Rapid Cooling After Device Annealing Suppresses Interface Losses in Cu 2 ZnSnS 4 Solar Cells Enabling High Efficiency and Stability

Wide‐bandgap kesterite (Cu 2 ZnSnS 4 , CZTS) is an earth‐abundant and environmentally friendly absorber, making it a promising candidate for both outdoor and indoor photovoltaic applications. However, device performance remains limited by multiple loss mechanisms, including recombination at the CdS/CZTS heterojunction and resistive back‐contact losses at the Mo/CZTS interface. Here, we report solution‐processed CZTS devices in which controlling the cooling dynamics after device annealing in ambient air provides an effective and scalable strategy to overcome these limitations and enable stable, high‐performance devices. By implementing a rapid cooling process (RCP), we suppress non‐radiative recombination at the CdS/CZTS heterojunction, resulting in a decrease in saturation current density ( J 0 ) by three orders of magnitude. Moreover, RCP improves the Mo/CZTS back contact by modifying the MoS 2 ‐related region, which may facilitate hole extraction and reduce back‐contact losses. As a result of these combined interface improvements, RCP‐treated devices retain a PCE of 11.1% after ∼1500 h of damp heat aging at 85°C, with a Voc corresponding to 62.4% of the thermodynamic limit. The RCP device also delivers an indoor PCE of 15.9% under 2700 K LED illumination, representing the highest reported performance for kesterite solar cells under indoor conditions at 1000 lux.