Manipulation of Morphology and Band Alignment Enables over 12% Efficient Wide‐Bandgap Solution‐Process Cu(In,Ga)S₂ Solar Cells

Abstract Sulfide chalcopyrite copper indium gallium sulfide (CIGS 2 ) solar cells show great potential as the top cell candidate for the tandem configuration due to their tunable wide band gap (1.6–2.4 eV), high‐efficiency potential, and excellent stability. However, the preparation of high‐quality sulfide chalcopyrite CIGS 2 presents a significant challenge, primarily due to the lack of atmospheric control in sulfurization reactions. In this study, based on the sol‐gel method, a novel approach is proposed utilizing double‐zone temperature sulfurization to control the sulfur atmosphere and sample temperature independently. With this strategy, the optimized sulfurization condition manages better crystallization and a lower defect density for the CIGS 2 absorber layer. Simultaneously, NaInS 2 is found to be synthesized on the upper surface of the CIGS 2 , enabling a better band alignment between CIGS 2 and the CdS buffer layer with less interfacial recombination. To further alleviate the mismatch of band alignment between the wide‐bandgap CIGS 2 absorber and buffer layer, an environmentally benign and more compatible ZnSnO (ZTO) buffer layer is utilized. As a result, an efficiency of 12.3% is attained for solution‐based wide bandgap CIGS 2 solar cells.