Controllable selenization kinetics via stabilizing Se concentration enables 14.33% CZTSSe solar cells

The early-stage uniformity and stability of selenium (Se) vapor during selenization critically determine the power conversion efficiency (PCE) of Cu2ZnSn(S,Se)4 (CZTSSe) devices. However, traditional large-volume graphite chambers (Ctrl) provide an overly spacious diffusion environment, leading to delayed Se supply and concentration fluctuations that promote void defects and Cu–Zn disorder. To address this, we propose a small-sized graphite box (S-box) that stabilizes Se delivery at the source by confining the diffusion volume, shortening the mass-transport path, and accelerating concentration equilibration. COMSOL Multiphysics simulations verify that this design markedly improves diffusion efficiency and rapidly establishes a stable concentration field; correspondingly, experiments show that S-box films exhibit higher crystallinity, reduced Sn2+ content, a denser bilayer structure with fewer voids, and a significantly lower overall defect density. Low-temperature photoluminescence further indicates that non-radiative recombination associated with VCu and band tail states is effectively suppressed. Accordingly, devices fabricated with the S-box achieve a champion efficiency of 14.33% (certified at 14.07%), establishing “restricting the Se vapor diffusion space” as the key point of selenization optimization.