efficiency maximum shown in Fig. 1, but still reasonable. It is, as discussed below, nearly ideal for the bottom cell of a cascade. The absorption constant of CuInSe2 is extremely high, allowing the possibility of cells as thin as one micron. Existing cells consist of a layer of the active copper indium selenide, typically about 3 microns in thickness; a front contact and heterojunction window of either cadmium-zinc sulfide or zinc oxide plus cadmium sulfide, thickness typically about one micron; and a back contact of molybdenum, typically several thousand angstroms thick. Thus, the material has an inherent low weight, and the major mass of an actual solar cell is that of the substrate onto which the film is deposited. A wide variety of manufacturing methods have produced >8% efficiency, including vacuum evaporation, reactive sputtering, and electroplating of the base material onto the substrate. In general, all these techniques either involve high temperature deposition, or a high temperature post-deposition anneal step. This could be a problem for space applications, where we would like to be able to deposit the cell onto a thin plastic (e.g. Kapton) substrate. Deposition onto a thin substrate has not been demonstrated to date. Copper indium selenide has the highest radiation tolerance of any known material. Other I-III- VI2 Compounds Related I-III-VI2 semiconductors have also been studied for solar cell use, although not as extensively as CuInSe2. The goal of investigations has been to identify related semiconductors which have the same ease of manufacturing thin-film solar cells, but have wider bandgaps and thus presumably higher ultimate efficiency. Copper gallium selenide is a major candidate for the proposed higher efficiency successor to copper indium selenide. The advantage of CuGaSe2 is the wider bandgap, 1.67 eV, which is much closer to the optimum for the solar spectrum (see Fig. 1), and nearly ideal for cascade upper cell. While the best experimental results to date are only 4.6% efficiency, the material has not been extensively developed. One known problem is that the CdS heterojunction used for CuInSe2 absorbs light in the short wavelength end of the spectrum. Since this is more important for the wider bandgap material, a different (wider bandgap) heterojunction material needs to be developed to reach maximum efficiency for CuGaSe2 [27,28], Unless CuGaSe2 differs electronically from CuInSe2 in some yet- unknown way, ultimate efficiency for CuGaSe2 cells should be about 18% better than for CuInSe2. Cu(InGa)Se2 quaternary compounds can be produced with bandgap intermediate between copper indium selenide and copper gallium selenide. This allows a bandgap variable 1.0-1.67 eV. Such materials can be tailored for a good match for AMO spectrum, yet be easier to work with than the wide bandgap CuGaSe2. Preliminary results with Cu(InGa)Se2 quaternaries are encouraging [29], with efficiencies of up to 10.7 AMI.5 for compositions of 25% Ga [30], Another proposed wide bandgap candidate is copper indium sulfide. CuInS2 has a bandgap of 1.55 eV, very close to the optimum. It is not a very well studied material, and until recently no good semiconductor properties had been made with the material. The results on CuInSe2 cells have restimulated interest in the material, and recently thin-film cells have been made with efficiency of 5.8% AMO [31]. Many other I-III-VI2 ternaries exist; only a minimum amount of research has been done.
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