Combined Precursor Engineering and Grain Anchoring Leading to MA-Free, Phase-Pure, and Stable α-Formamidinium Lead Iodide Perovskites for Efficient Solar Cells.

α-Formamidinium lead iodide (α-FAPbI ) is one of the most promising candidate materials for high-efficiency and thermally stable perovskite solar cells (PSCs) owing to its outstanding optoelectrical properties and high thermal stability. However, achieving a stable form of α-FAPbI where both the composition and the phase are pure is very challenging. Herein, we report on a combined strategy of precursor engineering and grain anchoring to successfully prepare methylammonium (MA)-free and phase-pure stable α-FAPbI films. The incorporation of volatile FA-based additives in the precursor solutions completely suppresses the formation of non-perovskite δ-FAPbI during film crystallization. Grains of the desired α-phase are anchored together and stabilized when 4-tert-butylbenzylammonium iodide is permeated into the α-FAPbI film interior via grain boundaries. This cooperative scheme leads to a significantly increased efficiency close to 21 % for FAPbI perovskite solar cells. Moreover, the stabilized PSCs exhibit improved thermal stability and maintained ≈90 % of their initial efficiency after storage at 50 °C for over 1600 hours.