Interface Formation between CdS and Alkali Postdeposition-Treated Cu(In,Ga)Se₂ Thin-Film Solar Cell Absorbers—Key To Understanding the Efficiency Gain

A combination of X-ray photoelectron/Auger electron spectroscopy and soft X-ray emission spectroscopy has been employed to investigate the impact of different alkali postdeposition treatments (PDTs) on the chemical structure of the (buried) CdS/Cu(In,Ga)Se2 heterojunction: the key interface in chalcopyrite-based thin-film solar cells. Chemical bath deposited (CBD) CdS layers of different thicknesses on NaF PDT (CIGSeNaF) and NaF + KF PDT (CIGSeNaF+KF) Cu(In,Ga)Se2 absorbers prepared at low temperature (to facilitate the use of flexible, e.g., polyimide, substrates) were studied. While we find the CdS/CIGSeNaF interface to be mainly free of significant chemical interaction, in the proximity of the CdS/CIGSeNaF+KF interface, an elemental redistribution involving Cd, In, K, S, and Se is revealed. For the early stages of the CBD-CdS process, our findings are in agreement with the conversion of the K–In–Se-type layer present on the CIGSeNaF+KF surface into a mixed Cd–In–(O,OH,S,Se)-type layer, probably having some Cd–In and (S,O)–Se composition gradients. For long CBD times—independent of employed PDT—we find the buffer material to be best described by a Cd(O,OH,S)-like species rather than by a pure CdS buffer. These findings shed light on the observed performance leap of corresponding CdS/CIGSeNaF+KF-based solar cells.