SPECIAL REPORT
Photovoltaic Cells New technologies are moving from lab to manVetplace Kenneth Zweibel, Solar Energy Research Institute
Photovoltaic cells, which have been used to produce electricity in space for more than two decades, are one of the most promising and practical alternatives now being developed for supplementing—or even replacing—conventional fuels for generating electricity in large-scale power plants. Recent megawatt demonstration projects show that computerized photovoltaic installations, operating with little or no human supervision or maintenance, can generate electricity during more than 30% of the time they are in place. Their operation, moreover, is well matched to the summer peak-load demand for air conditioning. In addition, photovoltaic capacity can be increased as needed, since megawatt facilities can be built in less than a year, thus relieving public utility planners of the problem of forecasting demand for electricity decades in advance. Many people do not realize how close photovoltaic cells are to widespread practical use. A new generation of technologies now is graduating from the laboratory into the marketplace. Cost reduction is the last major hurdle limiting their expansion into the public utility market. But substantial progress has been made in this 34
July 7, 1986 C&EN
area during the past decade through the cooperative research and development activities of industry, universities, and the various programs of the Department of Energy, including those of Solar Energy Research Institute (SERI) in Golden, Colo. The new photovoltaic technologies involve a departure from the traditional crystalline silicon materials long used to power communications satellites. They are based on thin films, which require much less material to absorb sunlight and thus provide the opportunity to reduce costs. Those thin films must be deposited as coatings on a substrate carrier to be fabricated into modules of photovoltaic cells packaged into a unit. Proper packaging is essential for long life in the field; today's photovoltaic modules usually carry a 10-year warranty and are designed to operate for 20 to 30 years. New photovoltaic technologies are based on four major types of materials: • Amorphous silicon (a-Si). • Copper indium diselenide (CuInSe2). • Cadmium telluride (CdTe). • Gallium arsenide (GaAs). Each of those categories actually covers a range of materials that can be tailored for specific electrical and optical characteristics to maximize electrical output while minimizing costs. All of the materials are being considered for "flat-plate" applications, in which the photovoltaic module intercepts sunlight while mounted on a support structure, which itself may be tracking the sun's path across the sky. Gallium arsenide and thick crystalline silicon also are being developed for "concentrator" applications, which use optical lenses or reflectors to focus direct sunlight on the photovoltaic device. This tactic permits the cost of relatively expensive solar cells to be counterbalanced by less expensive optics. Another strategy for enhancing photovoltaic technology is based on so-called cascade or multijunction cells. In that approach, cells are stacked atop one anoth-
Workers use a crane to moun t photovoltaic panels and reflectors on a steel support at Arco Solar's Carrisa Plains project in San Luis Obispo County, California. The panels, which track the sun, use single-crystal silicon cells to generate electricity
er so that the sun's spectrum is used more effectively. For example, the top cell can be designed to collect blue light and transmit red light to a device that is more efficient in converting red light to electricity. The two most important variables determining a photovoltaic module's economic competitiveness are its cost per square meter and the efficiency with which it converts sunlight to electricity. Some solar cells based on single-crystal GaAs or silicon can convert sunlight into electricity with efficiencies of more than 25%. The conversion efficiencies of many of the new thin-film photovoltaic cells are still close to 10%. But low-efficiency photovoltaic cells actually may produce electricity more cheaply than high-efficiency cells if they cost less. The relationship between output electricity cost and a photovoltaic system's performance is complicated by expenses that are not directly associated with the solar cell. Those include costs required for support struc tures, sun-trackers, electrical interconnections, and dcto-ac power conversion. Those costs, plus the cost of the array's required land area, are called the balance-ofsystem costs. They are nearly proportional to the array area and give an additional incentive for high efficiency. The goals set by DOE's recent five-year research plan can be used as ball-park figures for judging the relative merits of photovoltaic technologies. DOE's goals for cost-competitive photovoltaic electricity generated by flat-plate systems call for conversion efficiencies of 13 to 17% and module costs of $40 to $75 per sq m. Its goals for concentrator systems require module conversion efficiencies of 23 to 29% and module costs of $90 to $160 per sq m. Those goals, developed jointly by DOE and industry, are optimistic but achievable.
phous silicon cells were made in 1974 at RCA Corp. In 1981, the technology had 3% of the photovoltaic mar ket; now it has 32%. Most of the rest of the market is accounted for by crystalline silicon. Amorphous silicon differs physically from crystal line silicon in that amorphous silicon has no lattice structure. When bonded with hydrogen or fluorine for improved electrical properties, amorphous silicon is two orders of magnitude more light-absorbing than crystalline silicon. Hence, it can be made into very thin films (0.5 μπι) for solar cells. Crystalline silicon cells are typically 100 to 300 μπι thick. The cost reduction result ing from this two-orders-of-magnitude difference in thickness is the key advantage of a-Si. Amorphous silicon has attracted a great deal of atten tion in the U.S. and Japan. Photovoltaic cells based on this material are used in such consumer items as calcu lators, watches, and clocks. Many researchers are inter ested in developing nonphotovoltaic uses for the mate rial, such as xerography drums and thin-film transis tors. Large-area solar cell modules of a-Si have been fabri cated during the 1980s by various U.S. and Japanese companies. Some firms are developing continuous-roll
Amorphous silicon Amorphous silicon is the first of the new technol ogies actively to enter the marketplace, with last year's production equivalent to about 8 MW. The first amor-
Amorphous silicon modules made by Energy Conversion Devices in con tin uous sheets on flexible substrates July 7, 1986 C&EN
35
Special Report
Photovoltaics: converting sunlight to electricity Photovoltaic cells are semiconductor devices that transform light energy continuously into direct-current electricity. The smallest-unit photovoltaic device is called a cell. Several electrically interconnected cells are called a module and may range in area from about 100 to 10,000 sq cm. When photovoltaic devices are used to generate power, many modules are wired together into an array. Photovoltaic cells have been used in space since 1958 to power orbiting satellites. A proven photovoltaic technology now exists for such applications. The cells used in space are highly efficient at converting incident sunlight into electricity, but they are quite expensive. (Efficiency is defined as the cell's energy output, divided by the energy of the sunlight incident on the cell.) Despite their high efficiency (close to 2 0 % ) , the energy they produce is too costly to be practical on Earth. In one sense, the entire research pro-
gram for developing solar cells to produce electrical power on Earth has been an effort to take the proven but expensive space technology and make it cheap. To do so, the cost of the output electricity must be reduced several orders of magnitude. To help achieve that goal, entirely new photovoltaic technologies have been invented in the past 10 years. The principles by which solar cells convert light into electricity are easy to understand. Semiconductors can have their electrical properties dominated by free charges of either the negative (electrons) or positive (holes in the valence band) type. When two semiconductors dominated by opposite electrical charges are in contact with one another, free charge leaks across their common boundary and becomes fixed as ions in the regions adjacent to the common interface. The fixed but opposite ions produce at the interface a local region with an electric field. That electric field sends
