Boosting Open‐Circuit Voltage to a Record 0.692 V in Solution‐Processed CIGSSe Solar Cells Through Surface Ga‐Rich Engineering

Abstract

ABSTRACT The open‐circuit voltage ( V OC ) deficit remains a primary bottleneck hindering the efficiency of solution‐processed Cu(In,Ga)(S,Se) 2 (CIGSSe) solar cells, largely due to the uncontrolled elemental diffusion during high‐temperature selenization that prevents the formation of a beneficial bandgap gradient. Here, we propose a novel and highly effective low‐temperature surface treatment strategy to overcome this long‐standing challenge. By spin‐coating a gallium‐thiourea complex solution onto the pre‐fabricated CIGSSe absorber, we construct a strategic Cu‐poor, Ga‐rich surface layer. Such design widens the surface bandgap, leading to an optimized band alignment at the CIGSSe/CdS heterojunction with a reduced conduction band offset (0.15 eV) and an enlarged valence band offset (1.13 eV). This synergistic effect drastically suppresses interface recombination. As a consequence, we achieve a champion device with a V OC of 0.692 V—the highest value reported to date for solution‐processed CIGSSe—and a power conversion efficiency of 16.65% (17.93% with an MgF 2 anti‐reflection layer). This work presents a simple yet powerful pathway for targeted front interface engineering, paving the way for high‐efficiency, low‐cost chalcopyrite photovoltaics.