Black Phase of Inorganic Perovskite Stabilized with Carboxyimidazolium Iodide for Stable and Efficient Inverted Perovskite Solar Cells

As all-inorganic perovskite (CsPbI3–xBrx) is prone to phase transition from the α phase (black phase) to the δ phase (yellow phase) in a humid environment or under heating, improving the phase stability of all-inorganic perovskite of the black phase is one of the urgent problems to solve. Herein, 1,2-dimethyl-3-acetylimidazolium iodide (DMAII) is spin-coated onto the surface of CsPbI3–xBrx perovskite for use in p–i–n perovskite solar cells (PSCs). We find that the DMAII coating has two effects on the CsPbI3–xBrx perovskite film: surface passivation and phase stabilization of perovskite. Traps in the CsPbI3–xBrx perovskite film can be reduced significantly by DMAII passivation, resulting in enhanced hole extraction and suppressed charge recombination. Consequently, the power conversion efficiency (PCE) is improved from 10.81 to 13.14%. Moreover, the DMAII coating can significantly inhibit the phase transition from the α phase to the δ phase in a humid environment or under heating, as characterized by the X-ray diffraction pattern, UV–vis absorption spectrum, and film color. After exposing the CsPbI3–xBrx perovskite films to a humid atmosphere (relative humidity = 40–60%) for 6 h, the PCE decreases dramatically to only 0.12% of the initial PCE for the PSC without the DMAII coating, while the PCE maintains 80% of the initial PCE for the PSC with the DMAII coating. In addition, when the PSC devices are heated at 120 °C for 4 h, the control PSC shows a 96% decrease in PCE, while the PCE decay is only 9% for the DMAII-coated PSC. These findings indicate that carboxyl-substituted imidazolium iodide is a kind of promising material to not only passivate traps but also stabilize the black phase of all-inorganic perovskite.