Alkali-metal-mediated control of phase segregation for flexible kesterite solar cells and modules with improved efficiency

Flexible kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) photovoltaics are attractive for lightweight and portable applications, but their efficiency remains limited by uncontrolled alkali-metal incorporation. Here we elucidate and exploit the distinct and complementary roles of Na and Li in controlling CZTSSe crystallization on flexible substrates. Our results show that Na promotes crystal growth, but its induced Se enrichment simultaneously drives large-scale SnSe x phase segregation. The incorporation of Li reshapes the free-energy landscape of Cu-related phases, promoting the formation of Cu x Se that consumes Se and thereby suppressing SnSe x phase growth while driving ordered phase evolution. This kinetic competition strategy yields high-quality CZTSSe films with reduced charge recombination loss and enables power conversion efficiencies of 14.5% (certified 14.2%) for flexible cells and 12.7% (certified 12.0%) for shingled modules. Our results provide mechanistic insights into alkali-metal regulation in chalcogenide solar cells and demonstrate a kinetic competition strategy that can be generalized to regulate crystallization in complex multinary materials.