Composition and film quality of perovskite are crucial for the further improvement of perovskite solar cells (PSCs), including efficiency, reproducibility, and stability. Here, it is demonstrated that by simply mixing 50% of formamidinium (FA(+)) into methylammonium lead iodide (MAPbI(3)), a highly crystalline, stable phase, and compact, polycrystalline grain morphology perovskite is formed by using a solvent-mediated phase transformation process via the synergism of dimethyl sulfoxide and diethyl ether, which shows long carrier lifetime, low trap state density, and a record certified 21.8% power conversion efficiency (PCE) in pure-iodide, alkaline-metal-free MA(0.5)FA(0.5)PbI(3)perovskite-based PSCs. These PSCs show very high operational stability, with 85% PCE retention upon 1000 h 1 Sun intensity illumination. A 17.33% PCE module (6.5 x 7 cm(2)) is also demonstrated, attesting to the scalability of such devices.