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

As all-inorganic perovskite (CsPbI 3- x Br x ) 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 CsPbI 3- x Br x perovskite for use in p-i-n perovskite solar cells (PSCs). We find that the DMAII coating has two effects on the CsPbI 3- x Br x perovskite film: surface passivation and phase stabilization of perovskite. Traps in the CsPbI 3- x Br x 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 CsPbI 3- x Br x 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.