Tunable Crystallization and Nucleation of Planar CH₃NH₃PbI₃ through Solvent-Modified Interdiffusion

A smooth and compact light absorption perovskite layer is a highly desirable prerequisite for efficient planar perovskite solar cells. However, the rapid reaction between CH₃NH₃I methylammonium iodide (MAI) and PbI₂ often leads to an inconsistent CH₃NH₃PbI₃ crystal nucleation and growth rate along the film depth during the two-step sequential deposition process. Herein, a facile solvent additive strategy is reported to retard the crystallization kinetics of perovskite formation and accelerate the MAI diffusion across the PbI₂ layer. It was found that the ultrasmooth perovskite thin film with narrow crystallite size variation can be achieved by introducing favorable solvent additives into the MAI solution. The effects of dimethylformamide, dimethyl sulfoxide, γ-butyrolactone, chlorobenzene, and diethyl ether additives on the morphological properties and cross-sectional crystallite size distribution were investigated using atomic force microscopy, X-ray diffraction, and scanning electron microscopy. Furthermore, the light absorption and band structure of the as-prepared CH₃NH₃PbI₃ films were investigated and correlated with the photovoltaic performance of the equivalent solar cell devices. Details of perovskite nucleation and crystal growth processes are presented, which opens new avenues for the fabrication of more efficient planar solar cell devices with these ultrasmooth perovskite layers.