substrates consisting of stainless steel (Energy Conversion Devices in Troy, Mich.) or polyimide (3M Co.) on which a-Si can be deposited by a method called radiofrequency glow discharge. Production of high-quality material in large areas (1 sq ft and greater) has been the most important development in recent years in a-Si technology. This advance has opened the way to true thin-film solar modules. Several problems remain, however, before a-Si can be used for generating electric power. Higher sunlight-to-electricity conversion efficiencies and lower production costs must be attained and the instability of present devices overcome. Relatively low efficiency always has been a problem for the new thin films. The more mature but expensive materials (crystalline Si and GaAs) have impressive efficiencies (more than 20%) but high costs. The thin films are tailored to provide reasonable efficiencies at low cost. As a rule of thumb, a thin film with an efficiency of about 10% can be considered a serious contender. However, the DOE goals indicate that, unless module cost can be driven below $40 per sq m, efficiencies of about 15% will be needed for economical power production. Progress in improving the efficiency of a-Si cells has been impressive. Since 1974, more than a dozen research groups have developed a-Si devices with 10% efficiency. The most successful a-Si producers in the U.S. are Arco Solar of Chatsworth, Calif.; Solarex Corp. of Rockville, Md.; Chronar Corp. of Princeton, N.J.; and Energy Conversion Devices. Arco Solar recently 36
July 7, 1986 C&EN
free electrons one way and free holes the other. Normally, no current flows in this diodelike device. If light shines on the device, electrons are freed from their bound states in the valence band and move into the conduction band. The light-generated free charges can then be separated by the built-in electric field. Since the light-generated electrons and holes are continuously separated in opposite directions, a current can flow if the solar cell is illuminated and electrically connected to an external load. The entire process of designing a photovoltaic technology boils down to selecting appropriate semiconductors to produce a strong, homogeneous built-in field; making sure the semiconductors can absorb as much sunlight as possible; designing the cell so that the sunlight can be absorbed very close to or in the electric field region; and fabricating the device so that its electric properties favor good performance. The success of
introduced several large-area (1 sq ft and 4 sq ft) a-Si modules commercially. Energy Conversion Devices claims the highest efficiency (13%), with its three-junction cascade cell. In theory, a cascade cell design should convert light to electricity more efficiently because cascade cells can use the solar spectrum more efficiently
Facilities at Energy Conversion Devices produce photovoltaic modules based on amorphous silicon
Solar cells have a built-in electric field that separates light-generated charges and creates a current Sunlight
Positively ionized donors
Negatively ionized acceptors
Semiconductor A, containing a high concentration of free electrons
Semiconductor B, containing a high concentration of free holes
photovoltaic devices depends partly on achieving good sunlight-to-electricity conversion efficiencies. Another parameter of major interest is cost. Because only roughly 1000 watts per sq m of sunlight (depending on the location) falls on Earth's surface, innova tive means of making cells in very large areas cheaply must be found. Not many semiconductor devices can be made for $50 to $200 per sq m, but those are the goals of cost-effective photovoltaics.
Fixed electric field region (junction)
The need for cheap cells has spawned the so-called thin-film and concentrator technologies. These are strategies for producing cells inexpensively either by making the cells so thin that they require little cost for raw materials or process ing, or by using cheaper lenses to con centrate sunlight on small-area cells. Those two approaches form the bulk of the efforts to design cost-effective pho tovoltaic systems for generating com mercial electricity.
than can individual cells. Thin-film amorphous silicon was used in the first cascade cells. Stacked amorphous silicon cells are relatively easy to make because the different semiconductor layers can be deposited sequentially with only minor variations in composition from one layer to the next. Fabrication temperatures are all similar (about 200 to 250 °C) and are low enough to avoid inducing interdiffusion dam age to the various layers. Several research groups are pursuing a multijunction strategy to improve amorphous silicon efficiencies. One of the key research goals is to adjust the various component subcells to different parts of the solar spec trum. The threshold energy at which solar photons are usable in a photovoltaic material is called the material's bandgap. Photovoltaic materials absorb light whose energy is greater than their bandgap. They are trans parent to light with energy below their bandgap. Amorphous silicon absorbs light with energy above about 1.75 eV. By adding tin or germanium to a-Si, its bandgap can be reduced. Amorphous silicon alloy devices have been made with bandgaps as low as 1.5 eV. In fact, a three-junction cascade cell made by Energy Conversion Devices in cludes a high-performance a-Si:Ge bottom cell contain ing about 40% germanium. Development of even lower-bandgap a-Si alloy cells is important for the future development of high-efficiency (more than 15%) cas cade cells based on amorphous silicon. A number of companies are producing large-area amorphous silicon devices in batch processes or on
A final parameter of importance for photovoltaic cells is long-term durability. Most photovoltaic arrays for making electricity will require lifetimes of 20 to 30 years. Actually, existing photovoltaic demonstration projects have shown that megawatt arrays can work for years al most without human supervision. The potential for 30-year, maintenance-free electricity is a powerful plus for photo voltaics. But the new technologies are only now emerging from lab-scale develop ment, and experience in the field will be required before they can demonstrate dependable long-term performance. Even mature photovoltaic technologies based on crystalline silicon cells origi nally had problems out of doors. It may be assumed that the newer thin-film and concentrator technologies will have to pass through a gestation period before demonstrating the rock-stable perfor mance of currently available crystalline silicon arrays.
continuous roll-to-roll coaters. These advances are leading to superior photovoltaic module design and to large-scale processing. They are important steps in demonstrating the overall efficacy of the thin-film ap proach. However, because a-Si is produced in low vol ume, costs (about $400 per sq m) are still much too high. Production processes will have to be optimized and automated to allow costs to approach $50 per sq m. Existing fabrication methods must be improved to in crease fabrication rates and to make large-scale module production possible. Although several laboratories have reported making amorphous silicon devices that are stable, most groups working with this material observe an initial change in its electronic properties upon exposure to light. The change levels off within several weeks of exposure, after which efficiency is constant for periods exceeding three years. Several competing models are now being considered to explain the kinetics of the light-induced effect. Thus far, no clear explanation has emerged. In a practical sense, the cell's performance is stabi lized in devices that are thinner than 0.5 μπ\ or that include electronic compensation to preserve a strong electric field across the device's entire thickness. Those approaches are encouraged by the cost reduction asso ciated with reduced raw material requirements and with cascade structures that are necessarily thin to opti mize light transmission and efficiency. Recent progress in stabilizing a-Si devices is con vincing many skeptics of the long-term potential of the technology. That is evident in the joint effort of July 7, 1986 C&EN 37
Special Report Chronar Corp. and Alabama Power & Light Co., which have built a 100-kW a-Si demonstration power plant near Birmingham, Ala. In addition, Chronar Corp. and Southern Co., a large utility holding company, have agreed to build a facility to make a-Si modules capable of producing a total of 1 MW of electricity annually. Amorphous silicon is the leading thin-film photovoltaic material. It has been used to produce the largest-area, most uniform semiconductor coatings in the world. Commercial photovoltaic devices have been made in the U.S. and Japan for consumer electronics and in larger-scale power packs for lighting and battery charging. A near-term market for amorphous silicon is in automobile sunroofs to circulate air and cool parked vehicles. But the interest of power companies demonstrated by the joint programs of Chronar with Alabama Power & Light and Southern Co. shows that aSi is becoming an option for public utilities.
