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Thin-film solar cells: An overview
WRECI~6
THIN-FILM SOLAR CELLS: AN OVERVIEW
S. K. DEB National Renewable Energy Laboratory Golden, CO 80401-3393 USA
ABSTRACT Thin-film solar cells offer the most promising options for substantially reducing the cost of photovoltaic systems. A multiplicity of options, in terms of materials and devices, are currently being developed worldwide. Some of the leading contenders are: amorphous and polycrystalline silicon, compound semiconductor thin films such as CulnSe2-based alloys, and CdTe thin-film solar cells. Enormous progress in device performance has been made in most of these technologies, and considerable effort is devoted to commercialization of these technologies. Exciting new developments are happening in some relatively new materials and devices.
KEYWORDS Thinfilms,solarcells,photovoltaics,amorphoussilicon,copperindiumdiselenidealloys,cadmium telluride,thinsilicon,galliumarsenide,titaniumdioxide.
INTRODUCTION The primary objective of the worldwide photovoltaic (PV) solar-cell research and development is to reduce the cost to a level that it will be a viable alternative to conventional ways of generating electric power. To do that, it will be necessary to bring the cost of photovoltaics to about $1.50/Wp at the system level. Today's world PV market of about 80 MW/yr is sold at prices close to $10/Wp which is almost six times the long-term cost goal. PV technology in the marketplace today is dominated by crystalline silicon. It is generally believed that even with greatly increased market and production volume, the price of crystalline or polycrystalline silicon cannot be reduced to meet the long-term cost goal for large-scale power production. In fact, various analyses suggest that thin-film solar cells are the only viable alternative that has the potential to meet this long-term cost goal. If one accepts this rationale, it is instructive to analyze whether thin-film technology can realistically meet these goal. There is reason to be very optimistic, particularly in view of the remarkable progress that has been made in recent years, both in terms of cell efficiency and long-term stability in several thin-film solar-cell technologies. At the present time, thin-film solar-cell research and development involves several materials, such as amorphous silicon (a-Si), polycrystalline thin films consisting of CuInSe2-based alloys and cadmium telluride, thin-film crystalline silicon, and other novel materials and advanced concepts. In this paper, a brief overview of some of these thin-film materials and device technologies will be given.
AMORPHOUS SILICON THIN FILM Among the thin-film PV technologies, a-Si holds the promise for low cost. It is by far the most mature and
375
W R E C 1996 commercially viable technology. Well-defined production processes over very large areas have been implemented. Since the first a-Si solar cell was fabricated by the RCA group in 1976, considerable progress has been made in improving the cell efficiency and module fabrication processes and in reducing overall cost. Worldwide, a-Si devices accounted for about 15% of all PV sales. In 1982, 1.5 MWp of a-Si solar cells were produced primarily for consumer products. Since then, the production volume has grown almost tenfold, and the market has expanded to include terrestrial power generation. The U.S. utilities are showing commitment to this technology by establishing outdoor test facilities. The largest facility is a 400-kWp system by PV for Utility-Scale Applications (PVUSA) in California. Most recently, companies such as Solarex Enron and United Solar are in the process of building 10-MW production facilities. The technology of a-Si is based on two types of device design: one involving single-junction and the other involving multijunction p-i-n cell structure. In the single junction p-i-n cell, the p- and n-type doped layers are made very thin, and the absorber layer is the intrinsic (i) layer. Because of the lower photocarder diffusion length, efficient solar cells require separation of charge carders by the electric field in the i-layer. This puts an inherent limit to the thickness of the i-layer to around 0.5 ~rn and to practically achievable efficiency due to poorer light absorption. Using a thicker absorber layer reduces the efficiency because of poor photocarrier collection. To circumvent this problem, various schemes of light-trapping and enhanced reflection at the contacting interface have been used. It is generally believed that efficiency in a singlejunction device has reached its optimum value around 12%. Therefore, any significant increase