Reversible correlation between subnanoscale structure and Cu content in co-evaporated Cu(In,Ga)Se2 thin films

Thin film solar cells based on Cu-poor Cu(In,Ga)Se₂ absorbers grown by a three-stage co-evaporation process have been shown to strongly benefit from a Cu-rich intermediate growth stage and the presence of alkali elements such as Na, K, and Rb, pushing the current record efficiency to 22.6%. However, some details of the mechanisms underlying these improvements are not yet fully understood, particularly on a very local scale. We therefore used extended X-ray absorption fine structure spectroscopy to study element-specific local structural parameters of Cu(In,Ga)Se₂ thin films with varying final Cu content, varying history of the Cu content, and varying alkali treatment. We find that the bulk structure on a subnanometer scale is unaffected by the presence of alkali elements, confirming that their beneficial effect mostly stems from improving the electronic properties of the material. In contrast, local structural disorder clearly increases with decreasing Cu content but does not depend on whether or not the material has passed through a Cu-rich intermediate stage. While the latter is known to improve film morphology and microstructure, it obviously has no effect on the subnanoscale structure, which exhibits a fully reversible correlation with the final Cu content of the Cu(In,Ga)Se₂ absorber.