Hexamethylphosphoramide-Assisted Structure and Morphology Regulation of a PbI2 Film for Air-Processed Efficient Perovskite Solar Cells via a Two-Step Deposition Method

The two-step deposited method with the advantages of high reproducibility and controllability is widely used in fabricating highly efficient perovskite solar cells (PSCs). However, it is still a challenge to fabricate air-processed PSCs through this method due to the existing techniques with glove box circumstance that are not well extended to the ambient air-condition fabrication process. Here, hexamethylphosphoramide (HMPA) used as a solvent additive is incorporated into the first-step PbI2 precursor solution, which can effectively modify the morphology and crystallinity of the PbI2 layer, thereby creating more opportunities for the infiltration of the second-step organic salt. This two-step coating leads to the full conversion from PbI2 to the perovskite crystal and large grains, reduced defects, and enhanced uniformity for the final perovskite film. Consequently, a champion efficiency of 20.84% of the HMPA-based device fabricated using this procedure is achieved. The HMPA-based device shows long-term stability of the remaining 85.7% of the initial efficiency after 1000 h of storage in an ambient air environment. This method demonstrates the great potential toward air-processed efficient PSCs.