ABSTRACT The performance and stability of perovskite solar cells are strongly governed by the crystallization behavior of their active layer. In two‐step sequential deposition, early‐stage film formation plays a decisive role in determining final phase purity and device quality. Guided by a data‐driven analysis of nearly 39 000 devices in the FAIR perovskite database, we identified solvent‐mediated quenching and thermal processing as key variables affecting power conversion efficiency (PCE), particularly in two‐step fabrication. To investigate these effects in real time, we designed and implemented a custom‐built, temperature‐controlled spin‐coating system, enabling precise thermal modulation during precursor deposition. Using this platform, we performed in situ GIWAXS measurements to study the crystallization dynamics of FA 0.5 MA 0.5 PbI 3 films over a temperature range of 30°C–90°C. Our results reveal a non‐monotonic relationship between spin‐coating temperature and α‐phase formation, governed by the interplay between precursor interdiffusion, PbI 2 crystallinity, and δ‐phase suppression. The custom thermal control enabled us to isolate and quantify these competing effects during the earliest stages of film formation, providing mechanistic insight into how spin‐coating temperature governs both phase purity and kinetic pathways in two‐step perovskite systems. Temperature‐dependent SEM and photovoltaic device measurements further demonstrate that early‐stage crystallization pathways directly translate into differences in morphology, charge‐transport continuity, and device performance. These findings inform targeted strategies for optimizing deposition protocols to balance rapid nucleation, phase stability, and device performance.
Saadawy et al. (2026) studied this question.