Annealing CsPbX₃ (X = Cl and Br) Perovskite Nanocrystals at High Reaction Temperatures: Phase Change and Its Prevention

Annealing perovskite nanocrystals at high reaction temperature changes their crystal phase, shape, and optical properties. Carrying out reactions between 180 and 250 °C, the impact of thermal annealing for CsPbCl 3 and CsPbBr 3 nanocrystals in a reaction flask was investigated here. At higher temperature, a phase change was observed instantly, which could not be trapped even with ice-bath cooling. Interestingly, using a calculated amount of preformed alkylammonium halides as dual passivating agents, the nanocrystals of both CsPbCl 3 and CsPbBr 3 could even be stabilized for hours of annealing at 250 °C. CsPbCl 3, which was reported to be a poor emissive nanocrystal in comparison to CsPbBr 3, could sustain even more than 5 h of annealing at 250 °C and recorded ∼51% absolutely quantum yield. Details of the interface chemistry and the role of the used dual passivating agent for providing thermal stability are studied and reported in this Letter.