in efficiency can only be achieved by using multijunction devices containing as many as three different absorber layers with differing bandgaps. Today, the design of multijunction for high-efficiency solar cells is based on the use of a high-bandgap alloy of a-Si and carbon (a-SiC:H) and a low-bandgap alloy such as silicon and germanium (a-SiGe:H). Modeling of multijunction devices based on a triple tandem cell with 2.0/1.7/1.45 eV bandgap combinations predicts a theoretical efficiency of ~24%. The highest measured initial active-area efficiency to date is 13.7% for a small area of such a cell structure. A great variety of deposition methods have been used to fabricate amorphous-silicon-based alloy materials and devices, but among these, only the radio frequency (RF) and direct current (DC) glow-discharge techniques are being used by industry. Some of the newer techniques, such as ECR, remote plasma-assisted chemical vapor deposition (CVD), and hot-wire deposition, have produced materials with interesting physical properties such as lower defect density, greater diffusion length, and lower hydrogen concentration. Some of these techniques may hold promise for the future. The key research issues that affect the future of a-Si technology are: (1) much lower device efficiency than those predicted by theory, and (2) photodegradation due to the Staebler-Wronski (S-W) effect. The largest improvement in device efficiency will most likely come from improvement of the a-SiGe:H and a-SiC:H alloys. Research has concentrated on developing a-SiGe:H films with bandgap near 1.4 eV. This alloy is still not as good as those with higher bandgaps. Nevertheless, improvement in material and quality devices is constantly being made. Significant improvement in a-SiC:H alloy material has been made by using hydrogen dilution of methane. However, there is room for significant improvement of the materials, particularly for bandgaps greater than 1.9 eV. Current research efforts include establishing a correlation between microstructure and electrical properties; study of different deposition methods to achieve changes in performance, various approaches to improve Voc, fill factor and short-circuit current; innovative device designs to overcome the limitations in the current alloy materials; extensive use of computer simulations to model multijunction devices; and use of poly- or microcrystalline layer for the low-gap absorber layer. Light-induced degradation of a-Si remains as one of the challenging problems needing to be solved. To date, there is no solution to this problem. Although degradation can be completely removed by annealing the device at elevated temperatures, it starts anew after the annealing process. Fortunately, an engineering solution has been found that allows degradation to be controlled at a manageable level. It has been found that a thinner a-Si:H intrinsic layer results in either less or slower degradation, and this approach has become the mainstream strategy for managing the stability problem. The instability issue is intrinsic to aSi films. The effect is manifested in films by a decrease in photoconductivity and an increase in mid-gap defect density. Although many explanations have been proposed, none appears to be completely satisfactory. Two models that have gained much support include: (1) a weak Si-Si bond that is broken by light, and (2) charge dangling bonds that can be converted to metastable neutral dangling bonds by trapping of photogenerated carders. The hot-wire deposition technique, which results in lower hydrogen 376
WREC 1996 content in the film, appears to greatly improve the photostability. However, fabrication of efficient solar cells by this technique still remains a challenging issue. The strategy to reduce the cost of a-Si:H solar cells includes a combination of efficiency increase and unitarea cost reduction. By increasing the stabilized module efficiency to 10% from the present commercial module efficiency of 5% and by reducing the price by 50%, a-Si:H is expected to meet the 12C/kWh cost goal. The PV Manufacturing Technology (PVMaT) project is designed to reduce the cost by 50%. Eventually, the module efficiency has to be around 15% to meet the long-term cost goal of 6C/kWh. The overall market share for a-Si:H technology has been shrinking for several years, from a high of 39% in 1988 to about 16% at the present time. The hope is that this declining trend will be reversed by bringing on-line several 10-MW/yr Capacity plants. The commitment made by the major a-Si PV manufacturers in constructing several 10-MW/yr capacity plants provides strong credibility to the commercial viability of this technology.
