Formation of conductive CdO layer on CdS thin films during air heating

A thin layer of CdO is formed on a chemically deposited CdS thin film through reaction with atmospheric oxygen during heating in air for 5-240 min at 370-500 °C. The sheet resistance of the film drops from about 10 13 Ω-3.5 kΩ (370 °C) and 470 Ω (500 °C). From optical transmittance and reflectance spectra, the thickness of the CdO layer was found to be about 10 nm; the crystalline grain size is 12-25 nm, depending on the temperature and duration of heating; and the estimated electrical resistivity is 10 -4 -10 -3 Ω cm. The optical bandgap of the CdS-CdO layer is effectively that of the underlying CdS thin film, about 2.45 eV in the annealed film. The CdS, which remains under the conductive CdO top layer, is photosensitive, with a photo-to-dark current ratio of 10 3 and crystalline grain diameter of about 10 nm in the case of a film heated at 500 °C for 5 min. These results are discussed in the context of window layers in solar cells.