BackgroundOptimizing perovskite solar cells depends heavily on controlling the crystalline quality of the active layer during annealing. Synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS) enables effective in situ monitoring of structural evolution and additive-regulated phase transitions.PurposeThis study aims to systematically investigate how ammonium chloride (NH4Cl) and methylammonium acetate (MAAc) additives regulate the crystallization behavior of formamidinium lead iodide (FAPbI3) perovskite thin films during two-step sequential deposition using in situ GIWAXS.MethodsFirstly, PbI2 precursor films were prepared on ITO substrates via spin-coating, followed by annealing at 70 °C to form a stable intermediate phase. Then, three types of organic salt solutions (control, NH4Cl-doped, and MAAc-doped) were deposited onto the PbI2 films via a second spin-coating step to form perovskite intermediate films. Finally, in situ GIWAXS measurements were performed at the BL14B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) during annealing at 150 °C, collecting real-time two-dimensional diffraction patterns every 5 s to track the phase transformation and crystallographic orientation evolution.ResultsObservation results show that NH4Cl enhances the in-plane orientation of the δ-phase in the intermediate film, providing a structural template that facilitates rapid conversion to the optically active α-phase with a high orientation factor of ~0.28. In contrast, MAAc severely hinders α-phase formation, resulting in films where the δ-phase intensity exceeds that of the α-phase by more than 30 times after annealing stabilization, with disordered orientation. The phase transition from δ to α occurs within ~75 s for the NH4Cl-doped sample, significantly faster than the control.ConclusionsResults of this study demonstrate that NH4Cl additive promotes highly oriented FAPbI3 perovskite formation (orientation factor ~0.28) via enhanced δ-phase templating, while MAAc is unsuitable for two-step fabrication due to excessive δ-phase stabilization (δ/α intensity ratio >30). These quantitative insights provide structural kinetics guidance for optimizing perovskite solar cell fabrication processes.
WANG et al. (2026) studied this question.