Electrical characterization of annealed chemical-bath-deposited CdS films and their application in superstrate configuration CdTe/CdS solar cells

Application of chemical-bath-deposited CdS in the superstrate configuration of CdTe/CdS solar cells involving CdCl2:O2 heat treatment of CdTe/CdS structures at about 400 °C is problematic. Namely, the vertical capillary surfaces (grain boundaries) between the columnar CdS grains perform as fast diffusion channels leading to the emergence of short circuits between the absorber and front contact. It was assumed that the grain boundaries contain residual hydroxy-oxide type compounds and form electrical barriers between columnar grains in the lateral direction of the CdS layer and that the electrical methods should be indicative of the behavior of grain boundaries in the annealing process. All samples were characterized by temperature dependence of DC conductivity in a temperature range of 50-300 K, X-ray diffraction, and scanning electron microscope. It has been found that the deeper layers of H2 and N2 annealed CdS preserve residual hydroxide, which released the gas phase in the recrystallization process of the chloride processing and created porosity on the CdTe/CdS interface.