Copper indium diselenide Copper indium diselenide (CuInSe2) is another important new thin-film solar cell material. Boeing Aerospace Co., with SERI/DOE funding, pioneered work on the thin-film CuInSe2 cell. In June 1981, a Boeing team, led by Reid A. Mickelsen and Wen Chen, used it for the first 10%-efficient thin-film cell verified by SERI's rigorous cell-testing methods. The technology has progressed rapidly since then, with Boeing reporting 11.9% cell efficiency and Arco Solar reporting 11.2%. Laboratories at SERI and at the Institute of Energy Conversion (University of Delaware) also have report-
Thin films of amorphous silicon can be fabricated into modules of lame area Metal or oxide Amorphous , silicon
/
Transparent conductive oxide
Glass Thin-film techniques involving materials such as amorphous silicon permit large areas to be made without expensive steps to interconnect individual small cells. An amorphous silicon module can be produced by depositing a layer of transparent conductive metallic oxide on glass, etching grooves in the oxide, depositing and patterning the amorphous silicon layers, and then depositing a back contact made of metal or conductive oxide. The back contact of one cell can be made to touch the front contact of its neighbor, so that current can flow between them. This resulting connection of cells in series allows cell voltages to add to one another, while the current is eaual to that of one thin cell 38
July 7, 1986 C&EN
ed state-of-the-art CuInSe2 efficiencies. These efficiencies for single-junction devices are as good as or better than any achieved by a-Si technology. Until two years ago, the biggest barrier to wider use of CuInSe2 was the lack of a potentially low-cost, largescale fabrication method. Vacuum evaporation of copper, indium, and selenium had been used to make all of the best CuInSe2 cells. Although evaporation still has strong advocates at Boeing and at the Institute of Energy Conversion, which has demonstrated uniform evaporation of cadmium sulfide onto continuous-roll substrates, serious corporate support for commercializing CuInSe2 emerged only when Arco Solar pioneered a potentially lower-cost, nonvacuum method. The development of a low-cost method for fabricating high-quality CuInSe2 dates back to 1979, when Steven P. Grindle and Charles W. Smith of the University of Maine developed a promising method of sputtering metallic copper and indium and transforming them to copper indium disulfide (CuInS2) by annealing with hydrogen sulfide. A similar method was used by J. J. M. Binsma and H. A. Vander Linden of Catholic University in Nijmegen, the Netherlands, in 1981. They deposited copper and indium by molecular-beam epitaxy and then transformed the metals into CuInS2 with hydrogen sulfide. In the 1980s, an Arco Solar team headed by Vijay K. Kapur (now with International Solar Electric Technology in Ingle wood, Calif.) invented and patented a similar method for making CuInSe2, using hydrogen selenide in place of hydrogen sulfide. Arco Solar is now making high-efficiency CuInSe2 cells experimentally and is planning a pilot facility for producing commercial modules. Another company investigating a selenization process for making CuInSe2 is International Solar Electric Technology, whose work in this area is supported by SERI. In the company's method for producing this material, a layer of copper about 2000 A thick is first plated from an aqueous solution of copper sulfate onto an electrode made of molybdenum-coated glass. Then, a 4000-A layer of indium is plated on the copper from an aqueous solution of indium sulfamate. The plating steps, which take less than a minute, are much faster than competing processing methods for making CuInS2 and thus permit significant cost savings. The plated layers of copper and indium are transformed into CuInSe2 by annealing them for about two hours at about 400 °C in an atmosphere of hydrogen selenide and argon. Films made by International Solar Electric Technology that have been completed as cells at SERI have yielded active-area efficiencies of about 9%. While efforts are under way to build practical largearea CuInSe2 modules, work also is being done to increase cell efficiency. An efficiency of about 15% probably will be needed to produce cost-effective power. The efficiency of CuInSe2 can be improved by broadening its response to the solar spectrum. At present, most devices use a top layer of cadmium sulfide (CdS) to form the heterojunction window. This blocks about 15% of the available solar spectrum. The CdS layer can
be reduced or entirely replaced by a more transparent semiconductor. Arco Solar has reported using a very thin CdS layer (about 200 A thick) and a transparent zinc oxide conductor on top to collect the current that is generated in the CuInSe2 cell. Operating efficiency also can be enhanced by increasing the cell's open-circuit voltage. Typical voltages are about 400 mV. The highest reported voltage in a thin-film polycrystalline CuInSe2 device is 487 mV, achieved by Arco Solar in 1985. Experience with other solar cell materials suggests that voltages approaching 550 mV will be realized in the future on a routine basis. Improving the compositional uniformity and crystallite size are being investigated for increasing both the current and voltage. Also under study are alloys of CuInSe2 with gallium or sulfur to raise the bandgap and enhance the voltage output. Efficiency also can be improved by the multijunction strategy. Last year, as Charles F. Gay, vice president for R&D at Arco Solar points out, his company used a highperformance CuInSe2 device as a bottom cell in a cascade unit with an a-Si top cell to produce the highest reported efficiency yet (13.1%) for any all-thin-film device. Copper indium diselenide devices have passed a variety of tests confirming their inherent stability. Both Boeing Aerospace and SERI have shown that CuInSe2 cells can be exposed to light for 9000 hours without degrading. To date, outdoor testing has been minimal, because few encapsulated devices have been available. In the past, concern has been expressed about the cost of CuInSe2 because of its indium content. Actually, not only is the cost of indium for these cells small (about $1.00 per sq m), but the amount needed is only about 25 kg per MW. Indium is about as plentiful as silver. Almost all indium consumed today is a by-product of zinc recovery. But other sources would become available if the market for indium were to expand. In summary, CuInSe2 technology has emerged as one of the most powerful new thin-film materials. It offers high efficiencies, permits relatively simple large-area fabrication, and is stable. Arco Solar plans to build a pilot plant for making CuInSe2 panels. The U.S. now dominates this technology because progress has been almost exclusively the result of direct DOE /SERI support. As its success is becoming better known, interest in it in the U.S. is growing. Moreover, recent reports suggest that the Japanese have begun serious efforts to catch up.