COPPER INDIUM DISELENIDE AND ITS ALLOYS (CIS/CIGS) World-record efficiency of 17.7% has been recently achieved by the National Renewable Energy Laboratory (NREL) group by using CIGS alloy films. This is a remarkable achievement that matches the best efficiency of a polycrystalline solar cell. The CIGS thin-film absorber in this cell is fabricated by the physical vapor deposition of the constituent elements onto a heated Mo-coated soda-lime glass substrate. The absorber film is deposited in several stages. First, 2000 ~, of a near-stoichiometric layer of CuGaSe2 (CGS) is deposited at 350°C substrate temperature. The composition of the layer is designed to manage the overall Ga profile by minimizing the out-diffusion of Ga into the bulk and in-diffusion of In from the bulk. The CIGS film was deposited by a single two-stage plx~cess, whereby a Cu-rich CIGS precursor is converted to high-quality PV material by exposure to In, Ga, and Se at 550°C, which enhances the formation of (In,Ga)-rich surface phases. The device is completed by chemical bath deposition (CBD) of about 500 ~ CdS, followed by RF sputtering of 500/~ of intrinsic ZnO and 3000 ,~ of Al-doped ZnO. Nickel-aluminum grid contacts are then deposited with -5% coverage, followed by a 100-nm MgF2 antireflection coating. The 17.7% total-area efficiency represents a new world-record for all thin-film solarcell technologies that are grown non-epitaxially. This new result is an improvement over the previously reported record efficiency of 17.1%. It is reasonable to believe that a further optimization of device structure will lead to a small-area cell efficiency well over 18%. The major challenge before those active in this area is how to close the gap in efficiency between the small. area devices to large-area modules. In 1988, ARCO Solar (now Siemens Solar Industries) made an 11% square-foot module. Since then, no major advances in large-area module efficiency in CIGS-type devices has been reported. Even though the fabrication process for small-area devices is quite forgiving, it is suspected that transferring this process technology for large-area devices is not straightforward. Considerable development effort is necessary before the technology is ready for large-scale production. There is no doubt that this will be accomplished in time and that CIGS thin-film solar cells will be a major contender for large-scale power generation. One of the major strengths of this technology is the remarkable stability shown by these devices. NREL has been testing the ARCO Solar CIS module since November 1988, with no significant degradation during the past 7 years. Good stability and high efficiency are the strength of this technology, while manufacturability of these devices remains an issue. Another area of concern is that the cost of manufacturing CIS devices appears to be higher compared to the manufacturing costs of other competing thin-film technologies. This is largely because of the complexity of various processing steps. There are possible approaches to reducing the cost by developing alternative processes that are non-vacuum and involve simpler processing steps. Besides cost, there is also some concern regarding the availability of materials, such as indium, particularly for large-scale deployment.
377
W R E C 1996 CADMIUM TELLURIDE THIN FILM Enormous progress has been made in recent years on thin-film solar cells based on cadmium teUuride. Record efficiencies close to 16% have been achieved in laboratory-scale, small-area devices. One of the major advantages of cadmium telluride/cadmium sulfide thin-film solar cells is the low-cost fabrication options. A number of relatively simple, low-cost methods have been used to fabricate solar cells with efficiencies greater than 10%. Some of the low-cost deposition methods that show promise include: (l) close-space sublimation, (2) spray deposition, (3) electredeposition, and (4) screen printing. All of these techniques are being considered for large-scale manufacturing by several industries. The highest-efficiency cells (~ 16%) are fabricated in small-area devices using the close-space sublimation technique by a group at the University of South Florida. In this process, the source containing CdTe and the conducting substrate are maintained within close proximity (a few mm apart) and heated by an external radiant source. A small temperature gradient is maintained between the source and the substrates. When the temperature of the source is raised to about 550°C, in an inert atmosphere, the CdTe sublimes and is deposited on the conducting substrate, usually in a fine-grain structure. Subsequent heat treatment of the film with a fluxing agent, like CdCI 2, leads to considerable enhancement of grain-size and significant improvement of the electronic properties. The film is then immersed in a chemical bath to deposit the CdS layer as the heterojunction partner. The device is completed by depositing the metallic grid structure by screen printing or by similar low-cost processes. The entire process is relatively simple and amenable to large-scale production without major investment in capital-equipment cost. The process is typical of other competing low-cost, processing techniques that are being seriously considered for commercialization. In spite of this inherent simplicity, the problems in CdTe are somewhat different from those in a-Si technology. Like CIS technology, the manufacturing processes for this technology has to be further developed. Although several companies (Golden Photon, Inc., and Solar Cell, Inc., in the U.S. and BP Solar in the UK) are setting up several MW-scale production plants. It is not clear what kind of manufacturing problems they will encounter. The other concern is the considerable efficiency gap between a one-of-a-kind laboratory cell and the large-area modules (A eff is in the range of 8%-10% absolute percentage point). Considerable development work needs to be done to close this gap in order for this technology to be cost-competitive. In addition to the above, there are two other issues of concern: long-term stability and environmental sensitivity of cadmium. Cadmium-telluride-based solar cells show varying degrees of stability; some show excellent stability while others have poor stability. The stability, although poorly understood, appears to be related to poor contacts, sensitivity to humidity, diffusion of copper, and lack of rigorous control on materials quality. Because some of the cells show excellent stability, it raises hope that the problem is solvable. The toxicity of cadmium and its impact on the environment is another area of concern. At this stage, it is not clear whether disposal of CdTe modules will be classified as hazardous waste. Different countries appear to react differently with regard to the use of Cd-containing material. The appropriate safety procedure can be applied in the work environment to prevent any possible health hazards to workers. It is generally believed that the problem is manageable, particularly because the amount of Cd in thin-film PV modules will remain a small fraction (under 10%) of the world's use of cadmium in numerous other applications.
