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

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.