Cadmium telluride Another potentially successful polycrystalline material for thin-film solar cells is cadmium telluride (CdTe). Several research groups claim about 10% efficiency for CdTe solar cells made by a variety of lowcost methods. Eastman Kodak led the development of thin-film CdTe using a film-fabricating method called closespaced sublimation. In this procedure, a bulk CdTe source material is placed close to a conductively coated glass substrate. When the CdTe is heated and a thermal gradient is set up between the source and the substrate,
Scientists at International Solar Electric Technology examine an electroplated copper/indium film before it is converted to CuInSe2 for a photovoltaic cell the CdTe sublimates and is deposited on the substrate. In 1984, Yuan-Sheng Tyan of Kodak reported making 11%-efficient CdS/CdTe cells in this way. Matsushita Co. in Japan has developed a method for screen printing and sintering CdTe. Last year, the company reported making a 5%-efficient 0.5-sq-m CdTe module and a 12.8%-efficient 1-sq-cm laboratory cell. This year, Matsushita signed a contract with Texas Instruments to supply CdTe cells for calculators at a rate of a million units a month. Those cells are the first commercial CdTe products. Monosolar, a small Los Angeles company now owned by Ohio's Standard Oil, has developed allelectrodeposited CdS /CdTe and CdS/Cdi_xHgxTe cells that are more than 10% efficient. The company's research group, led by Bulent M. Basol (now with International Solar Electric Technology), developed HgCdTe cells as an alternative to CdTe to improve stability. The electrodeposition technology for CdTe' was transferred to Standard Oil and then to British Petroleum, which now has majority ownership of the American oil company. Recently, Arco Solar announced a 10.5%-efficient 4-sq-cm Sn02/CdTe cell made by close-spaced sublimation. In that cell, SnÛ2 replaces the more frequently used CdS and allows more photons to reach the active portion of the device. Ting Li Chu at Southern Methodist University has made 10.5%-efficient CdTe cells by sublimation. John Jordan of Photon Energy in El Paso, Tex., has developed a proprietary method for making CdTe and is investigating the fabrication of 1-sq-ft panels. Peter Myers of Ametek in Paoli, Pa., recently verified at SERI the performance of a 10.4%-efficient CdTe cell made by electrodeposition. The multitude of techniques available for making high-efficiency CdTe cells is the technology's greatest strength. July 7, 1986 C&EN
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Special Report The greatest weakness of CdTe technology is the difficulty in electrically contacting the material, as well as an associated instability in cell performance. CdTe used in solar cells is intrinsically difficult to control electrically; it tends to be resistive rather than highly conductive. Yet, contacting CdTe requires a highly conductive layer. Chemical methods have been developed to alter an exposed CdTe surface to make it more conductive, but the resulting contacts are not always stable. Usually, an extrinsic impurity, such as copper, is applied to the CdTe to make it more conductive. The copper atoms, however, tend to diffuse and ruin the cell junction. The advantages and problems of CdTe technology can be summed up as follows: Many potentially lowcost methods exist for making 10%-efficient CdTe solar cells, and some research groups have demonstrated the effectiveness of square-foot-sized modules. The problem of stabilizing the back contact has hampered this technology, but Matsushita has recently demonstrated the reliability of its CdTe modules by outdoor stability testing. The Japanese company's sale of CdTe cells for Texas Instruments' calculators moves CdTe into direct competition with a-Si and CuInSe2 for the thin-film market.
Concentrator systems In photovoltaic concentrator systems, lenses focus sunlight on small-area solar cells. The use of relatively low-cost lenses reduces the need for potentially expensive solar cells. Since the solar cells are only a small portion of the total cost of a concentrator system, more elegant and expensive cells can be used. Several highly efficient solar-cell technologies that would be prohibitively costly in flat-plate applications are good candidates for concentrator systems. Actually, most solar cells improve in performance as more sunlight is focused on them. Solar cells that are 20% efficient under normal sunlight (one sun) may become 25% efficient under sunlight magnified to 500 suns. This presumes that the cells are kept reasonably cool (about 50 °C) by a heat sink, since high temperatures severely reduce conversion efficiency. Although concentrator cells can function more efficiently than their flat-plate counterparts, they have some disadvantages. For example, the best focusing lenses may absorb or reflect 5 to 10% of the incident direct-beam light. In addition, diffuse sunlight (the blue light from the sky), which makes up about 20% of the available solar energy, is not usable because concentrators cannot focus it. Thus, any comparison between flat plates and concentrators must take into account the difference in usable sunlight. This problem is further compounded by the fact that concentrators cannot use the diffuse light produced on cloudy days. Flat plates, on the other hand, can generate a significant portion of their usual output even on cloudy days. Studies of concentrator systems have shown that losses caused by clouds and indirect sunlight are significant (about 40% on an annual basis). Consequently, concentrators may be competitive with flat-plate units only in sunny locales, such as in the 40
July 7, 1986 C&EN
Southwest, where losses from clouds are generally minimal. Even in the desert, concentrator systems would have to be carefully protected from output variability (possibly severe) that would result from clouds obscuring part of a photovoltaic array. The promise of concentrators is that cells may become available with efficiencies high enough to outweigh the losses resulting from unusable sunlight. The leading concentrator cells are made from single-crystal silicon or gallium arsenide (GaAs). These are very high-performance and stable devices with efficiencies in the 20 to 26% range. Single-crystal silicon solar cells are perhaps the bestunderstood devices in photovoltaics. They perform well and stably outdoors. Their disadvantage always has been their relatively high cost. Nevertheless, production of large areas of single-crystal silicon for flat plates is an important segment of the current photovoltaic industry. When single-crystal silicon devices are used in concentrator cells, the difficulties of large-area production and high cell costs are reduced. Very high efficiencies have been achieved by silicon concentrator devices. This year, Richard M. Swanson and coworkers at Stanford University demonstrated 27.5% efficiency for such a device under 500-sun concentration. They developed a special design called the point-contact cell to achieve this record result. Continued work in this area is supported by Electric Power Research Institute in Palo Alto, Calif. In another very successful effort to demonstrate high-efficiency single-crystal silicon devices, Martin A. Green of the University of New South Wales, Australia, developed a 20%-efficient cell, measured at one sun. Theoretical work done to explain the loss mechanisms in silicon concentrator devices has led to very sophisticated cell designs. The performance of this technology probably will continue to improve. Single-crystal GaAs technology is close to that involving silicon in terms of the sophistication of the concentrator systems. Several groups have made GaAs cells with efficiencies of more than 20% under normal sunlight. Varian Associates has developed a GaAs cell with 26% efficiency under nearly 800 suns. The potential peak efficiency of the GaAs technology is about 30%. Concentrators already have shown efficiencies approaching 30%. One can assume that an actual module efficiency of about 20% (after reduction for the previously mentioned losses) will soon be demonstrated. Actually, the long-term promise of concentrators can be greatly enhanced by using cascade cells. That may ultimately allow them to achieve efficiencies in the 40% range. Electric Power Research Institute, which considers concentrators the leader among the various solar-cell technologies, is championing them for use by public utilities. Cascade cells can be designed for either flat-plate or concentrator systems. The concept of a cascade cell is simple: Stack two or more solar cells on top of each other and use the light passing through the top cell in the lower ones. Theoretically, any cell can be improved
Solar cells are built in several designs The design of photovoltaic cells varies to counteract the limitations in the cell material. The four basic designs are: homojunction cells (using crystalline silicon), heterojunction cells (CulnSe2, CdTe, GaAs), p-i-n cells (amorphous silicon), and cascade combinations of these structures Homojunction cell
Sunlight Front contact
Front contact
η-Type Si Junction region p-Type Si
Homojunction cells are formed by altering a single material (in this case, crystalline silicon) so that one side is dominated by holes (p-type) and the other by electrons (η-type). The junction is located so that maximum light is absorbed in the semiconductor. Free electrons and holes created by light deep in the silicon diffuse to the junction and separate to produce a current if the silicon is a high-quality single-crystal material
Back contact Heterojunction cell
Sunlight
Transparent conductive window Heterojunction partner
η-Type Sn0 2 η-Type CdS
Junction region
p-Type CdTe
Absorber
p-l-n cell
Sunlight
p-Type a-Si
"Intrinsic" semicon ductor (resistive)
i-Type a-Si
Electric field region
η-Type a-Si
Cascade cells
Cell!
