Solid-State Anion Exchange Reactions for Color Tuning of CsPbX3 Perovskite Nanocrystals

Herein, we report on a room-temperature anion exchange reaction of highly emitting, all-inorganic CsPbBr3 nanocrystals (NCs) taking place entirely in the solid state. A fast exchange from Br to I and Br to mixed Br/Cl without exertion of additional energy is observed within minutes to hours, taking place by immobilization of the perovskite NCs on pure potassium halide salts (KCl, KBr, and KI). Via adjustment of the halide ratios of the embedding salt matrix, the bright fluorescence of the CsPbX3 (X = Cl, Br, or I) NCs can be tuned over a wide spectral range (400-700 nm) while maintaining the initial photoluminescence quantum yields of similar to 80% and narrow full widths at half-maximum. We found that combinations of different initial CsPbX3 NCs and KX matrices result in different final halogen contents of the NCs. This is explained with a host-lattice limiting exchange mechanism. The anion exchange rate can be accelerated by pressing the soft, NC-loaded salts under pressure of 2.2 GPa. Because of the "cold flow" behavior of the potassium salts during the pressing, a complete embedding of the NCs into transparent salt pellets is achieved. This strategy allows for an easy adjustment of the NC loading as well as the form and thickness of the resulting composite. An encapsulation of the NC-salt pellets with silicone yields robustness and stability of the embedded NCs under ambient conditions. The ease of handling and the superior stability make the resulting perovskite composite materials attractive for various photonic and optoelectronic applications as demonstrated in a proof-of-concept color-converting layer for a light-emitting diode.