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

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.