Mutual Composition Transformations Among 2D/3D Organolead Halide Perovskites and Mechanisms Behind

Post-deposition processing of organolead halide films, by exposure to alkyl amine vapor and composition transformation, is an attractive approach for the formation of high-quality and large-scale perovskite films of desired compositions. However, the generalization of such composition transformation requires further understanding of the mechanism of the process. In this work MAPbI₃, BA₂PbI₄, and OA₂PbI₄ (MA⁺: CH₃NH₃ ⁺, BA⁺: C₄H₉NH₃ ⁺, and OA⁺: C₈H₁₇NH₃ ⁺) are chosen as typical representatives of 3D and 2D organolead halide perovskites, and their transformations are systematically studied. It is found that the transformation can universally take place among these organolead iodides, irrespective of either from 3D to 2D or from 2D to 3D form. The important intermediates in these transformations have been identified to be alkyl amine-bound lead iodides of well-defined chemical composition and crystalline nature in solid state. Furthermore, such intermediates have unexpected good solubility in alcohol, enabling the use of ethanol as a processing solvent for fabrication of high-quality perovskite films and solar cells. The so-prepared perovskite solar cells display an average power conversion efficiency of 14.08% with the best one of 15.79%. These findings not only gain new insights on perovskite transformation mechanism, but also lead to developing an environmentally benign processing method for perovskite films and devices.