Defects from grains and boundaries and the random phase distribution of 2D perovskite films generally cause nonradiative recombination and a reduction in carrier transport and stability. Herein, we find that pseudohalides can promote the growth of oriented large-grained 2D perovskite films. It is shown that pseudohalide additives can induce the aggregation and the assembly of 2D perovskites, effectively facilitate the assembly of random low-n phases into pure high-n phases, and enlarge the corresponding grain sizes of films. Concurrently, the phase distribution, crystalline quality, and carrier transport of perovskite films are also improved. As a result, the pseudohalide-optimized perovskite solar cells achieve remarkably improved PCEs of 18.72% and a negligible hysteresis. Importantly, the unencapsulated devices retain 100% of the initial efficiencies at persistent heating at 55 °C for 1180 h. After they were kept operating under the maximum power point (MPP) with continuous light illumination for about 1200 h, the devices showed only 13.3% efficiency degradation.