Low-bandgap formamidinium-cesium (FA-Cs) perovskites of FA _{1- x} Cs _x PbI ₃ ( x < 0.1 ) are promising candidates for efficient and robust perovskite solar cells, but their black-phase crystallization is very sensitive to annealing temperature. Unfortunately, the low heat conductivity of the glass substrate builds up a temperature gradient within from bottom to top and makes the initial annealing temperature of the perovskite film lower than the black-phase crystallization point (~150°C). Herein, we take advantage of such temperature gradient for the diffusional growth of high-quality FA-Cs perovskites by introducing a thermally unstable MA ⁺ cation, which would firstly form α -phase FA-MA-Cs mixed perovskites with low formation energy at the hot bottom of the perovskite films in the early annealing stage. The natural gradient annealing temperature and the thermally unstable MA ⁺ cation then lead to the bottom-to-top diffusional growth of highly orientated α -phase FA-Cs perovskite, which exhibits 10-fold of enhanced crystallinity and reduced trap density ( ~ 3.85 × 10 15 c m − 3 ). Eventually, such FA-Cs perovskite films were fabricated into stable solar cell devices with champion efficiency up to 23.11%, among the highest efficiency of MA-free perovskite solar cells.