CdS/Cu2S The first thin-film solar cell developed was the cadmium sulfide/copper sulfide heterojunction cells [23]. The best achieved efficiency on these cells is about 9% [24]. These cells were made obsolete by the discovery of better thin-film materials. Copper Indium Selenide Currently the leading technology for thin-film photovoltaics is copper indium selenide. As of 1988 11.2% AMO cell efficiency has been achieved (extrapolated from AMI.5 data; see above); 12% efficiency can be confidently predicted in the near term [25]. Figure 2 shows the electrical characteristics of the best CuInSe cell. Modules can be made with integral monolithic series interconnection of the deposited thin-film cells. ARCO Solar, for example, produces large area (4000 cm2) modules [26] with multiple cells series interconnected on a single substrate. The bandgap of copper indium selenide is 1.0 eV. This is on the low side of the
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