Tunnel junction Cell 2
In a heterojunction cell, two different semiconductors (for example, CdS and CdTe) form the junction. This type of structure frequently is chosen for producing cells made of thin-film materials that are much more light-absorbing than silicon. The use of a high-bandgap heterojunction partner is desirable because it allows most of the light to be absorbed in or near the junction, rather than being absorbed at the top of the cell
Sunlight
The p-i-n structure can be used to make efficient cells with amorphous silicon. An electric field set up between the pand η-type regions stretches across the middle "intrinsic" resistive region. Free electrons and holes generated in the intrinsic region are separated by the electric field. Because amorphous silicon has many atomic-level electrical defects when it is highly conductive, very little current would flow if an a-Si cell had to depend on diffusion. But in a p-i-n cell, current flows because the free electrons and holes are gen erated within the influence of an electric field Sunlight
Sunlight
n-Type
Front contact High-bandgap window
p-Type
— High-bandgap absorber —
Highly conductive p-type Highly conductive n-type η-Type f p-Type Back contact
High-bandgap window
Sunlight
n-Type P-Type Transparent back contact Front contact
Low-bandgap absorber
Two-terminal cascade cell (equal current flows in both cells)
n-Type p-Type Back contact Four-terminal cascade cell (current from each cell flows to separate circuit)
In a cascade structure, high-energy photons are absorbed in a cell that is tuned to high energies, and low-energy photons in a cell that is tuned to low energies. This results in more efficient use of the solar spectrum. Each component cell uses essentially all of the light above its bandgap, thereby producing power proportional to the number of photons times a voltage that is a frac tion of its bandgap
by adding another one beneath it. But the additional power provided by the lower cells must be worth the extra cost. Often it is, because an improvement in effi ciency can more than pay for an increase in cost. The cascade system is based on the fact that cells built
of different materials absorb different parts of the solar spectrum. For instance, amorphous silicon absorbs light with energy above 1.75 eV, and CuInSe2 absorbs light above 1.0 eV. Thus, a CuInSe2 cell placed beneath an amorphous silicon cell will absorb solar photons July 7, 1986 C&EN
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Efficiency of cascade cells increases as cell bandgaps approach 1.0 and 1.75 eV ! Bottom-cell bandqap, eV
1.50
Efficiency
1.25
1.00
0.75 1.50
1.75 2.00 Top-cell bandgap, eV
2.25
2.50
The above diagram indicates how a cascade cell improves in efficiency for converting sunlight to electrical energy as the bandgap of the top cell approaches 1.75 eV and the bandgap of the bottom cell approaches 1. For four-terminal cells, maximum efficiency occurs over quite a broad range, from about 1.6 to 1.85 eV for the top cell and about 0.9 to 1.1 eV for the bottom cell. The efficiency data shown here were calculated by John C. C. Fan while at Massachusetts Institute of Technology's Lincoln Laboratory
with énergie» Between 1.0 and 1.75 eV. The number of photons in this range is about half the number that a CuInSe2 cell would use if the amorphous silicon were not obscuring the rest of the spectrum. Since a solar cell's power is approximately proportional to the number of incident photons, a CuInSe2 cell under an amorphous silicon cell provides about half its normal power. Thus, if the efficiency of each cell separately (fully exposed to normal sunlight) is 10%, combining them achieves 10% from the top cell and 5% from the bottom cell to attain 15% as a cascade cell. The resulting additional power may easily pay for itself in savings because balance-of-system costs in an actual photovoltaic array depend heavily on module efficiency. The combination of two cells into one cascade structure can be cost-effective even when the separate cells are not. John C. C. Fan, now at Kopin Corp. in Taunton, Mass., has calculated that maximum efficiencies for a two-cell cascade combination require a top cell with a bandgap of about 1.75 eV and a bottom cell with a bandgap of about 1.0 eV. Such bandgaps depend on specific cell characteristics, but these figures give a general idea of the requirements. Many materials have those bandgaps, and the composition of others can be altered to achieve them. Cascade cells can be designed in a variety of ways. One major variable is the manner in which the top and bottom cells are "wired" together. Two or more cells in a cascade structure can be connected to each other elec42
July 7, 1986 C&EN
trically, or their individual outputs can be sent to separate circuits. Both methods have been used extensively. Stacked solar cells can easily be connected electrically in series by making a transparent electrical contact between them. Such contacts can be achieved with highly conductive, transparent oxide semiconductors, such as indium oxide (Ιη 2 θ3), zinc oxide (ZnO), and tin oxide (SnOz). Electricity can pass between the cells, and sunlight also is able to reach each of them. This is called a two-terminal cell because it has one contact on the top cell and another on the bottom cell. A four-terminal cell can be built if the two stacked cells are electrically isolated by a transparent insulator and if each cell has its own top and bottom electrical contacts. When exposed to sunlight, two- and four-terminal cascade cells behave differently. In a two-terminal cell, optimal performance occurs when the light-generated current in both the top and bottom cells is equal. If light-generated current in one cell is more than in the other, the total output current is limited by the smaller of the two. Hence, optimal performance requires care ful matching of the current of the component cells. Even so, the outdoor performance of two-terminal cells can be impaired when spectral conditions differ from those to which the device is tuned. Cells that are perfectly matched (with equal current in the top and bottom layers) at noon on a dry day will not be perfect ly matched at other times of the same day or at noon on a humid day. The solar spectrum varies with atmo spheric conditions, and the top and bottom cells respond differently. In most sunny climates, the losses resulting from normal climatic variations cause the performance of a two-terminal structure to be about 5% less (on an annu al basis) than for a similar four-terminal cascade cell. Four-terminal cascade cells do not require currentmatching between cells, and they are much less sensi tive to spectral variations. Both structures have their advocates. Usually, the choice between them is based on engineering consid erations. For example, because it may be easier or cheaper to make a two-terminal cell, it may be favored, despite the approximately 5% loss in performance.