THIN-FILM SILICON SOLAR CELLS In recent years, we have seen a renewed interest worldwide in thin-film silicon solar cells. It is well-known that thin-film silicon solar cells, with active-layer thicknesses in the range of 10-50 ~, have the potential to achieve efficiencies around 20% with suitable light-trapping. Moreover, the thin-film approach offers major cost advantages in view of reduced material consumption and less stringent requirements for material quality. In many ways, thin-film silicon is an ideal choice, primarily because the technology can borrow heavily from the well-established technology of its single crystal or polycrystalline thick-cell analogue. The validity of this approach has already been demonstrated by achieving over 16% efficiency on epitaxially grown thin film (20 la thick) on a heavily doped, single-crystal substrate. The effort is now directed toward the growth of device-quality thin-film silicon on low-cost substrates and the design and implementation of an effective light-trapping scheme. Significant progress in this direction has been made by AstroPower Corporation through a project funded by DOE under the PVMaT program. 378
W R E C 1996 GaAs THIN-FILM SOLAR CELLS GaAs semiconductors having an optimum direct bandgap of 1.43 eV is a leading material for fabricating very-high-efficiency solar cells. In fact, efficiency over 25% has been achieved on solar cells fabricated in epitaxially grown GaAs on single-crystal substrates. For solar ceils made of GaAs to be used for terrestrial applications, it is necessary to make thin-film devices at a cost that is competitive to other thinfilm options. In the past, considerable success was achieved in this direction by fabricating high-efficiency, thin-film, single-crystal GaAs solar cells in materials grown either on germanium-coated silicon singlecrystals or by using various schemes for reusable single-crystal substrates. Moderate success was also achieved in the early 1980s in fabricating reasonably efficient (~ 11%) polycrystalline thin-film GaAs solar cells in materials deposited on low-cost substrates such as tungsten-coated graphite. Sadly, however, the approach was abandoned too prematurely before its full potential could be realized. In view of the enormous progress that has been made in recent years in the technology of thin-film single-crystal GaAs, it would be inslnactive to revisit the approach of polycrystalline GaAs on a low-cost substrate.
DYE-SENSITIZED TiO 2 THIN-FILM SOLAR CELLS One of the exciting new developments in the low-cost thin-film solar-cell area is the achievement of over 10% efficiency in a dye-sensitized, nanocrystalline thin-film TiO 2 photoelectrochemical solar cell. The device structure is extremely simple. It involves deposition of a highly porous thin film of nanostructure TiO 2 on a transparent conducting substrate. The TiO 2 film, which is normally transparent to most of the solar spectrum, is sensitized by dipping it in a solution containing a photostable inorganic dye. A counter electrode, consisting of a conducting glass coated with tiny amounts of platinum, is positioned in front of the TiO 2 layer, with thin spacers separating the two electrodes. The inter-electrode spacing is then filled with an electrolyte containing a I/l 3-redox couple. The device is completed by hermetic sealing of the whole assembly. The idea is not new since the first U.S. patent on an essentially similar device was issued to this author in 1978. Major innovation in this technology was done recently by Prof. Michael Gretzel in Switzerland, which involves the use of nanostructured TiO z and a stable inorganic dye material. The device has reportedly been cycled over 50 milfion cycles (10 years equivalent life). The cost of such a device is estimated to be around 60¢/Wp.
CONCLUSION The future of thin-film solar cells looks very promising. Major improvements in efficiency, stability, and cost are continuously being made. A multiplicity of options are available, and a clear cost winner is hard to discern at this stage. Industries are gearing up for large-scale production in some of these technologies. It is hoped that in their enthusiasm for commercializing these new technologies, they do have the sticking power to go through the development processes necessary to close the performance gap between the laboratory-scale devices and commercial modules--in other words, not to push some of these technologies too prematurely. If the present trend continues, thin-film solar cells will certainly meet the challenge of the future as the most desirable alternative to the conventional source of power generation.
FURTHER READING A series of recent articles appearing in the journals "Progress in Photovoltaics: Research and Applications," Vol. 3 (1995), published by John Wiley & Sons, Ltd.; 13th NREL Photovoltaics Program Review, AIP Conference Prec. 353, ed. H.S. Ullal and C. E. Witt, 1995; Solar Energy Materials and Solar Ceils, 32, No. 3, March 1994; 24th IEEE PVSC, 1994, Vols. 1 & 2.
