Thermal-decoupled selenization enables kesterite solar cells with 15.3% certified efficiency

Cu₂ZnSn(S,Se)₄ kesterite solar cells, while promising for sustainable photovoltaics, are constrained by undefined crystallization kinetics during selenization, which introduces bilayer crystallization with detrimental horizontal grain boundaries, voids, and secondary phases. This work traces this issue to early-stage Se-driven reaction at the back interface, which forms a low-melting-point Cu(S,Se) phase and triggers uncontrolled reverse crystallization. To address this, we propose a thermal-decoupled selenization strategy that creates a vertical Se concentration gradient in the initial stage. This approach decouples Se supply from the Cu(S,Se) formation temperature range, thereby suppressing bilayer crystallization. Consequently, it enables the growth of top-down columnar grains, which enhance carrier transport and suppress recombination, achieving a champion power conversion efficiency of 15.7% (certified 15.3%) in the resultant devices. This approach offers critical insights into crystallization kinetics and is also applicable to solution-processed Cu(In,Ga)Se₂ solar cells, highlighting its great significance for diverse copper-based chalcogenides.