A High-Resolution X-ray Photoelectron Spectroscopy Study of Carbon-Nitrogen Impurity in Chemical Bath Deposited CdS Thin Films

By means of X-ray Photoelectron Spectroscopy (XPS) we show the existence of a carbon-nitrogen (CN) impurity in chemical bath deposited (CBD) CdS thin films grown on Cu(In,Ga)Se2. The very broad nitrogen N(1s) feature ( ca. 3 eV) indicated a high degree of disorder around the carbon-nitrogen bond in as-deposited films. By vacuum-annealing the disorder decreased. This interpretation was supported by Infrared Spectroscopy (FTIR). At the same time as the CdS growth started the CN incorporation began and it continued throughout the deposition. The CN impurity is fairly water-soluble and effectively reduced by water treatments at 60° C. Cadmium cyanamide (CdNCN) has been proposed as a possible impurity, but our XPS analysis (N/C-ratio etc.) could not confirm this compound. Other decomposition products of thiourea ( e.g. Cd(SCN)2) were considered. The CBD-CdS film investigated here results in high-efficiency Cu(In,Ga)Se2/CdS thin film solar cells. However, if the presence of the CN impurity is responsible for the enhanced cell efficiency remains to be investigated.