379
THIN-FILM SOLAR CELLS: AN OVERVIEW
S. K. DEB National Renewable Energy Laboratory Golden, CO 80401-3393 USA
ABSTRACT Thin-film solar cells offer the most promising options for substantially reducing the cost of photovoltaic systems. A multiplicity of options, in terms of materials and devices, are currently being developed worldwide. Some of the leading contenders are: amorphous and polycrystalline silicon, compound semiconductor thin films such as CulnSe2-based alloys, and CdTe thin-film solar cells. Enormous progress in device performance has been made in most of these technologies, and considerable effort is devoted to commercialization of these technologies. Exciting new developments are happening in some relatively new materials and devices.
KEYWORDS Thinfilms,solarcells,photovoltaics,amorphoussilicon,copperindiumdiselenidealloys,cadmium telluride,thinsilicon,galliumarsenide,titaniumdioxide.
INTRODUCTION The primary objective of the worldwide photovoltaic (PV) solar-cell research and development is to reduce the cost to a level that it will be a viable alternative to conventional ways of generating electric power. To do that, it will be necessary to bring the cost of photovoltaics to about $1.50/Wp at the system level. Today's world PV market of about 80 MW/yr is sold at prices close to $10/Wp which is almost six times the long-term cost goal. PV technology in the marketplace today is dominated by crystalline silicon. It is generally believed that even with greatly increased market and production volume, the price of crystalline or polycrystalline silicon cannot be reduced to meet the long-term cost goal for large-scale power production. In fact, various analyses suggest that thin-film solar cells are the only viable alternative that has the potential to meet this long-term cost goal. If one accepts this rationale, it is instructive to analyze whether thin-film technology can realistically meet these goal. There is reason to be very optimistic, particularly in view of the remarkable progress that has been made in recent years, both in terms of cell efficiency and long-term stability in several thin-film solar-cell technologies. At the present time, thin-film solar-cell research and development involves several materials, such as amorphous silicon (a-Si), polycrystalline thin films consisting of CuInSe2-based alloys and cadmium telluride, thin-film crystalline silicon, and other novel materials and advanced concepts. In this paper, a brief overview of some of these thin-film materials and device technologies will be given.
AMORPHOUS SILICON THIN FILM Among the thin-film PV technologies, a-Si holds the promise for low cost. It is by far the most mature and
375
W R E C 1996 commercially viable technology. Well-defined production processes over very large areas have been implemented. Since the first a-Si solar cell was fabricated by the RCA group in 1976, considerable progress has been made in improving the cell efficiency and module fabrication processes and in reducing overall cost. Worldwide, a-Si devices accounted for about 15% of all PV sales. In 1982, 1.5 MWp of a-Si solar cells were produced primarily for consumer products. Since then, the production volume has grown almost tenfold, and the market has expanded to include terrestrial power generation. The U.S. utilities are showing commitment to this technology by establishing outdoor test facilities. The largest facility is a 400-kWp system by PV for Utility-Scale Applications (PVUSA) in California. Most recently, companies such as Solarex Enron and United Solar are in the process of building 10-MW production facilities. The technology of a-Si is based on two types of device design: one involving single-junction and the other involving multijunction p-i-n cell structure. In the single junction p-i-n cell, the p- and n-type doped layers are made very thin, and the absorber layer is the intrinsic (i) layer. Because of the lower photocarder diffusion length, efficient solar cells require separation of charge carders by the electric field in the i-layer. This puts an inherent limit to the thickness of the i-layer to around 0.5 ~rn and to practically achievable efficiency due to poorer light absorption. Using a thicker absorber layer reduces the efficiency because of poor photocarrier collection. To circumvent this problem, various schemes of light-trapping and enhanced reflection at the contacting interface have been used. It is generally believed that efficiency in a singlejunction device has reached its optimum value around 12%. Therefore, any significant increase in efficiency can only be achieved by using