The composition of perovskite has been optimized combinatorially such that it often contains six components (A_xB_yC_1−x−yPbX_zY_3−z) in state-of-art perovskite solar cells. Questions remain regarding the precise role of each component, and the lack of a mechanistic explanation limits the practical exploration of the large and growing chemical space. Here, aided by transient photoluminescence microscopy, we find that, in perovskite single crystals, carrier diffusivity is in fact independent of composition. In polycrystalline thin films, the different compositions play a crucial role in carrier diffusion. We report that methylammonium (MA)-based films show a high carrier diffusivity of 0.047 cm² s⁻¹, while MA-free mixed caesium-formamidinium (CsFA) films exhibit an order of magnitude lower diffusivity. Elemental composition studies show that CsFA grains display a graded composition. This curtails electron diffusion in these films, as seen in both vertical carrier transport and surface potential studies. Incorporation of MA leads to a uniform grain core-to-edge composition, giving rise to a diffusivity of 0.034 cm² s⁻¹ in CsMAFA films. A model that invokes competing crystallization processes allows us to account for this finding, and suggests further strategies to achieve homogeneous crystallization for the benefit of perovskite optoelectronics.

钙钛矿的成分经过组合优化，通常包含六种成分（A _x B _y C _{1- x - y} PbX _z Y _{3- z}) 在最先进的钙钛矿太阳能电池中。关于每个成分的确切作用仍然存在疑问，并且缺乏机械解释限制了对庞大且不断增长的化学空间的实际探索。在这里，借助瞬态光致发光显微镜，我们发现，在钙钛矿单晶中，载流子扩散率实际上与成分无关。在多晶薄膜中，不同的成分在载流子扩散中起着至关重要的作用。我们报告了基于甲基铵 (MA) 的薄膜显示出 0.047 cm ²  s ^{-1的高载流子扩散率}，而不含 MA 的混合铯-甲脒 (CsFA) 薄膜表现出低一个数量级的扩散率。元素组成研究表明，CsFA 晶粒具有分级组成。这减少了这些薄膜中的电子扩散，如垂直载流子传输和表面电位研究中所见。MA的加入导致均匀的晶粒核心到边缘组成，在CsMAFA薄膜中产生0.034 cm ²  s ^{-1的扩散率。}一个调用竞争结晶过程的模型使我们能够解释这一发现，并提出进一步的策略来实现均匀结晶，从而有利于钙钛矿光电子学。