Phase-Stable Red-Emitting CsPbI₃ Nanocrystals: Successes and Challenges

The semiconducting bulk α-cubic CsPbI3 phase is stable at high temperature. However, recent developments concluded that in nanodimensions this phase can also be stable even at room temperature. The unique feature of these α-CsPbI3 nanocrystals is their low-energy red color emission, which remained an essential part for the perovskite family of nanocrystals to cover the entire visible spectrum. Even though these were reported to be stable at room temperature, it is only under certain conditions. These are mostly phase-sensitive, and under ambient conditions, the α phase is transformed to a nonemitting phase. Hence, the phase stability in these nanocrystals remained one of the major challenges in current research. In this Perspective, the origin of phase instability, observations of change in optical properties along with phase transformation under different environmental conditions, insights of possible modulations in A, B, and X sites of the perovskites, precaution in the purification process, and the ligand shell chemistry adopted during and postsynthesis for obtaining the stability of these CsPbI3 nanocrystals were systematically analyzed and reported. In addition, different possible aspects of future research for retaining the phase stability are also discussed.