Cascade concentrator cells A combined cascade/concentrator cell design is a natural choice for reasonably priced power generation because the cascade cell's higher cost per unit can be offset by higher efficiency and the lower cost of lenses. High-performance cascade-cell combinations using single-crystal materials, such as gallium arsenide alloys and silicon, are promising candidates for a concentra tor system. A number of potentially attractive cascadecell designs for concentrators made with these and related single-crystal materials use AlGaAs, GaAsP, or GaAsSb as the top cell and GalnAs as the bottom cell or AlGaAs, GaAsP, or GaAsSb as the top cell and silicon as the bottom cell. These combinations are well matched because they split the solar spectrum evenly between the top and bottom cells. Theoretically, matching a 30%-efficient single-crystal Si bottom cell and a 30%-efficient GaAs-
alloy top cell should produce a cascade cell with an efficiency of more than 40% under concentrated sunlight. Such extremely high efficiencies can overcome the losses associated with concentrators. Three kinds of single-crystal cascade structures look most promising for concentrator systems: cells with Si as a bottom cell and a GaAs alloy as the top cell; cells made of GaAs alloys grown on top of one another; and cells made with GaAs alloys using a process commonly known as CLEFT (cleavage of lateral epitaxial film technology). The first choice builds on the already substantial success of single-crystal silicon cells. Several options exist for the top cell in such structures. One approach being pursued by Spire Corp. of Bedford, Mass., is to grow single-crystal GaAs-alloy cells directly on highefficiency single-crystal silicon cells to produce a twoterminal cell. The problem is to make an optimal GaAsalloy top cell, despite the lattice mismatch between it and silicon. An alternative is to produce a four-terminal structure in which the high-bandgap GaAs alloy is grown on a lattice-matched transparent material such as single-crystal gallium phosphide (GaP). This device is then used as the top cell. The second approach for making cascade cells uses GaAs alloys grown sequentially in a monolithic structure. In this procedure, single-crystal GaAs-alloy cells are formed on top of single-crystal GalnAs cells. One of the successes of SERI-supported research and development on concentrator units was the creation by one of its subcontractors, Varian Associates, of a single-junction GalnAs low-bandgap cell that is about 24% effi-
CLEFT method allows reuse of expensive GaAs substrates Interlayer
Single-crystal GaAs film
Direction of crystal growth
Single-crystal GaAs substrate
In the CLEFT process, gallium arsenide film Is formed on top of a single-crystal GaAs substrate by chemical vapor deposition. An interlayer, made of carbon, facilitates subsequent cleavage of the film from the substrate. The interlayer contains narrow gaps that allow the GaAs film to start growing on the GaAs substrate. The film initially fills the gaps in the interlayer and then grows laterally across it. The use of the single-crystal GaAs substrate permits the film to be formed as a low-defect single crystal. After the film is produced, it is bonded to a glass support and then cleaved from the reusable GaAs substrate. The solar cell is then completed by growth and diffusion processes on the exposed surface. Because the cost of the single-crystal substrate is a major component of the total cost of GaAs technology, the CLEFT process may reduce this cost considerably
cient at 175 suns concentration. GalnAs, with a bandgap near 1.1 eV, is an attractive alternative to silicon as an optimal bottom cell. The all-GaAs-alloy monolithic cascade cell is a system in which cells of near-perfect crystallinity are grown to achieve near-optimal efficiencies. The monolithic approach presents problems, however. A successful electrical interconnection must be made between the cells. An interlayer is needed to reduce lattice mismatch and thus enable the top cell to be grown successfully as a low-defect single crystal. Finally, damage to the bottom cell must be avoided during fabrication of the top cell. The innovative CLEFT technology provides a third alternative. In this technique, developed by John C. C. Fan, then at Massachusetts Institute of Technology's Lincoln Laboratory, a high-performance GaAs cell is grown on and then cleaved from a reusable singlecrystal GaAs substrate. That procedure avoids the consumption of an expensive, thick GaAs substrate with each individual cell. Future research may lead to CLEFT being used to form GaAs-alloy structures in which the bottom and top cells are grown separately and then assembled into four-terminal devices. In all cases, high-bandgap GaAs-alloy technologies, using AlGaAs, GaAsP, or GaAsSb, are not so developed as GaAs technology. Work will have to be done on these higher-bandgap materials to make them as efficient (25%) as existing GaAs cells. Despite the inherent problems of concentrators (such as their inability to use indirect sunlight), the extreme efficiencies already achieved by concentrator cells—and their potential for ultimately achieving nearly 40% efficiency—make them promising.
Flat-plate cascade cells There are many likely combinations of photovoltaic materials for flat-plate cascade cells. Most have been developed only in the past five years. When two cells are well matched, the efficiency of the combination is about the efficiency of the top cell plus 50% of the efficiency of the bottom cell. The top cell has the favored position because it is exposed to the entire solar spectrum. Fan has categorized various combinations of top/ bottom cascade cells, based on the cells' efficiency and cost, with "high" denoting high efficiency but high cost and "low" meaning low efficiency but low cost. Of the four Fan categories, one is least likely to provide valuable cascade cells: "low" (efficiency) on "high" (efficiency) combinations. In the "low-high" category, an inexpensive but relatively inefficient cell is placed above an expensive but high-performance bottom cell. That combination is the worst of all possible worlds. It may be less efficient than the bottom cell alone, and it is more costly than the top cell alone. The other Fan combinations are more promising. The "high-high" strategy is attractive because it can provide about 50% more efficiency than a high-performance single-junction device without doubling its cost. Fan also favors the "high-low" strategy, since the costly top cell can provide its full power, while the July 7, 1986 C&EN 45
Special Report inexpensive bottom cell can add a small efficiency boost at relatively little cost. The "low-low" strategy is also very attractive, since it provides a 50% increase in efficiency for cells that are already very inexpensive. Such a gain may put a low-cost combination of otherwise low-efficiency cells over the top. The most promising combination in the high performance-high performance category for flat plates is CLEFT GaAs-alloy top cell matched with single-crystal silicon. Alloying GaAs with phosphorus or aluminum increases its bandgap the necessary amount to match it perfectly with silicon. Assuming an efficiency of 20% for each cell, the combination would be 30% efficient as a cascade cell. Such a high-performance flat plate combination could be quite expensive but still provide electricity at competitive costs. One advantage of the CLEFT cell is that it can be bonded to a supporting glass layer, eliminating the need for an extra cover glass. One drawback of the combination is that single-crystal silicon cells are less efficient for long-wavelength light. Long-wavelength light penetrates deeply into silicon devices, and some of the light-generated electron-hole pairs are lost before they can diffuse to the junction. This is an inherent problem because of silicon's weak light absorption. Although the loss can be reduced, it cannot be eliminated. A highly light-absorbing low-bandgap (about 1 eV)
Materials can be used in combinations to form flat-plate cascade cells Top cell
Bottom cell
Fan category
Amorphous Si
CulnSe2
Low-low
Amorphous Si
Amorphous Si:Ge
Low-low
CdTe (Mn, Mg, Zn) CdTe (Mn, Mg, Zn)
CulnSe2
Low-low
CdHgTe
Low-low
Hg^Zn/Te CuGaSe2
HgxZn^Te CulnSe2
Low-low Low-low
Amorphous Si
CdHgTe
Low-low
GaAs (P, AI) made by CLEFT process GaAs (P, AI) made by CLEFT process GaAs (P, AI) made by CLEFT process
CulnSe2
High-low
CdHgTe
High-low
Crystalline Si
High-high
Comments
13.1% efficient; work by Arco Solar 12.7% efficient; work by Energy Conversion Devices CdTe alloys in early research stage CdHgTe in early research stage but promising Experimental CuGaSe2 in early research stage CdHgTe in early research stage but promising GaAs alloys in early research stage
CdHgTe and GaAs alloys in early research stage but promising GaAs alloys in early research stage
Note: All of these combinations permit near-optimal cascade cell efficiencies.
