Electrical properties of CdTe films and junctions

Abstract

Polycrystalline films of CdTe have been deposited on graphite substrates, and homoepitaxial films on single-crystal CdTe:P substrates, with hole densities as high as 1.5×1016 cm−3 without intentional doping of the films, using close-spaced vapor transport. The dependence of hole density in the films on the properties of the source, the properties of the substrate, the growth rate, and the substrate temperature, indicates that the doping of the films is the result of a complex interaction between out-diffusion of mobile impurities (if present) from the substrate and intrinsic defects. The capacitance-versus-voltage-indicated hole density decreases slowly with time under a Schottky barrier, but not at a free film surface or under a CdS/CdTe heterojunction. CdS/CdTe solar cells have been prepared using these films, with solar efficiencies greater than 6%. The junction properties of CdS/CdTe heterojunctions are compared with those of In/CdTe Schottky barriers as a function of temperature; transport is dominated by tunneling below room temperature and by interface recombination above room temperature.