In the ongoing intense quest to increase the performance of perovskite solar cells, optimizing the morphology of perovskite material has become imperative to achieve high power conversion efficiencies. Despite the fact that nucleation plays a key role in controlling the crystal morphology, very little is known about the nucleation and crystal growth processes. Here, we perform metadynamics simulations of nucleation of methylammonium lead triiodide (MAPI) in order to unravel the atomistic details of perovskite crystallization from a γ-butyrolactone solution. The metadynamics trajectories show that the nucleation process takes place in several stages. Initially, dense amorphous clusters mainly consisting of lead and iodide appear from the homogeneous solution. These clusters evolve into lead iodide (PbI₂)-like structures. Subsequently, methylammonium (MA⁺) ions diffuse into these PbI₂-like aggregates triggering the transformation into a perovskite crystal through a solid–solid transformation. These enticing results allowed us to design new experimental strategies to rationally control the dimensions of MAPI grains using the spin-coating technique and to engineer homogeneous nucleation and growth of MAPI single crystals. The experimental results amply support our unprecedented observations related to the critical role of monovalent cations in inducing the nucleation process in lead halide perovskites.

中文翻译：

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溶液中甲基铵碘化钙钛矿形核的原子机理

为了提高钙钛矿太阳能电池的性能而进行的强烈要求中，优化钙钛矿材料的形态已成为实现高功率转换效率的必要条件。尽管成核在控制晶体形态方面起着关键作用，但对成核和晶体生长过程知之甚少。在这里，我们进行甲基碘化三碘化铅（MAPI）成核的元动力学模拟，以揭示从γ-丁内酯溶液中钙钛矿结晶的原子细节。元动力学轨迹表明成核过程发生在几个阶段。最初，从均相溶液中出现主要由铅和碘化物组成的致密无定形簇。这些团簇演变成碘化铅（PbI ₂）状的结构。随后，甲基铵（MA ⁺）离子扩散到这些类似PbI ₂的聚集体中，从而通过固-固转变触发转变为钙钛矿晶体。这些诱人的结果使我们能够设计新的实验策略，以利用旋涂技术合理地控制MAPI晶粒的尺寸，并设计出MAPI单晶的均匀成核和生长。实验结果充分支持了我们与单价阳离子在诱导卤化钙钛矿中成核过程中的关键作用有关的空前观察。