46
July 7, 1986 C&EN
material such as GalnAs would be a better choice than silicon if it could match silicon's performance. Silicon cells are attractive, however, because they generally are less costly than single-crystal GaAs-alloy cells (especially if ribbon silicon can be used), and are now produced commercially. The CLEFT technique makes possible another attractive and unique combination: a relatively costly top cell with a less efficient, but less expensive, bottom cell (a high-low option). Additional power provided by the cheap bottom cell can make the top cell more competitive. The leading candidate for a bottom cell in this category is polycrystalline thin-film CuInSe2. That material has a better response to long-wavelength light than does silicon—in some cases, achieving 90% external quantum efficiency at wavelengths to which silicon does not respond. If a 20%-efficient GaAs-alloy CLEFT cell and a 12%-efficient CuInSe2 cell were combined in a cascade structure, the combination would achieve 26% efficiency at a small additional cost over the CLEFT cell alone. With that arrangement, the cost of output power could be significantly less than that with either device alone. Boeing Aerospace and Kopin Corp. are investigating that combination of materials, primarily for use in space. The low-low Fan category consists of combinations that are among the most promising of the flat-plate photovoltaic technologies. As single-junction devices, their near-term potential is about 15% efficiency. Combined into cascade structures, they already come very close to 15% efficiency. As they approach their potential, thin-film cascade cells should be highly efficient (more than 20%) and economical. Research will be needed to develop the necessary thin-film cascade structures. But their potential efficiency at low cost suggests they are possible winners in the race for the multibillion-dollar central power market. Among the many all-thin-film cascade-cell structures currently available, three show the greatest near-term promise: a-Si on a-Si alloy, a-Si on CuInSe2, and CdTe alloy on CuInSe2. Amorphous silicon was the first thinfilm material used in cascade-cell structures. The main emphasis in future research will be to reduce the bandgap of the lower cells by alloying a-Si with germanium or tin. Some success has been achieved in lowering the bandgap from about 1.8 to 1.5 eV. Energy Conversion Devices has developed an a-Si:Ge cell with 10.2% efficiency. However, a-Si materials richer in germanium or tin and lower in bandgap have poor electrical properties, and cells made from them do not perform so well. The low-cost potential of amorphous silicon suggests that, if a breakthrough to lower bandgaps with high efficiency can be made, the overall cell will be economical. ' As a top-cell material, amorphous silicon is quite attractive because it has an optimal bandgap (1.7 to 1.8 eV) and provides reasonable efficiencies (more than 10%). In addition, a large technical/industrial community is working with this material. As a result, amorphous silicon cells soon will be available as top cells for use in a four-terminal design. The glass superstructure
SERI suDDOrts work on ohotovoltaics, other renewable energy sources Govemment-owned Solar Energy Re search Institute (SERI) was founded in 1977 in Golden, Colo., in response to the realization that solar energy could provide a significant renewable portion of U.S. total energy supply. The objec tive in developing solar energy always has been to replace or supplement conventional energy with cleaner, more reliable alternatives. With a present staff of about 500, the institute is managed by Midwest Research Institute for the Department of Energy. SERI operates many re search projects, including those devel oping such energy options as solar heating, photovoltaics, biomass, wind, and ocean thermal gradients. The pho tovoltaics portion of the work consists of an in-house effort and a subcontract ed program. The subcontracted work predates the in-house work. It is an R&D effort that is a major portion of DOE's overall photovoltaics program.
Over the years, this program has mainly involved three areas: crystalline silicon work at Jet Propulsion Labora tory, concentrator R&D at Sandia Na tional Laboratory, and advanced cell research on newer technologies at SERI and its subcontractors. Recently, the budget for photovoltaics has shrunk from about $150 million a year to less than $50 million, and management of the reduced program has become cen tered at SERI. SERI-funded and -directed R&D on thin-films and cascade cells has led to many advances. SERI gave financial support and technical guidance to RCA when that company led the develop ment of the amorphous silicon ceil. More recently, SERI helped Chronar Corp., Spire Corp., and Solarex estab lish themselves in the forefront of re search on amorphous silicon ceils. SERI-supported work at Boeing Aero space Co. in the late 1970s almost
on which the amorphous silicon is coated can act as the encapsulating cover. The only potentially low-cost thin-film material that has the proper bandgap for a bottom cell and conver sion efficiencies of at least 10% is CuInSe2. Several groups, including Arco Solar, are working on a hybrid cascade cell in which an amorphous silicon cell is stacked on a CuInSe2 cell in a four-terminal design. Other laboratories, including Energy Conversion De vices and SERI, are investigating two-terminal cells in which the amorphous cells are fabricated directly on the CuInSe2 cells. The Arco Solar technology is furthest along. Arco projects efficiencies in excess of 20% for aSi/CuInSe2 panels. Clearly, the amorphous silicon/CuInSe 2 combina tion is very attractive, but some scientists question the use of amorphous silicon as the top cell material. The long-term stability of amorphous silicon has not yet been determined, and other thin films have higher ultimate efficiencies. An alternative is a CdTe-alloy top cell. Cadmium telluride is a highly developed polycrystalline thinfilm material with efficiencies at least as high as those of a-Si. CdTe usually is fabricated in a structure on glass and thus is suitable for a four-terminal combination with CuInSe2. However, there are two substantial re search problems: The bandgap of CdTe needs to be increased slightly from 1.6 to 1.8 eV to make it an optimal top cell, and no stable, transparent back con tact exists for CdTe cells. One approach to the first problem is CdTe alloys containing zinc, magnesium, or manganese. Those al
single-handedly established copper in dium diselenide as a viable thin-film technology. R&D on that material sup ported by SERI still provides the great er fraction of the available expertise in this field worldwide. In the high-effi ciency area, SERI helped Massachu setts Institute of Technology's Lincoln Laboratory develop the CLEFT process for making gallium arsenide cells and also supported the pioneering efforts of Varian Associates on gallium indium arsenide cells. The in-house work at SERI involves research in several major new technol ogies, so that the institute can add to fundamental knowledge supporting these technologies and can help solve some of the more crucial technical problems. SERI groups are doing im portant research on amorphous sili con, copper indium diselenide, gallium arsenide, and other materials for pho tovoltaic cells.
