Effects of processing on the properties of polycrystalline CDTE grown by various deposition techniques

Enhanced performance of CdS/CdTe polycrystalline heterojunction solar cells has been observed following post-deposition chemical and temperature processing. However, correlation of the effects of the CdCl ₂ and 300°-500°C treatments with the resulting film properties seems to vary with the origin of the material. This paper examines and compares the effects of these treatments on the physical, structural, chemical, compositional, and electro-optical properties of polycrystalline CdTe films produced by sputtering, close-spaced sublimation, physical vapor deposition and metallo-organic chemical vapor deposition. The chemical and heat treatments are shown to decrease Cd-vacancy levels (PL measurements) and quench deep electronic levels (DLTS). These defect level determinations are correlated with increases in the minority-carrier lifetimes. Differences in grain growth and densification are reported for various deposition techniques. The possibility of grain boundary passivation associated with Cl and oxygen diffusion at that defect is reported. Atomic-force microscopy (AFM) is used to investigate nanostructural differences among the films, especially the presence of small grains in the vicinity of grain boundaries. The change in structure in these intergrain regions and annihilation of these <300-Å grains following processing are observed and correlated with the passivation and change of electronic defect levels