alpha-Formamidinium lead iodide (alpha-FAPbI(3)) 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 alpha-FAPbI(3) 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 alpha-FAPbI(3) films. The incorporation of volatile FA-based additives in the precursor solutions completely suppresses the formation of non-perovskite delta-FAPbI(3) during film crystallization. Grains of the desired alpha-phase are anchored together and stabilized when 4-tert-butylbenzylammonium iodide is permeated into the alpha-FAPbI(3) film interior via grain boundaries. This cooperative scheme leads to a significantly increased efficiency close to 21 % for FAPbI(3) perovskite solar cells. Moreover, the stabilized PSCs exhibit improved thermal stability and maintained approximate to 90 % of their initial efficiency after storage at 50 degrees C for over 1600 hours.