Stabilization of a black CsPbI₃ Perovskite phase via octahedral tilting control with pressure

The air-stable, optically active CsPbI₃ perovskite phase is one of the promising candidates for applications in solar cells. However, the black CsPbI₃ perovskite is thermodynamically unstable and spontaneously converts to a yellow non-perovskite phase at room temperature. We report that a black perovskite phase can persist at room temperature by tuning the tilt of [PbI₆]^4- octahedra of high temperature perovskites with pressure, which shows improved stability and remains unchanged after releasing pressure to ambient conditions. Synchrotron X-ray diffraction, Raman spectroscopy, and photoluminescence measurements indicate that the preserved CsPbI₃ crystallizes into an orthorhombic perovskite structure and has a robust PL signal at ~702 nm. First-principles calculations reveal that the tilt of the [PbI₆]^4- octahedra play a significant role on stabilizing CsPbI₃ perovskite to room temperature. Our results present a promising approach to prepare superb stable black CsPbI₃ for perovskite solar cells.