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

Formamidinium lead triiodide (FAPbI) 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-FAPbI 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-FAPbI by sequential blade coating. It is found that depositing organic cation FAI at elevated temperatures facilitates the growth of α-FAPbI, 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 PbI. Solar cells prepared with the blade-coated α-FAPbI yield a champion efficiency of 18.41%, which is among the highest values for FAPbI-only solar devices. These results suggest that two-step sequential blade deposition offers a viable approach to fabricate high-quality α-FAPbI films for optoelectronic applications.