OPTICAL AND ELECTRICAL CHARACTERIZATION OF CDS THIN-FILMS

Thin CdS films have been grown by chemical bath (CdCl2, thiourea, ammo nia) deposition (CBD) on SnO2 (TO)-coated glass substrate for use as w indow materials in CdS/CdTe solar cells. High-resolution transmission electron microscopy revealed grains with an average size of 10 nm. The structure was predominantly hexagonal with a high density of stacking faults. The film crystallinity improved with annealing in air. Anneal ing in a CdCl2 flux increased the grain size considerably and reduced the density of stacking faults. The optical transmission of the as-dep osited films indicated a band gap energy of 2.41 eV. Annealing in air reduced the band gap by 0.1 eV. Annealing in CdCl2 led to a sharper op tical absorption edge that remained at 2.41 eV. Similar band gap value s were obtained by photocurrent spectroscopy and electroabsorption spe ctroscopy (EEA) using an electrolyte contact. EEA spectra were broad f or the as-deposited and air-annealed samples, but narrower for the CdC l2-annealed films, reflecting the reduction in stacking fault density. Donor densities of ca. 10(17) cm(-3) were derived from the film/elect rolyte junction capacitance.