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-FAPbI3) has become one of the most promising candidate materials for high-efficiency and thermally stable perovskite solar cells (PSCs) owing to their outstanding optoelectrical properties and high thermal stability. However, achieving a stable form of α-FAPbI3 where both the composition and the phase are pure is very challenging. Herein, we first report on a combined strategy of precursor engineering and grain anchoring to successfully prepare MA-free and phase-pure stable α-FAPbI3 films. Engineering the precursor solutions by incorporating volatile FA-based additives completely suppresses the formation of non-perovskite δ-FAPbI3 during film crystallization. Grains of the desired α-phase are anchored together and stabilized by permeating 4-tert-butyl-benzylammonium iodide into the α-FAPbI3 film interior via grain boundaries. This cooperative scheme leads to a significantly increased efficiency close to 21% for FAPbI3 perovskite solar cells (PSCs), which is among one of the highest values to date. Moreover, the stabilized PSCs exhibit improved thermal stability and maintained ~90% of their initial efficiency after storing at 50 °C for over 1600 hours. These findings should open new routes towards fabrication of high-quality perovskite films and optoelectronic devices.