Temperature dependence study of FAPbI3 thin films annealed under controlled humidity and light conditions

Perovskite solar cells are emerging as promising alternatives to conventional silicon-based photovoltaics due to their superior optoelectronic properties. Formamidinium lead iodide (FAPbI 3 ) is a widely used active layer material, offering enhanced thermal stability and a lower bandgap. However, achieving phase-pure α-FAPbI 3 remains challenging due to its sensitivity to external environmental conditions. Recent studies have revealed that, in addition to humidity, light exposure during the annealing process significantly influences phase formation. In this work, we present a comparative study on the effect of annealing temperature using two different approaches: (i) dark annealing (DA) and (ii) vacuum-assisted dark annealing (VADA). FAPbI 3 films were prepared via a two-step spin coating method and characterized using XRD, UV–Vis spectroscopy, FTIR, SEM, and TEM analyses. Our results reveal that the VADA method, when annealed at 175 °C, yields phase-pure, highly crystalline α-FAPbI 3 with a bandgap of 1.53 eV and minimal δ-phase impurities. The improved crystallinity and suppression of the δ-phase are attributed to controlled humidity and elimination of light exposure during annealing. This approach offers valuable insights into a scalable and efficient route for fabricating high-quality α-FAPbI 3 films suitable for next-generation perovskite solar cells. • α-FAPbI 3 films were optimized at an annealing temperature of 175 °C for PSC applications. • Structural and morphological analysis was performed using XRD, FTIR, and TEM techniques. • The 175 °C annealed film exhibited a bandgap close to the ideal value for PSCs under controlled humidity and light conditions. • Light and humidity conditions were optimized to enhance α-FAPbI 3 phase formation.