Vapor-phase Annealing toward Homogeneous Bottom-Up Crystallization in Perovskite Solar Cells

High-quality perovskite films are essential to device efficiency and stability, yet inhomogeneous crystallization and interfacial defects remain major challenges. Here, we introduce a vapor-phase annealing (VPA) strategy to regulate crystallization kinetics and film morphology. By establishing a uniform thermal-humidity field, VPA is proposed to promote a bottom-up crystallization tendency, in contrast to the conventional top-down mode often observed in hot-plate annealing. This shift in growth direction could facilitate homogeneous nucleation, reduce interfacial void formation, and enable more complete precursor conversion with suppressed PbI₂ residue. As a result, the films exhibit improved optoelectronic properties, and additive-free inverted perovskite solar cells achieve a champion power conversion efficiency (PCE) of 22.15% with excellent long-term stability. These results suggest that VPA may provide a scalable route to mitigating crystallization inhomogeneity in perovskite photovoltaics.