An effective method to control the rate of perovskite crystallization by incorporating rationally chosen additives into the perovskite precursor solutions is demonstrated. The processing additives simultaneously facilitate nucleation and modulate the kinetics of crystal growth during crystallization, leading to much smoother perovskite morphology with improved coverage area and crystal uniformity. As a result, it enables very high PCE (similar to 12%) planar-heterojunction solar cells to be fabricated through the low-temperature solution processes (under 150 degrees C). This study opens up a new direction for optimizing perovskite active layer properties to expand device performance ceilings.