loys can be made so that their bandgaps are in the optimal range for top cell use. Little work has yet been done, however, on making solar cells with CdTe alloys. The problem of producing a transparent back contact for CdTe is made more difficult because the material does not readily form an electrical contact. Making the same contacts transparent is a challenge. However, the University of Delaware Institute of Energy Conversion has made transparent contacts to CdTe and has deliv ered samples to SERI for optical analysis. Other cascade combinations are more speculative but deserve continued research. One combination uses CdTe alloyed with mercury as a potential bottom cell. High-efficiency (10.6%) CdHgTe cells have been fabri cated at Monosolar/Standard Oil by Bulent M. Basol (now with International Solar Electric Technology) and more recently at British Petroleum's lab in Middle sex, England. The bandgap of such cells is less than that of CdTe but not so low as that of CuInSe2. Therefore, CdHgTe cells have not yet been found to be highly efficient when made with a bandgap low enough for an optimal bottom cell. A new material of some interest is HgZnTe. It is a very flexible substance that can be adjusted to any bandgap between 0 and 2.2 eV. It can be used for either top or bottom cells, depending on its composition. For instance, Hg x Zni_ x Te, with χ = 0.27 or 0.54, can be made with bandgaps of about 1.6 or 1.0 eV, respective ly. Thus, altering the mercury content during fabrica tion allows materials appropriate either for top or bot tom cells to be made sequentially in the same equip ment. July 7, 1986C&EN
47
Special Report The HgZnTe alloy appears to be promising for an other reason: In comparison to CdTe, its conductivity is greater and more easily controlled, thus making elec trical contacting easier. Although HgZnTe is still rela tively unexplored, it holds promise as a candidate for flat-plate cascade cells. Single-junction thin-film solar cells show increasing promise for producing inexpensive electric power. As they are developed further, they will provide a basis for new opportunities based on cascade cell combina tions. A few years ago, before the development of the thin-film photovoltaic technologies, opportunities for producing low-cost, 20%-efficient cells did not exist.
Looking to the future A clear need exists to expand the nation's options for producing electricity. The potential for nuclear acci dents, pollution from coal, or supply problems with oil makes this evident. Sometimes, our imaginations are strained by the un usual characteristic of a new technology. Can we con ceive of harnessing large, silent arrays of photovoltaic cells to power cities? The answer should be yes. Photovoltaics is a real science, with real devices that make electricity. Although problems with the technique ex ist, there are rational ways to solve them. As a society, we have spent a great deal of money developing such well-publicized energy technologies as synfuels and fission. Despite several orders of magnitude less finan cial support, photovoltaics are on the threshold of prac tical use. The people who are making decisions about how to supply future electricity needs should not have their choices limited to the commonly known alternatives. A strong research commitment to photovoltaics would accelerate the development of improved technologies and guarantee the production of economical photovol taic-generated electricity on a large scale before the end of the century.
Suggested readings Chopra, K. L, Das, S. R., "Thin Film Solar Cells," Plenum Publishing Co., New York, 1983. Ciszek, T. F., "Silicon for Solar Cells," in "Crystal Growth of Electronic Materials," Aldis, Ε. Κ., Ed., Elsevier Sequoia, Oxford, England, 1985. Fan, J, C. C , "Photovoltaic Cells," in "Kirk-Othmer Encyclope dia of Chemical Technology," Third Edition, John Wiley & Sons, New York, Vol. 17, p. 709-32, 1982. Green, Μ. Α., "Solar Cells," Prentice-Hall, Englewood Cliffs, N.J., 1982. Maycock, P.D., Shimada, K., Stirewalt, Ε. Ν., Hunt, V. D., "Amer ica Challenged: Photovoltaics in Japan," Photovoltaics Ener gy Systems, Alexandria, Va., 1982. Zanio, K., "Semiconductors and Semimetals: Cadmium Telluride," Vol. 13, R. K. Willardson, A. C. Beer, Eds., Academic Press, New York, 1978. Zweibel, K., Hersch, P., "Basic Photovoltaic Principles and Methods," Van Nostrand Reinhold Co., New York, 1984. 48
July 7, 1986 C&EN
At present, the greatest enemy slowing the progress of photovoltaics is ignorance. People generally—and energy decision-makers in particular—do not realize the potential for photovoltaics to replace conventional energy sources. Their ignorance is obvious, given the public's response to the recent Chernobyl nuclear acci dent. People do not say, "Let's provide photovoltaic R&D with the research funding needed to bring this new technology to fruition." Instead, they grit their teeth and ask, "What else can we do but pour more money into the conventional choices?" Photovoltaics is a technology that eventually may rival any other energy technology in size. Yet, in the U.S. today, the drop in oil prices and the squeeze on energy profits are limiting the interest of oil companies in photovoltaic R&D. Meanwhile, except for some pio neering small companies, few in the private sector are taking the risks needed to develop this field. The utili ties are interested but have substantial inertia. The U.S. government, at the level where policy is made, is com mitted to traditional choices. Japan is not so hesitant. It has a powerful Sunshine Project that provides significant governmental support for photovoltaic R&D. Indeed, photovoltaics may someday be our best ener gy option. At a recent SERI meeting, a New England utility company representative stated that photovol taics may be the nation's only practical method for producing additional electricity in the late 1990s. How ever, the major question remains: Who in the U.S. will provide the financial support for the R&D needed to make this technology commercially successful? D HegEte of fH»Ci«* specialreportwW be available at $6.00 per eepy. ^ r 10 or more copiée. $3.00 per copy. Send requeststo: M r J M f e M | o o m 2 ^ ety, 1155—16th SL.N.W., Washington, DC. 20038. On orders of i&eriees, please send checkor money eider with request
Kenneth Zweibel is manager of the polycrystalline thin films program at Solar Energy Research Institute in Golden, Colo. He received a B.S. degree in physics from the University of Chicago in 1970 and has been employed at SERI since 1979. He has written many scientific articles on photovoltaics and is coauthor of the book "Basic Photovoltaic Principles and Methods" published by Van Nostrand Reinhold in 1984. The research program he manages is concerned with the development ofCulnSei and CdTe solar cells, as well as their combined use in high-performance cascade structures. The opinions expressed in this article, he points out, are his own and do not necessarily represent those of either SERI or the federal government.