multijunction devices containing as many as three different absorber layers with differing bandgaps. Today, the design of multijunction for high-efficiency solar cells is based on the use of a high-bandgap alloy of a-Si and carbon (a-SiC:H) and a low-bandgap alloy such as silicon and germanium (a-SiGe:H). Modeling of multijunction devices based on a triple tandem cell with 2.0/1.7/1.45 eV bandgap combinations predicts a theoretical efficiency of ~24%. The highest measured initial active-area efficiency to date is 13.7% for a small area of such a cell structure. A great variety of deposition methods have been used to fabricate amorphous-silicon-based alloy materials and devices, but among these, only the radio frequency (RF) and direct current (DC) glow-discharge techniques are being used by industry. Some of the newer techniques, such as ECR, remote plasma-assisted chemical vapor deposition (CVD), and hot-wire deposition, have produced materials with interesting physical properties such as lower defect density, greater diffusion length, and lower hydrogen concentration. Some of these techniques may hold promise for the future. The key research issues that affect the future of a-Si technology are: (1) much lower device efficiency than those predicted by theory, and (2) photodegradation due to the Staebler-Wronski (S-W) effect. The largest improvement in device efficiency will most likely come from improvement of the a-SiGe:H and a-SiC:H alloys. Research has concentrated on developing a-SiGe:H films with bandgap near 1.4 eV. This alloy is still not as good as those with higher bandgaps. Nevertheless, improvement in material and quality devices is constantly being made. Significant improvement in a-SiC:H alloy material has been made by using hydrogen dilution of methane. However, there is room for significant improvement of the materials, particularly for bandgaps greater than 1.9 eV. Current research efforts include establishing a correlation between microstructure and electrical properties; study of different deposition methods to achieve changes in performance, various approaches to improve Voc, fill factor and short-circuit current; innovative device designs to overcome the limitations in the current alloy materials; extensive use of computer simulations to model multijunction devices; and use of poly- or microcrystalline layer for the low-gap absorber layer. Light-induced degradation of a-Si remains as one of the challenging problems needing to be solved. To date, there is no solution to this problem. Although degradation can be completely removed by annealing the device at elevated temperatures, it starts anew after the annealing process. Fortunately, an engineering solution has been found that allows degradation to be controlled at a manageable level. It has been found that a thinner a-Si:H intrinsic layer results in either less or slower degradation, and this approach has become the mainstream strategy for managing the stability problem. The instability issue is intrinsic to aSi films. The effect is manifested in films by a decrease in photoconductivity and an increase in mid-gap defect density. Although many explanations have been proposed, none appears to be completely satisfactory. Two models that have gained much support include: (1) a weak Si-Si bond that is broken by light, and (2) charge dangling bonds that can be converted to metastable neutral dangling bonds by trapping of photogenerated carders. The hot-wire deposition technique, which results in lower hydrogen 376
WREC 1996 content in the film, appears to greatly improve the photostability. However, fabrication of efficient solar cells by this technique still remains a challenging issue. The strategy to reduce the cost of a-Si:H solar cells includes a combination of efficiency increase and unitarea cost reduction. By increasing the stabilized module efficiency to 10% from the present commercial module efficiency of 5% and by reducing the price by 50%, a-Si:H is expected to meet the 12C/kWh cost goal. The PV Manufacturing Technology (PVMaT) project is designed to reduce the cost by 50%. Eventually, the module efficiency has to be around 15% to meet the long-term cost goal of 6C/kWh. The overall market share for a-Si:H technology has been shrinking for several years, from a high of 39% in 1988 to about 16% at the present time. The hope is that this declining trend will be reversed by bringing on-line several 10-MW/yr Capacity plants. The commitment made by the major a-Si PV manufacturers in constructing several 10-MW/yr capacity plants provides strong credibility to the commercial viability of this technology.
