Several thin film solar cell materials have demonstrated greater than 10% conversion efficiency, including amorphous silicon, polycrystalline silicon, cadmium sulfide/copper sulfide, cadmium sulfide/copper indium diselenide, gallium arsenide (CLEFT), cadmium sulfide/cadmium telluride, and gallium arsenide/silicon. The generic category of thin film solar cells is examined to determine prerequisites for use of these materials for large quantities of competitive electrical energy production. The future extrapolated performance, low cost potential, and areas for further research are discussed.
One square foot Ovonic amorphous photovoltaic devices are already in commercial production and are manufactured through a continuous web process. The next levels of commercialization required to achieve a large-volume power market will be discussed, and the device specifications correlated with the chemical and electronic properties of the materials that we are developing to achieve even higher efficiencies. It has been long considered a utopian dream to harness the energy of the sun to create electricity that would be competitive in cost to that produced from the conventional sources of energy such as oil, gas, and uranium. The impact on our society of stand-alone power generators without moving parts using the continually available, ubiquitous energy of the sun could certainly lead to a new age with consequences comparable to the first introduction of electricity which greatly accelerated the Industrial Revolution. Low cost, nonpolluting energy not dependent upon or limited by transmission costs could again make DC electricity a realistic option. The relatively young field of photovoltaics suffers from certain dogmas that are just now being questioned. For example, it is thought by many that solar cells utilizing crys-talline materials have inherently higher efficiencies than those using amorphous materials, and that somehow crystalline solar cells, whether fabricated from single crystals or polycrystalline material, in round or rectangular geometries, grown from the melt or by a rib-bon process, can be reduced in cost sufficiently that the economics become attractive enough for large-scale terrestrial generation of power. In this paper, we shall show that amorphous materials can have much higher efficiencies than do crystalline and that the answer to our power generation needs lies not in crystalline but in amorphous technology. At Energy Conversion Devices, Inc. (ECD), we have designed and built a production machine (described by my colleague, Dr. Izu, in a subsequent paper) which has clearly demonstrated that the basic barrier to low-cost production has been broken through and that one can now speak realistically of delivering power directly from the sun for under a dollar per peak watt merely by making larger versions of this basic continuous web, large-area thin-film machine. We have made one square foot amorphous silicon alloy PIN devices with conversion efficiencies in the range of 7%, and in the laboratory, we have reported smaller area PIN de-vices in the 10% conversion efficiency range. In addition, much higher energy conversion efficiencies can be obtained within the same process by using multi-cell layered or tandem thin-film solar cell structures (see Figure 1). These devices exhibit enhanced efficiency by utilizing a wider range of the solar spectrum. Since the theoretical maximum efficiency for multi-cell structures is over 60%, one can certainly realistically anticipate the pro-duction of thin-film amorphous photovoltaic devices with efficiencies as high as 30%. Our production device is already a two-cell tandem, as we have solved not only the problems of interfacing the individual cell components but also the difficulties associated with a one foot square format deposited on a continuous web. Figure 2 shows a continuous roll of Ovonic solar cells. Realistic calculations for a three-cell tandem thin-film device using amorphous semiconductor alloys with 1.8eV, 1.5eV, and 1.0eV optical band gaps indicate that solar energy conversion efficiencies of 20-30% can be achieved.
The performance of amorphous silicon solar cells has improved steadily over the last several years, and recently conversion efficiencies as high as 10.1% have been achieved at RCA Laboratories. This increase in performance is partly due to an improvement in the quality of undoped amorphous silicon and partly due to the development of new doped amorphous silicon alloys. We project that the conversion efficiencies will continue to improve and that single junction amorphous silicon solar cells will attain efficiencies in the 12 to 14% range within the next few years.
