Balancing Crystallization Kinetics and Lattice Strain Enables Thermally Stable Slot‐Die‐Coated Narrow‐Bandgap Perovskite Solar Cells

ABSTRACT Unbalanced crystallization kinetics in mixed Sn‐Pb perovskite cause film heterogeneity and high defect densities, thereby constraining overall performance and stability, particularly under scalable processing conditions. Here, we investigate nanocrystal‐mediated crystallization regulation in ambient‐processed, slot‐die‐coated narrow‐bandgap Sn‐Pb perovskites by combining CsPbI 3 nanocrystal‐assisted processing with time‐resolved in situ grazing‐incidence wide‐angle X‐ray scattering (GIWAXS). After nine ISOS‐T‐3 thermal cycles, the perovskite solar cells (PSCs) fabricated from seeded perovskite films retain 80% of their initial efficiency at 25°C. Operando GIWAXS reveals differences in lattice evolution, residual PbI 2 formation, and microstrain accumulation between the control and nanocrystal‐assisted films. By linking scalable processing and crystallization kinetics to the subsequent lattice response under thermal cycling, this work establishes a processing, crystallization, structure, stability relationship for narrow‐bandgap Sn‐Pb perovskites and highlights the importance of controlling early‐stage film formation for thermally robust, scalable perovskite photovoltaics.