Temperature-Assisted Crystal Growth of Photovoltaic α-Phase FAPbI 3 Thin Films by Sequential Blade Coating.

Formamidinium lead triiodide (FAPbI ₃ ) 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 ₃ 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 ₃ by sequential blade coating. It is found that depositing organic cation FAI at elevated temperatures facilitates the growth of α-FAPbI ₃ , 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 ₂ . Solar cells prepared with the blade-coated α-FAPbI ₃ yield a champion efficiency of 18.41%, which is among the highest values for FAPbI ₃ -only solar devices. These results suggest that two-step sequential blade deposition offers a viable approach to fabricate high-quality α-FAPbI ₃ films for optoelectronic applications.