities in remote regions. Photovoltaic systems are modular and use energy more efficiently,
21 while the smallest available and reliable diesel pumps are found to be t o o large for many applications. Thus the photovoltaic option may prove to be more attractive at lower capacities. Ground-level pumps with a capacity of 40 000 1 per day at a water head of 12 m, have been considered for purposes of drinking, washing and irrigation, while submersed pumps for tube-wells, with a capacity of 27 000 1 per day at a water head of 30 m, have been considered for small rural communities, especially in the northeast of Brazil. Directly coupled diesel pumps have been considered for comparison with ground-level pumps, while diesel generators have been considered for comparison with submersed water pumps. Each photovoltaic system consists of a solar panel and a centrifugal d.c. motor. The costs of 444 Wp panels for a ground-level pump (1920 OTN) and 980 W~ panels for a submersed pump (4480 OTN) ( 4 . 3 2 - 4.57 OTN Wp-l) represent the largest fractions of the costs of photovoltaic systems. Photovoltaic systems are therefore cheaper to maintain (1% p. a.) than diesel systems (10% p. a.). It has been shown that the ratio of total installation and operational costs of conventional diesel systems to the corresponding costs of photovoltaic systems, over the solar panel lifetime of 20 years, would be in the range 0.56 - 1.25. For the ground level pump, the solar option would become competitive at diesel prices of over 0.08 OTN 1-1 and interest rates of below 8% p. a., while for the submersed pumps it would become competitive near diesel prices of 0.09 - 0.1 OTN 1-1 and interest rates of 6% - 7% p. a. The photovoltaic option may still be considered on the basis of maintenance, reliability, transport and ambient impact of the diesel systems.
5.2. Refrigerators The refrigerators considered are used to preserve vaccines in remotely located health clinics or to preserve f o o d in farm houses. The photovoltaic option would require 111 Wp solar pannels (480 OTN), a refrigerator (110 OTN) and batteries (74 OTN), compared with a kerosene refrigerator (60 OTN) which would need 1.25 1 of kerosene per day. The third o p t i o n - that of a diesel g e n e r a t o r - turns o u t to have excessive capacity, with running costs from 2.1 to 5.5 times greater than those of photovoltaic solar options. It may, therefore, be seriously considered only with electrification as described below. It is shown that the cost of running single isolated refrigerators on kerosene, with no other electrical requirements, would range from 0.33 to 0.8 that of the photovoltaic option. Here again, the photovoltaic option may be considered on the basis of reliability, transport and maintenance of o p t i m u m temperature range.
5.3. Electrification The electrification of small rural properties in remote regions has been considered for the following applications {daily consumption in the parentheses}: illumination (60 W h), radio communication (33 W h), television (45 W h), refrigeration (300 W h), pumping 1000 1 of water per day (200 W h), for a total of 668 W h per day. It would require 258 Wp
22 solar panels (1120 OTN), power regulation (42 OTN), 540 A h/12 V batteries (148 OTN) and other components. Even the smallest available diesel generator that is reliable would still be utilized below its capacity. In this case the installation and operational cost of the conventional diesel generator is 1.6 - 5.5 times that of the photovoltaic option.
6. Final comments
PRO-SOLAR is based on careful study and strives to address the problem of the development and utilization of solar energy in Brazil. The participation of different ministries, together with the broad base of industries, universities, research centres, scientific associations and funding agencies, should assure the success of this endeavour. Experience with the earlier highly successful PRO-ALCOOL programme shows that when planned and executed properly such programmes bring great benefits, utilizing locally available resources and reducing the chronic dependence on external resources. PRO-ALCOOL was n o t without its economic cost. With the decrease in petroleum prices in the world market, the early hope of attaining self-sufficiency has not materialized, and PRO-ALCOOL constitutes a huge drain on the exchequer. Pursued properly, the photovoltaic option would turn o u t to be very beneficial and, in five to ten years, could even become economically competitive. For this to happen, the development efforts need to be directed towards reducing the high cost of solar photovoltaic panels and components in the Brazilian market. There must also be testing of the long-term p.erformance of the local solar panels and components. Demonstration projects are an essential ingredient of any new energy technology and should receive a proper share of the funds. However, the solar option should n o t be pursued only on the basis of universal availability and abundance. Efficiency and economic competitiveness within the next decade should be the goal of PRO-SOLAR. The efficiency of thin film solar cells and panels has been increasing continuously. More importantly, work with different materials is showing promising results [ 17 ]. Single-heterojunction CuInGaSe2 thin film solar cells have recently shown an active area, photovoltaic conversion efficiency of 14.1%. Multijunction a-Si:H:F solar cell efficiencies are greater than 12.4%. The efficiencies of four-terminal a-Si:H/CuInSe2 cascade solar cells have reached 14.6%. Stable thin film CdTe panels have been fabricated with aperture area of 938 cm: and module efficiency of 11.1%. It has therefore been predicted that the cost of solar cell panels could be reduced to U S $ 1 Wp-1. These promising results on several fronts suggest that the emphasis should be placed on research and development directed at economically promising amorphous and polycrystalline thin film solar.cells and modules, and that major industries should be encouraged to participate in the manufacture of solar equipment, all of which will lead to higher efficiency, lower costs and economic competitiveness.
23
Acknowledgments This work was supported by the U.S. Department of Energy under contract DE-AC02-83CH10093. It was also partially supported by the Brazilian Ministry of Education, through CAPES. The author is grateful to Dr. Jorge Cals Coelho, Coordinator of the group that prepared the programme PRO-SOLAR, for permission to utilize the information. The opinions expressed in this paper are those of the author and do not represent the opinions of SERI, DOE or the Brazilian Ministry of Mining and Energy.
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