Identifying the Molecular Structures of Intermediates for Optimizing the Fabrication of High-Quality Perovskite Films

Abstract

During the past two years, the introduction of DMSO has revolutionized the fabrication of high-quality pervoskite MAPbI₃ (MA = CH₃NH₃) films for solar cell applications. In the developed DMSO process, the formation of (MA)₂Pb₃I₈·2DMSO (shorted as Pb₃I₈) has well recognized as a critical factor to prepare high-quality pervoskite films and thus accomplish excellent performances in perovskite solar cells. However, Pb₃I₈ is an I-deficient intermediate and must further react with methylammonium iodide (MAI) to be fully converted into MAPbI₃. By capturing and solving the molecular structures of several intermediates involved in the fabrication of perovskite films, we report in this work that the importance of DMSO is <b>NOT</b> due to the formation of Pb₃I₈. The use of different PbI₂-DMSO ratios leads to two different structures of PbI₂-DMSO precursors (PbI₂·DMSO and PbI₂·2DMSO), thus dramatically influencing the quality of fabricated perovskite films. However, such an influence can be minimized when the PbI₂-DMSO precursor films are thermally treated to create mesoporous PbI₂ films before reacting with MAI. Such a development makes the fabrication of high-quality pervoskite films highly reproducible without the need to precisely control the PbI₂:DMSO ratio. Moreover, the formation of ionic compound (MA)₄PbI₆ is observed when excess MAI is used in the preparation of perovskite film. This I-rich phase heavily induces the hysteresis in PSCs, but is readily removed by isopropanol treatment. On the basis of all these findings, we develop a new effective protocol to fabricate high-performance PSCs. In the new protocol, high-quality perovskite films are prepared by simply treating the mesoporous PbI₂ films (made from PbI₂-DMSO precursors) with an isopropanol solution of MAI, followed by isopropanol washing. The best efficiency of fabricated MAPbI₃ PSCs is up to 19.0%. As compared to the previously reported DMSO method, the devices fabricated by the method reported in this work display narrow efficiency distributions in both forward and reverse scans. And the efficiency difference between forward and reverse scans is much smaller