Depth profile composition studies of thin film CdS:Cu2S solar cells using XPS and AES

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Abstract
Surface composition and depth profile studies of chemiplated thin film CdS:Cu2S solar cells have been carried out using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. These studies indicate that the junction is fairly diffused in the as-prepared cell. However, heat treatment of the cell at 210 degrees C in air relatively sharpens the junction and improves the cell performance. Using the Cu(2p32/)/S(2p) ratio as well as the Cu(LVV)/(LMM) Auger intensity ratio, it can be inferred that the nominal valency of copper in the layers above the junction is Cu+ and it is essentially in the Cu2S form. Copper signals are observed from layers deep down in the cell. These seem to appear mostly from the grain boundary region. From the observed concentration of Cd, Cu and S in these deeper layers and the Cu(LVV)/(LMM) ratio it appears that the signals from copper essentially originate partly from copper in CuS and partly from Cu2+ trapped in the lattice. It is significant to note that the nominal valence state of copper changes rather abruptly from Cu+ to Cu2+ across the junction.