COPPER INDIUM DISELENIDE AND ITS ALLOYS (CIS/CIGS) World-record efficiency of 17.7% has been recently achieved by the National Renewable Energy Laboratory (NREL) group by using CIGS alloy films. This is a remarkable achievement that matches the best efficiency of a polycrystalline solar cell. The CIGS thin-film absorber in this cell is fabricated by the physical vapor deposition of the constituent elements onto a heated Mo-coated soda-lime glass substrate. The absorber film is deposited in several stages. First, 2000 ~, of a near-stoichiometric layer of CuGaSe2 (CGS) is deposited at 350°C substrate temperature. The composition of the layer is designed to manage the overall Ga profile by minimizing the out-diffusion of Ga into the bulk and in-diffusion of In from the bulk. The CIGS film was deposited by a single two-stage plx~cess, whereby a Cu-rich CIGS precursor is converted to high-quality PV material by exposure to In, Ga, and Se at 550°C, which enhances the formation of (In,Ga)-rich surface phases. The device is completed by chemical bath deposition (CBD) of about 500 ~ CdS, followed by RF sputtering of 500/~ of intrinsic ZnO and 3000 ,~ of Al-doped ZnO. Nickel-aluminum grid contacts are then deposited with -5% coverage, followed by a 100-nm MgF2 antireflection coating. The 17.7% total-area efficiency represents a new world-record for all thin-film solarcell technologies that are grown non-epitaxially. This new result is an improvement over the previously reported record efficiency of 17.1%. It is reasonable to believe that a further optimization of device structure will lead to a small-area cell efficiency well over 18%. The major challenge before those active in this area is how to close the gap in efficiency between the small. area devices to large-area modules. In 1988, ARCO Solar (now Siemens Solar Industries) made an 11% square-foot module. Since then, no major advances in large-area module efficiency in CIGS-type devices has been reported. Even though the fabrication process for small-area devices is quite forgiving, it is suspected that transferring this process technology for large-area devices is not straightforward. Considerable development effort is necessary before the technology is ready for large-scale production. There is no doubt that this will be accomplished in time and that CIGS thin-film solar cells will be a major contender for large-scale power generation. One of the major strengths of this technology is the remarkable stability shown by these devices. NREL has been testing the ARCO Solar CIS module since November 1988, with no significant degradation during the past 7 years. Good stability and high efficiency are the strength of this technology, while manufacturability of these devices remains an issue. Another area of concern is that the cost of manufacturing CIS devices appears to be higher compared to the manufacturing costs of other competing thin-film technologies. This is largely because of the complexity of various processing steps. There are possible approaches to reducing the cost by developing alternative processes that are non-vacuum and involve simpler processing steps. Besides cost, there is also some concern regarding the availability of materials, such as indium, particularly for large-scale deployment.
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W R E C 1996 CADMIUM TELLURIDE THIN FILM Enormous progress has been made in recent years on thin-film solar cells based on cadmium teUuride. Record efficiencies close to 16% have been achieved in laboratory-scale, small-area devices. One of the major advantages of cadmium telluride/cadmium sulfide thin-film solar cells is the low-cost fabrication options. A number of relatively simple, low-cost methods have been used to fabricate solar cells with efficiencies greater than 10%. Some of the low-cost deposition methods that show promise include: (l) close-space sublimation, (2) spray deposition, (3) electredeposition, and (4) screen printing. All of these techniques are being considered for large-scale manufacturing by several industries. The highest-efficiency cells (~ 16%) are fabricated in small-area devices using the close-space sublimation technique by a group at the University of South Florida. In this process, the source containing CdTe and the conducting substrate are maintained within close proximity (a few mm apart) and heated by an external radiant source. A small temperature gradient is maintained between the source and the substrates. When the temperature of the source is raised to about 550°C, in an inert atmosphere, the CdTe sublimes and is deposited on the conducting substrate, usually in a fine-grain structure. Subsequent heat treatment of the film with a fluxing agent, like CdCI 2, leads to considerable enhancement of grain-size and significant improvement of the electronic properties. The film is then immersed in a chemical bath to deposit the CdS layer as the heterojunction partner. The device is completed by depositing the metallic grid structure by screen printing or by similar low-cost processes. The entire process is relatively simple and amenable to large-scale production without major investment in capital-equipment cost. The process is typical of other competing low-cost, processing techniques that are being seriously considered for commercialization. In spite of this inherent simplicity, the problems in CdTe are somewhat different from those in a-Si technology. Like CIS technology, the manufacturing processes for this technology has to be further developed. Although several companies (Golden Photon, Inc., and Solar Cell, Inc., in the U.S. and BP Solar in the UK) are setting up several MW-scale production plants. It is not clear what kind of manufacturing problems they will encounter. The other concern is the considerable efficiency gap between a one-of-a-kind laboratory cell and the large-area modules (A eff is in the range of 8%-10% absolute percentage point). Considerable development work needs to be done to close this gap in order for this technology to be cost-competitive. In addition to the above, there are two other issues of concern: long-term stability and environmental sensitivity of cadmium. Cadmium-telluride-based solar cells show varying degrees of stability; some show excellent stability while others have poor stability. The stability, although poorly understood, appears to be related to poor contacts, sensitivity to humidity, diffusion of copper, and lack of rigorous control on materials quality. Because some of the cells show excellent stability, it raises hope that the problem is solvable. The toxicity of cadmium and its impact on the environment is another area of concern. At this stage, it is not clear whether disposal of CdTe modules will be classified as hazardous waste. Different countries appear to react differently with regard to the use of Cd-containing material. The appropriate safety procedure can be applied in the work environment to prevent any possible health hazards to workers. It is generally believed that the problem is manageable, particularly because the amount of Cd in thin-film PV modules will remain a small fraction (under 10%) of the world's use of cadmium in numerous other applications.
