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.

Fig. 1. (a) Schematic illustration of the two-step procedure for fabricating FAPbI₃ films. (b–e) SEM images of PbI₂ surfaces prepared from precursor solutions at different temperatures: 25 °C (b), 50 °C (c), 70 °C (d), and 90 °C (e).

Fig. 2. (a–d) Optical microscopy images of FAPbI₃ surfaces. (e–h) SEM images of FAPbI₃ surfaces. The PbI₂ precursor solution temperatures used to fabricate the underlayer were 25 °C (a and e), 50 °C (b and f), 70 °C (c and g), and 90 °C (d and h).

Fig. 3. Structural and spectroscopic characterizations of FAPbI₃ films fabricated by the two-step process at different PbI₂ precursor solution temperatures in the first step. (a) XRD patterns. The vertical axis is on a logarithmic scale. (b) Absorbance spectra around FAPbI₃ band edge.

Fig. 4. (a–d) Colormaps of in situ PL spectra recorded during the annealing process in the second step. The PbI₂ precursor solution temperatures in the first step were 25 °C (a), 50 °C (b), 70 °C (c), and 90 °C (d). The Roman numerals denoted in (a) denote three stages within perovskite crystallization process. (e) Schematic illustration of perovskite crystal growth and PL outcoupling during the second step annealing process. (f) Temporal evolution of in situ PL peak intensity, normalized to the maximum for each dataset. (g) Schematics of perovskite film formation models in the second step. The upper and lower panels correspond to growth models for first-step solution temperatures of 25 or 90 °C, and 50 or 70 °C, respectively.

Fig. 5. (a–d) Images of perovskite films prepared with different FAI precursor solution temperatures: 25 °C (a), 50 °C (b), 70 °C (c), and 90 °C (d). (e) UV-vis absorbance spectra of perovskite films as a function of the second-step solution temperature.