Grain boundaries and impurities in CdTe/CdS solar cells

Abstract

Near-interface grain effects and quantitative SIMS impurity profiles for close-space sublimation-deposited CdTe/CdS solar cells and test structures are described. TEM of the interfacial CdTe grains demonstrated them to be smaller than expected from extrapolation of the bulk grain size. Computer simulation showed this to be consistent with the development of grain sizes during additive growth processes, the grain size increasing approximately as the root distance from the polycrystalline hetero-interface. TEM of the near-interfacial CdTe before and after CdCl 2 processing revealed grain growth from 0.1 to 0.5 μm — even though the grain size distribution in the overlying CdTe is stable to processing. Quantitative SIMS of test structures comprising co-sputtered contacts on close-space sublimation-deposited cells (Sb–Te/CdTe/CdS/TCO/glass) is reported for the structures in their fresh, light-soaked and bake-tested states. High concentrations of impurities are present in the CdTe of the fresh cells as follows: S, 7×10 19; Cl, 8×10 18; Cu, 7×10 17; Na, 3×10 17; Sb, 4×10 18; and O, 7×10 18 cm −3. It is considered that the unintentionally introduced impurities are unlikely to originate from the source CdTe. The effects upon the impurity distributions of light soaking and baking in air at 400°C are reported. Accumulations of impurities near to the CdS layer are discussed.