Temperature-Assisted Crystal Growth of Photovoltaic α-Phase FAPbI₃ Thin Films by Sequential Blade Coating

Formamidinium lead triiodide (FAPbI₃) exhibits the smallest band gap among lead halide perovskites, which is more desirable for solar cell applications compared to methylammonium-based counterparts. However, it remains a big challenge to prepare phase-pure α-FAPbI₃ in addition to controlling the crystal morphology during film formation. Herein, we developed a temperature-assisted crystal growth to prepare high-quality thin films of α-FAPbI₃ by sequential blade coating. It is found that depositing organic cation FAI at elevated temperatures facilitates the growth of α-FAPbI₃, which otherwise yields mainly a yellow δ-phase at room temperature. In parallel, the crystal morphology of the perovskite films can be effectively manipulated by taking advantage of the porous structure of PbI₂. Solar cells prepared with the blade-coated α-FAPbI₃ yield a champion efficiency of 18.41%, which is among the highest values for FAPbI₃-only solar devices. These results suggest that two-step sequential blade deposition offers a viable approach to fabricate high-quality α-FAPbI₃ films for optoelectronic applications.