Thermal evolution of physical properties of vacuum evaporated polycrystalline CdTe thin films for solar cells

This paper reports the change in optical, structural, morphological and electrical properties of CdTe thin films with post-deposition thermal treatment in air atmosphere. The polycrystalline cadmium telluride (CdTe) thin films of thickness 850 nm were deposited on glass and ITO coated glass substrates by thermal vacuum evaporation technique followed by annealing at different temperature 150, 250 and 350 A degrees C. The physical properties have been investigated using characterization tools like UV-Vis spectrophotometer, X-ray diffraction, scanning electron microscopy coupled with EDS and source meter. The optical transmittance was observed to increase with post-deposition heat treatment which revealed the systematic reduction in optical energy band gap from 1.78 to 1.54 eV. The Swanepoel and Herve-Vandamme models as well as the dielectric theory were used to calculate various optical and dielectric constants. The thermal annealing enhances the crystallinity (zinc blende cubic structure) of films with preferred orientation (111). A number of crystallographic parameters were also evaluated and studied in depth with heat treatment. The surface morphology studies indicate that the films are homogeneous, densely packed, uniform and free from crystal defects. The current-voltage measurements exhibited ohmic behavior and the conductivity was found to increase with annealing temperature.