Complex compositional engineering of mixed halides​/mixed cations perovskites has recently fostered a rapid progress in perovskite solar cell technol. Here we demonstrate that when 10​% of formamidinium (FA+) is simply added into methylammonium lead iodide (MAPbI3) a highly cryst. and compositionally uniform perovskite is formed, self-​organizing into a stable "quasi-​cubic" phase at room temp. We reached power conversion efficiency of over 20.2​%, the highest value reported to date for FAxMA1-​xPbI3 perovskite.