Cesium Modulation in Cu(In, Ga)(S, Se) Solar Cells: Comprehensive Analysis on Interface, Surface, and Grain Boundary.

Cesium (Cs) incorporation and sulfurization on copper indium gallium selenide solar cells are the keys to improving the device quality. In this study, we explore the impact of Cs modulation on sulfur-containing Cu(In, Ga)(S, Se) (CIGSSe) absorbers, resulting in a performance increase of over 2%, reaching 18.11%. The improvement stems from a widened surface bandgap, grain boundary (GB) passivation, and a moderate injection blocking layer. The surface bandgap widens from 1.44 to 2.63 eV after Cs incorporation, confirmed by ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoemission spectroscopy (LEIPS) analysis. Cs presence and S depletion in GBs suggest a new phase that might mitigate carrier recombination. Heightened Cs incorporation introduces interface issues, including an augmented injection blocking layer and interface defects. Our study offers insights into interface challenges and GB engineering strategies in Cs-treated CIGSSe solar cells, illuminating the multifaceted impact of heavy alkali metal ion Cs in CIGS-based photovoltaics.