In recent years, remarkable progress has been made in improving the photovoltaic (PV) conversion efficiencies of thin film solar cells. The best active-area efficiencies (air mass 1.5) of thin film solar cells reported are as follows: polycrystalline CuInSe2, 14.1%; CuIn(Ga)Se2, 12.9%: CdTe, 12.3%, total area; single-junction hydrogenated amorphous silicon (a-Si:H), 12.0%; multiple-junction a-Si:H, 13.3%; cleaved epitaxial GaAs-Ga(1-x)Al(x)As, 21.5%, total area. Laboratory methods for preparing small thin film solar cells are evaporation, closed-space sublimation, closed-space vapor transport, vapor phase epitaxy and metallo-organic chemical vapor deposition, while economic large-area deposition techniques such as sputtering, glow discharge reduction, electrodeposition, spraying and screen printing are being used for module fabrication. The following aperture-area efficiencies have been measured, at the Solar Energy Research Institute, for thin film modules: a-Si:H, 9.8%, 933 cm2; CuIn(Ga)Se2, 11.1%, 938 cm2, CdTe, 7.3%, 838 cm2. The instability issue of a-Si:H continues to be a high priority area. It is necessary to improve the open-circuit voltage of CuIn(Ga)Se2 cells, which do not seem to exhibit any intrinsic degradation mechanisms. With continued progress and increased production, PV modules are likely to become competitive for medium-scale power requirements in the mid-1990s.
