In Situ PL and WAXS Insights into the Stability of α-FAPbI3 Phase

Formamidinium lead iodide (FAPbI3) remains limited by poor ambient stability, hindering its deployment in optoelectronic devices. Here, we combine in situ photoluminescence (PL) and wide-angle X-ray scattering (WAXS) to resolve the structural and optoelectronic evolution of FAPbI3 based films during crystallization across a range of annealing temperatures. WAXS reveals rapid formation and stabilization of the bulk α-phase irrespective of temperature, whereas PL uncovers distinct temperature-dependent defect kinetics. Above 140 °C, the additional dip in PL indicates a recrystallization process within the already-formed α-FAPbI3 that critically influences film stability. The extracted activation energy for this process (0.53 ± 0.02 eV) agrees with reported iodide-ion and formamidinium-cation diffusion barriers, implicating ion-mediated grain coarsening. These findings identify late-stage structural recrystallization as a key mechanism governing α-FAPbI3 stabilization and provide mechanistic insight for optimizing perovskite processing.