Impact of precursor solution temperature on two-step spin-coated FAPbI3 film elucidated by surface morphology and in situ photoluminescence dynamics

Abstract

Perovskite solar cells fabricated by the two-step spin-coating method are highly sensitive to processing conditions, yet the role of precursor solution temperature remains poorly understood. Here, we systematically investigate how the temperatures of PbI₂ and formamidinium iodide (FAI) precursor solutions affect crystallization dynamics, surface morphology, phase formation, and optoelectronic properties of FAPbI₃ thin films. By combining microscopic characterization with in situ photoluminescence (PL) monitoring during annealing, we reveal distinct temperature-dependent nucleation, ripening, and film densification processes. PbI₂ solutions at intermediate temperatures (50–70 °C) yield compact underlayers that promote homogeneous conversion and suppress residual PbI₂, while low-temperature FAI solutions favor stabilization of the photoactive α-phase. Quantitative analysis of the in situ PL evolution clarifies the correlation between crystallization stages and optical out-coupling behavior. These results establish precursor solution temperature as a critical and practical parameter for controlling perovskite film formation in two-step deposition processes.