Thin-film solar cells can be made from a wide variety of materials including ternaries and quaternaries; many of these have not been extensively studied. The achievable efficiency of a solar cell material will depend on the characteristic energy bandgap of the material. An idealized calculation of achievable efficiency versus bandgap is shown in Fig. 1, with the bandgaps of some of the important solar cell materials indicated (after Loferski [20]). For the technologically well developed materials, such as silicon and GaAs, the efficiencies on this chart are very close to the achieved efficiencies (e.g. 22.5% for GaAs, about 19% for Si). For thin-film materials, achieved efficiencies as yet fall well below these values. This is for two reasons. First, Si and GaAs have received the benefit of extensive materials development research done for the electronics industry, and are technologically very well understood materials, while thin-film materials are relatively new and have been comparatively little researched. Second, because the thin-film materials are polycrystalline or amorphous, there are additional sources of efficiency loss due to grain-boundary effects. It is as yet unknown whether the ultimate efficiencies of these materials will approach those of the single crystal materials. Since the absorption coefficients of all the materials discussed are very high, the cells can be made extremely thin, typically a few microns, compared to several hundred microns thickness required for conventional silicon solar cells. This means that the technology could potentially be extremely low weight, if the cells can be deposited on low mass substrates (or superstrates). However, the current technology development programs are directed at terrestrial use, for which the preferred substrate
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