Organic-inorganic hybrid metal-halide perovskites, such as methylammonium lead iodide, have emerged as amazing semiconductors with immense potential in thin film photovoltaic owing to their impressive diffusion lengths. However, the instability of these perovskites in ambient air, due to the presence of hydrophilic and volatile organic cation, hinders their further commercialization. Although low-dimensional perovskite solar cells (PSCs) show better stability than conventional three-dimensional (3D) devices, the low power conversion efficiency (PCE) is delivered, due to the decline of carrier mobility and diffusion length. Here, a large organic cation, tert-butylammonium (t-BA), is incorporated into the 3D perovskite, which not only enhances the crystal stability, but also greatly reduces the trap density and improves the mobility of the perovskite film, leading to ∼1.8 μm electron and hole diffusion lengths. High-performance PSCs based on t-BA [Cs (FA MA ) ] Pb(I Br ) with champion PCEs of 20.62% (19.8% ± 0.4%) for 0.04 cm and 14.54% for 20.8 cm are demonstrated. More importantly, with humidity of 45–55%, the solar cells could sustain 80% of their “post burn-in” PCE after continuous working under light (AM1.5G, 100 mW cm ) in air for 1174 h. This lifetime is 63% longer than that (718 h) of the control Cs (FA MA ) Pb(I Br ) PSCs.