Forming Intermediate Phase on the Surface of PbI₂ Precursor Films by Short-Time DMSO Treatment for High-Efficiency Planar Perovskite Solar Cells via Vapor-Assisted Solution Process

Morphology regulation is vital to obtain high-performance perovskite films. Vapor-assisted deposition provides a simple approach to prepare perovskite films with controlled vapor-solid reaction. However, dense PbI₂ precursor films with large crystal grains make it difficult for organic molecules to diffuse and interact with inner PbI₂ frame. Here, a surface modification process is developed to optimize the surface layer morphology of PbI₂ precursor films and lower the resistance of the induced period in crystallization. The vapor optimization time is shortened to several seconds, and the intermediate phase forms on the surface layer of PbI₂ films. We achieve porous PbI₂ surface with smaller grains through dimethyl sulfoxide vapor treatment, which promotes the migration and reaction rate between CH₃NH₃I vapor and PbI₂ layer. The PbI₂ precursor films undergo dramatic morphological evolution due to the formed intermediate phase on PbI₂ surface layer. Taking advantage of the proposed surface modification process, we achieve high-quality uniform perovskite films with larger crystal grains and without residual PbI₂. The repeatable perovskite solar cells (PSCs) with modified films exhibit power conversion efficiency of up to 18.43% for planar structure. Moreover, the devices show less hysteresis because of improved quality and reduced defect states of the films. Our work expands the application of morphology control through forming intermediate phase and demonstrates an effective way to enhance the performance of the PSCs.