FORMATION OF CONDUCTIVE CdO THIN FILMS ON PHOTOCONDUCTIVE CdS THIN FILMS FOR WINDOW LAYER APPLICATIONS IN SOLAR CELLS

We report the formation of a thin layer of CdO on chemically deposited CdS thin films during air anneal at 370°C to 500°C for 5 min to 120 min. During a 5 min anneal, the sheet resistance of the CdS thin films drops from about 1013 □/9 to 3.5 k□/□(370°C) and 470 □/□(500°C). X-ray diffraction studies showed that this is associated with the formation of a thin layer of CdO layer, which occurs at temperatures above 370°C. The CdS, which remains under the conductive CdO top layer, is photosensitive - with photo-to-dark current ratio of 103 - 104. Essentially the air annealing converts the highly resistive and highly photosensitive intrinsic (i) CdS thin film into a (i)CdS-( n +)CdO layer. The technique offers prospects to convert the top part of a chemically deposited CdS thin film window layer of high photosensitivity, deposited on an absorber layer, to a conductive layer. This is of interest in thin film solar cell technology.