Annealing CsPbX 3 (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.