THIN-FILM SILICON SOLAR CELLS In recent years, we have seen a renewed interest worldwide in thin-film silicon solar cells. It is well-known that thin-film silicon solar cells, with active-layer thicknesses in the range of 10-50 ~, have the potential to achieve efficiencies around 20% with suitable light-trapping. Moreover, the thin-film approach offers major cost advantages in view of reduced material consumption and less stringent requirements for material quality. In many ways, thin-film silicon is an ideal choice, primarily because the technology can borrow heavily from the well-established technology of its single crystal or polycrystalline thick-cell analogue. The validity of this approach has already been demonstrated by achieving over 16% efficiency on epitaxially grown thin film (20 la thick) on a heavily doped, single-crystal substrate. The effort is now directed toward the growth of device-quality thin-film silicon on low-cost substrates and the design and implementation of an effective light-trapping scheme. Significant progress in this direction has been made by AstroPower Corporation through a project funded by DOE under the PVMaT program. 378
W R E C 1996 GaAs THIN-FILM SOLAR CELLS GaAs semiconductors having an optimum direct bandgap of 1.43 eV is a leading material for fabricating very-high-efficiency solar cells. In fact, efficiency over 25% has been achieved on solar cells fabricated in epitaxially grown GaAs on single-crystal substrates. For solar ceils made of GaAs to be used for terrestrial applications, it is necessary to make thin-film devices at a cost that is competitive to other thinfilm options. In the past, considerable success was achieved in this direction by fabricating high-efficiency, thin-film, single-crystal GaAs solar cells in materials grown either on germanium-coated silicon singlecrystals or by using various schemes for reusable single-crystal substrates. Moderate success was also achieved in the early 1980s in fabricating reasonably efficient (~ 11%) polycrystalline thin-film GaAs solar cells in materials deposited on low-cost substrates such as tungsten-coated graphite. Sadly, however, the approach was abandoned too prematurely before its full potential could be realized. In view of the enormous progress that has been made in recent years in the technology of thin-film single-crystal GaAs, it would be inslnactive to revisit the approach of polycrystalline GaAs on a low-cost substrate.
DYE-SENSITIZED TiO 2 THIN-FILM SOLAR CELLS One of the exciting new developments in the low-cost thin-film solar-cell area is the achievement of over 10% efficiency in a dye-sensitized, nanocrystalline thin-film TiO 2 photoelectrochemical solar cell. The device structure is extremely simple. It involves deposition of a highly porous thin film of nanostructure TiO 2 on a transparent conducting substrate. The TiO 2 film, which is normally transparent to most of the solar spectrum, is sensitized by dipping it in a solution containing a photostable inorganic dye. A counter electrode, consisting of a conducting glass coated with tiny amounts of platinum, is positioned in front of the TiO 2 layer, with thin spacers separating the two electrodes. The inter-electrode spacing is then filled with an electrolyte containing a I/l 3-redox couple. The device is completed by hermetic sealing of the whole assembly. The idea is not new since the first U.S. patent on an essentially similar device was issued to this author in 1978. Major innovation in this technology was done recently by Prof. Michael Gretzel in Switzerland, which involves the use of nanostructured TiO z and a stable inorganic dye material. The device has reportedly been cycled over 50 milfion cycles (10 years equivalent life). The cost of such a device is estimated to be around 60¢/Wp.
CONCLUSION The future of thin-film solar cells looks very promising. Major improvements in efficiency, stability, and cost are continuously being made. A multiplicity of options are available, and a clear cost winner is hard to discern at this stage. Industries are gearing up for large-scale production in some of these technologies. It is hoped that in their enthusiasm for commercializing these new technologies, they do have the sticking power to go through the development processes necessary to close the performance gap between the laboratory-scale devices and commercial modules--in other words, not to push some of these technologies too prematurely. If the present trend continues, thin-film solar cells will certainly meet the challenge of the future as the most desirable alternative to the conventional source of power generation.
FURTHER READING A series of recent articles appearing in the journals "Progress in Photovoltaics: Research and Applications," Vol. 3 (1995), published by John Wiley & Sons, Ltd.; 13th NREL Photovoltaics Program Review, AIP Conference Prec. 353, ed. H.S. Ullal and C. E. Witt, 1995; Solar Energy Materials and Solar Ceils, 32, No. 3, March 1994; 24th IEEE PVSC, 1994, Vols. 1 & 2.
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