Comparative study of nano-structured CdS thin films prepared by CBD and spray pyrolysis : Annealing effect

CdS films were deposited on glass and ITO-covered glass substrates by two techniques: chemical bath deposition (CBD) and spray pyrolysis technique. CdCl2 and thiocarbamide were used as basic precursors in both cases. After deposition, films were isothermally annealed in evacuated (1.3 Pa) quartz ampoules at 450 °C for 5 min and slowly cooled down to room temperature. The morphology of the films was characterized by electron scanning microscopy (SEM), electrical resistivities were calculated from I-V measurements, optical properties by UV-Vis and PL, and the presence of the chlorine in the films was detected by wavelength-dispersive analysis (WDX). Annealing decreased the resistivity of the sprayed CdS films on glass up to two orders of magnitude (from 50 Ω·cm down to 0.1 Ω·cm), while the CBD CdS does not changed the resistivity. Moreover, with the aid of post-deposition annealing, we succeeded to reduce the series resistance of CdS/ITO structure by an order of magnitude. This should contribute to the smaller series resistance of the whole solar cell. As a result of annealing optical band gap (Eg) for CBD CdS shifted from 2.51 eV down to 2.42 eV, whereas the Eg of sprayed films (2.46 eV) did not change. All the films have high transparency over 80% in the visible spectrum (520- 850 nm). The further development of annealing process for the improvement of continuity and conductivity of ultra-thin CBD CdS films is discussed.