We have developed a new type of amorphous silicon (a-Si) solar cell, called the integrat-ed type a-Si solar cell (cascade type a-Si solar cell), and a new fabrication method for high conversion efficiency a-Si solar cells, called the consecutive, separated reaction chamber method. With this method, we were able to improve the conversion efficiency of Glass/SnO,/p(SiC)-i-n/Metal to a level of 9.28% with a size of 2 mm x 2 mm, to a level of 7.58% with a size of 10 cm x 10 cm, and to level of 6.14% with a size of 20 cm x 20 cm. We have also proposed a new type of a-Si solar cell, called the "multi-gap amorphous solar cell (stacked cell)" which consists of a-Si:N:H, a-Si:H, and a-Si:Sn:H to improve conversion efficiency. The theoretical limit efficiency of the three band-gap amorphous solar cell is estimated to be 24%. The calculation for the optimization of the integrated type multi-gap solar cell was done. Integrated type a-Si solar cells have been applied both to consumer electronics and power generating systems.
The present and future markets for photovoltaic devices are discussed in detail. It is pointed out that such devices are already a major industry rather than just a hope for the future. The existing commercial devices and other near-term possibilities are briefly described and some of the companies involved in research, development, and/or commercial production associated with each class of material are identified.
This paper deals with a technology for improving the conversion efficiency of glass/TCO/ p-i-n/metal a-Si:H solar cells. The improvement is discussed on the basis of expetimental results mainly for electrode formation involving the transparent conductive oxide(TCO) electrode with ITO/Sn02 double-layered configuration and the back-electrode with a high reflectance in the long wavelength region. Design parameters for the glass-substrate solar cell with an area of more than 100 cm2 are also discussed. Conversion efficiency of more than 7% is demonstrated for the cells with 100 cm2 area under AM1 (100 mW/cm2) illumination.
Large area three-stacked tandem type cells were fabricated and an efficiency of 6% has been obtained. The three-stacked structure has been proved to be stable under intense illumination. And an interpretation on spectral response of tandem type cells is discussed.
This paper discusses the components of a projected manufacturing process for amorphous silicon photovoltaic modules. The modules themselves are monolithic series-intraconnected panels. This design exploits the intrinsic advantages of the thin film technologies employed and will result in the simultaneous achievement of high module performance and low manufacturing costs. It is projected that modules with efficiencies in excess of 10%, at manufacturing costs of less than 0.50/W can be produced. The projected manufacturing process will be discussed with regard to its roduction cost components. The impact of anticipated amorphous silicon device performance improvements upon the manufacturing process and the manufacturing costs will also be discussed.
A roll-to-roll plasma deposition machine for depositing multi-layered amorphous alloys has been developed. The plasma deposition machine (approximately 35 ft. long) has multiple deposition areas and processes 16-inch wide stainless steel substrate continuously. Amorphous photovoltaic thin films (less than 1pm) having a six layered structure (PINPIN) are deposited on a roll of 16-inch wide 1000 ft. long stainless steel substrate, continu-ously, in a single pass. Mass production of low-cost tandem amorphous solar cells utilizing roll-to-roll processes is now possible. A commercial plant utilizing this plasma deposition machine for manufacturing tandem amorphous silicon alloy solar cells is now in operation. At Energy Conversion Devices, Inc. (ECD), one of the major tasks of the photovoltaic group has been the scale-up of the plasma deposition process for the production of amorphous silicon alloy solar cells. Our object has been to develop the most cost effective way of producing amorphous silicon alloy solar cells having the highest efficiency. The amorphous silicon alloy solar cell which we produce has the following layer structure: 1. Thin steel substrate. 2. Multi-layered photovoltaic amorphous silicon alloy layers (approximately 1pm thick; tandem cells have six layers). 3. ITO. 4. Grid pattern. 5. Encapsulant. The deposition of the amorphous layer is technologically the key process. It was clear to us from the beginning of this scale-up program that amorphous silicon alloy solar cells produced in wide width, continuous roll-to-roll production process would be ultimate lowest cost solar cells according to the following reasons. First of all, the material cost of our solar cells is low because: (1) the total thickness of active material is less than 1pm, and the material usage is very small; (2) silicon, fluorine, hydrogen, and other materials used in the device are abundant and low cost; (3) thin, low-cost substrate is used; and (4) product yield is high. In addition, the development of high efficiency cells in future time will further reduce the material cost. Secondly, the labor cost associated with the production of our solar cells is low because our process utilizes simple, high production rate, highly automated processing for the complete fabrication of photovoltaic modules. Specifically, six layers of tandem amorphous silicon alloy solar cell are plasma-deposited on a roll of wide stainless steel substrate, continuously in a single pass. Over one order of magnitude increase in the line speed is straightforward from an engineering point of view. Other downstream process steps for the fabrication of photovoltaic modules also utilize simple, high production rate, highly automated machineries.
