Technology
Performance of photovoltaic cells improved University of Delaware products have conversion efficiencies of 10% or more and can be mass-produced in continuous chemical process The acceptance of photovoltaic solar energy conversion in the U.S. mar ketplace has been hampered by un favorable economics. However, things are looking up for thin-film photo voltaic cells: They have been im proved to where they can provide conversion efficiencies of more than 10% and they can be mass-produced in a "chemical process." Both are crucial items in making the economics attractive. Among the goals for photovoltaic conversion set by the Department of Energy have been a cost for materials of 70 cents per watt and a conversion efficiency of 10%. It now appears that both goals can be reached or exceeded by products issuing from the Uni versity of Delaware's Institute of Energy Conversion. The work at Delaware has been in progress since 1972 with emphasis on cells made from cadmium and copper salts. The best-known photovoltaic converters are equipped with silicon wafers, usually cut from large single crystals. These have conversion ef ficiencies of about 15%. A CdS/Cu2S cell developed at Delaware has a demonstrated conversion efficiency of 9.2%, and institute director T. W. Fraser Russell believes that a practi cal upper limit might be 12%. Another cell being developed at Delaware is a Cd-ZnS/Cu2S converter that has a demonstrated efficiency of 10.2%. Russell believes that this can be pushed to 16%. The principal advantage of the polycrystalline sulfide cells over the more efficient single crystal cells is cost. Much of the advantage lies in the way the thin-film cells would be produced. Until now the manufacture of photovoltaic cells has been considered well entrenched in the semiconductor business. Russell doesn't think that is particularly good, because it means
that each cell is considered a separate, manufactured item, and the attitude of cell makers is roughly equivalent to that of any others making discrete, mass-produced items. Continuous chemical processing is a better way of viewing cell manu facture, Russell believes. The manu facture of thin-film cells, he says, is properly a chemical process under taken with continuous production in mind. In addition to better economic prospects, it also aids in uniformity of product quality and reliability. Rus sell and his colleagues now are de veloping a continuous manufacturing process with the aim of producing thin-film cells at the rate of 100 sq cm per minute. The key items in the process are vacuum coaters for foil substrates (C&EN, June 23, page 11). Most of the effort so far has been exerted in development of the CdS/Cu2S thin-film cell. The cell is five layered, and the particular cell geometry would be the same whether manufactured one at a time or cut from an endless thin film. The substrate for the cell is a cop per foil overplated with zinc to a thickness of up to 0.5 micrometer. This is an efficient ohmic contact and a serviceable light reflector. The col lector/converter layer is cadmium sulfide which is deposited on the substrate by sublimation in vacuum at 1000° C and condensation at 200° C. The absorber/generator layer is up to 0.3 micrometer thick and is com posed of cuprous sulfide obtained from topochemical ion exchange. Superimposed on the surface of the absorber generator are grid stripes for circuit control. The final layer is really two layers of antireflecting en capsulant glass. The first layer is sputtered on to a thickness of less than 1 micrometer. The outer layer is tempered glass bonded with polyvinyl butyral. The process for making these solar cells eventually will be scaled up to a level of 1000 Mw per year of plant capacity, assuming the commercial development succeeds. One of the more promising responses so far has been a contract signed with Chevron Research Corp. earlier this year. Under terms of the contract, Chevron
CdS/Cu2S solar cell has five functional layers Encapsulant
Grid ' tripes/
«ψ^Γ
1 Opaque contact and substrate: 0.5 micrometer zinc on copper 2 Collector/converter: 8 to 20 micrometers of CdS 3 Absorber/generator: 0.1 to 0.3 micrometer of Cu2S 4 Transparent contact: grid stripes 5 Encapsulant: antireflection coating
will provide $250,000 per year for the next three years to support the de velopment. Although the CdS/Cu2S cell has been the mainstay of the de velopment effort so far, most of the emphasis in the future will be on the Cd-Zn/Cu2S cell, which has an in trinsically higher efficiency. Two conceptual processes have been costed out by Russell and his colleagues. One is continuous and the other is the more traditional batch process. The continuous process should cost about 70% less than the batch process for a comparable product. For a plant producing 107 sq m per year of the thin-film cells, the raw materials costs are estimated to be from 10 to 17 cents per watt. The total cost is estimated to be about 50 cents per watt, which is considerably below the DOE goal for 1986. If photovoltaic conversion of solar energy eventually is to provide up to 10% of the nation's electrical power, economics demand that conversion lants with capacities of 1000 Mw be uilt. To support such an effort it would be necessary to make 10 mil lion sq m of the cells per year. Such a cell-making plant would cost about $400 million, which is comparable to a major chemical plant. D
E
Oct. 6, 1980 C&EN
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