• • Champion device achieved a power conversion efficiency (PCE) of 14.29%, with an open-circuit voltage (VOC) of 591 mV, the highest reported for CZTSSe solar cells to date, directly addressing the VOC deficit bottleneck.
• • The low-temperature annealing during ITO sputtering (SA) improved ITO crystallinity, carrier concentration, and optical transmittance, while promoting In diffusion into CdS and CZTSSe layers, leading to an optimized ITO/In:CdS/In&Cd:CZTSSe heterojunction with favorable band alignment.
• • The SA treatment enabled epitaxial growth at the CZTSSe/CdS interface by optimizing lattice matching, which reduces interfacial recombination and enhances carrier transport, critical for achieving high VOC.
• • The study demonstrates a synergistic approach to interface engineering and defect passivation, providing a pathway to overcome the efficiency gap between CZTSSe and CIGS technologies, with potential for industrial scalability.