A review is given of the optical, electronic and device properties of hydrogenated amorphous silicon (a-Si:H) prepared by chemical vapor deposition (CVD) from higher order silanes. Prepared in this way, a-Si:H possesses a smaller energy gap than its counterpart prepared by glow discharge (GD) techniques, and thus is a potentially interesting material for photovoltaic solar energy conversion. Topics discussed are the deposition mechanism, hydrogen concentration and bonding, the optical energy gap, dark conductivity and photo-conductivity, sub-bandgap absorption, the effect of hydrogen plasma treatment, doping properties, gap states, and photovoltaic devices.
This paper describes correlations between deposition parameters and photovoltaic properties of intrinsic amorphous silicon films produced by RF sputtering. We present data showing strong dependence between photovoltaic properties and structural and compositional inhomogeneity of the films. Studies on films produced at different hydrogen concentrations show that the ones with larger optical gaps have better photovoltaic potential. Small concentration of dopant impurities in the intrinsic films has significant effect on their photovoltaic properties. These optimization studies lead to intrinsic material, which when incorporated in a P-I-N solar cell generates external currents up to 13 mA/cm2 and open circuit voltages of between 0.85 to 0.95 volts.
The throughput and production costs associated with commercial-scale deposition of amorphous silcon solar-cell active layers have been examined. A methodology was developed to isolate and compare the economics of glow discharge deposition, sputter deposition, and conventional and low pressure chemical vapor deposition on a self-consistent basis. The imposition of restrictive assumptions was avoided by leaving key parameters as variables. We conclude that the largest cost differences between deposition processes are most likely to occur in the categories of equipment capital cost and silicon feedstock costs.
Computer analysis of crystalline solar cells indicates that a substantial increase in cell conversion efficiencies can be achieved by using two-cell, multi-bandgap tandem structures instead of single-junction cells. Practical AM1 efficiencies of about 30% at one sun and over 30% at multiple suns are to be expected. The further increases in efficiency calculated for a three-cell tandem structure are smaller and may not justify the added complexity. For inexpensive two-cell tandem modules, Si is preferred for the bottom cell, and the top cell material should have a bandgap of 1.75 to 1.80 eV. The GaAs-AlAs and GaAs-GaP systems are attractive candidates for the top cell. Significant advances have been achieved in growing GaAs on Ge-coated Si substrates (for the two-terminal, two-cell structure) and in growing free-standing ultrathin GaAs layers (for the two-terminal or four-terminal structures). These material advances should be transferable to the GaAs-AlAs and GaAs-GaP systems.
Progress in the development of thin-film polycrystalline CdTe solar cells is reviewed. A wide variety of film preparation techniques is currently being used: Vacuum Evaporation, Chemical Vapor Deposition (CVD), Spray Pyrolysis, Screen Printing/Sintering, and Electrodepositon. These techniques differ considerably with regard to the quality of the films produced and the level of maturity of the technologies. Only methods producing all-thin-film solar cells are considered. Progress has been rapid and, in 1982, a 10% efficient thin-film solar cell was demonstrated. Work is now in progress at a number of different laboratories to produce a commercially viable solar cell. A critical comparison is made of the various approaches.
