Examination of a Cu-Deficient Layer on Cu(In, Ga)Se₂ Films Fabricated by a Three-Stage Process for Highly Efficient Solar Cells

The carrier recombination at a heterointerface between an n-type CdS buffer and a p-type Cu(In, Ga)Se2 (CIGS) absorber in CIGS solar cells can be suppressed by introducing a Cu-deficient layer (CDL) onto the CIGS surface. We have demonstrated that the introduction of CDL fabricated during the three-stage process has a positive impact on the performance of CIGS solar cells. Also, we have found that the introduction of Se element via a Se irradiation process after the second stage in the three-stage process during CIGS growth leads to the formation of uniform CDL on the CIGS surface, despite nonuniform CDL unintentionally formed on the CIGS surface without Se irradiation. In this article, the characterization of the CDL formed by the Se irradiation process is performed by energy-dispersive X-ray spectroscopy using a transmission electron microscope for a cross section of the CIGS solar cells. It was found that CDL formed by introducing the Se irradiation process shows high In content, while the CIGS region near the CDL shows a high Cu content. The formation of a valence band offset of 0.15 eV resulting from CDL on the CIGS surface was observed by ultraviolet photoelectron spectroscopy, leading to the successful suppression of recombination at an interface between the CdS buffer and CIGS absorber. In addition, a buried homojunction near the CIGS surface region was directly observed at the interface between the CDL and CIGS regions by employing a differential phase contrast method, suggesting that CDL has n-type conductivity owing to the In-rich condition in CDL and the diffusion of the Cd element into CDL. This unique and simple method may help to further enhance the device performance of CIGS solar cells.