Oxygen in solution grown CdS films for thin film solar cells

A CdS layer prepared by chemical bath deposition (CBD) is included as part of the heterojunction in the best polycrystalline thin film solar cells, based on either CuInSe₂ or CdTe. The reason why the CBD grown CdS film gives the best cells may be related to its material properties. In this paper, we report about incorporation of impurities in CBD grown CdS films for CuInSe₂ thin film solar cells. Special attention is paid to oxygen impurities. The CBD-CdS films are shown by infrared spectroscopy (FTIR) to contain hydroxyl ions (OH^-) and water (H₂O) as well as carbon-nitrogen bonds, most probably cyanamide ions (CN₂^2-). At least on the surface (as demonstrated by X-ray Photoelectron Spectroscopy (XPS)) also carbonate ions (CO₃^2- are found. The distribution of oxygen throughout the CBD-CdS film is approximately uniform according to Secondary Ion Mass Spectrometry (SIMS) measurements. The average oxygen concentration is determined to 10-15 at.% by Rutherford Backscattering Spectrometry (RBS). For comparison evaporated CdS films were prepared. As expected, no detectable amount of oxygen is found in those films.