Recent results on CuInSe2 from the United States Department of Energy (DOE) Polycrystalline Thin-Film Device Program are presented. The program, managed by the Solar Energy Research Institute (SERI), encompasses mate-rials and device research in a variety of highly absorbing compound semiconductors with emphasis on CuInSe2, for which thin-film device efficiences of 11% (AM1) have been reported. This paper describes film deposition techniques including coevaporation from the elements, reactive sputtering, and spray pyrolysis. Electrical and optical characterization of the films is discussed, and experiments on control of carrier type and concentra-tion are outlined. Differences from corresponding results on single-crystal CuInSe2 are discussed in terms of defect-chemistry models. Experiments on heterojunction formation of CuInSe2 with CdS or (Cd,Zn)S are presented, and recent results in device characterization and modeling are discussed. Central problems in the achievement of higher efficiency devices are described. Low open-circuit voltages and the role of oxygen in post-deposition anneal are discussed. Scalability of techniques for the deposition of device-quality CuInSe2 is also discussed in light of the recent announcement of plans for commercialization of thin-film CdS/CuInSe2 devices.
A new ribbon growth technique has been developed for the manufacture of semiconductor substrates intended for photovoltaic application. In the Edge stabilized ribbon (ESR) growth method, ribbon is grown from the melt surface with the edges stabilized by capillary attachment to wetted strings. Edge stabilized ribbon has been grown in widths of 2.5, 5.6 and 10.0 cm. The growth is extremely stable and the dimensional control is excellent with the growth of 50 - 100 micron thick ribbon a standard procedure. Solar cells fabricated on edge stabilized ribbon material demonstrate AM1 conversion efficiencies of 11% (corrected for AR coating) on 4 cm2 area, with excellent consistency of performance. Infra-red laser scanner studies demonstrate that ESR performance is limited solely by high dislocation density. Future work will emphasize continuous melt replenished growth of low dislocation density material.
This paper reviews recent results obtained in the polycrystalline silicon solar cell task area at the Solar Energy Research Institute (SERI) and presents a description of the goals which are expected to be achieved in the next year and beyond.
The efficiency of devices made from cast silicon is limited by structural factors such as dislocations and grain boundaries. Improving the crystallinity of this cast material continues to be an area of great importance to the photovoltaic community. To improve the solar cell performance, single crystal silicon with lower dislocation densities and silicon carbide inclusions are necessary. This paper describes an ingot growth technique which is designed to yield a high quality ingot. In this method, the crucible containing the melt is rotationally accelerated and decelerated periodically. This oscillation is intended to homogenize the solution by effectively stirring the melt. Several ingots were directionally solidified using this Oscillating Crucible Technique (OCT) with the result that single crystallinity was achieved to the top of the ingot and approximately 70% of the ingot was single crystal. Using an IR spectrophotometer, the interstitial oxygen and substitutional carbon content was measured and found to be uniform over the entire ingot. Details of the crystal growth conditions and resultant structure with chemical analysis is presented.
Although the power conversion of energy fluctuation is a basic physical concept, the theoretical analysis of the performance for all the wide ranging design options for each application, as for example, solar energy conversion, is not simple. The basic physics of the reversible energy fluctuation (REF) converter is to transfer the energy fluctuations of the hot charged particles in the first layer across the thermal barrier of the second layer to the cool rectifying diodes of the third layer with an intrinsic efficiency in a reversible cycle for independent particles that is limited by the efficiency of the Carnot cycle. Because of the questions on this surprising high efficiency, an extended analysis of each question has been made. One question is whether impractically small circuit sizes limited to a few nanometers are required. The answer is there is no limitation on circuit size and that simple REF design options exist to achieve a conversion efficiency of 85% independent of circuit sizes. Another question is whether practical amounts of power can be obtained from energy fluctuations. The answer shows the inherent ability of the REF converter to transfer energy fluctuations across the thermal barrier at a power rate orders of magnitude larger than for any practical application. As a result of finding that several feasible and practical design options are available as solutions to each question, it is reasonable to conclude that near-term production is a feasible prospect.
Chevron recently completed a comprehensive market study on the applications and economics of photovoltaic power systems. In this paper we report on the manufacturing cost goals and performance requirements that we believe must be met to ensure a significant domestic market in the 1990's and beyond. We conclude that a market on the order of $1 billion per year for central station photovoltaic (PV) systems can only result if PV power cost is competitive with the costs of intermediate load power generation. Even more efficient modules, manufactured at lower cost, are required for residential PV application due to the higher associated marketing costs and the fairly immediate benefit homeowners will require from such systems. This will hinder the development of a large, residential market.