How to Slow Down the Phase Transformation of CsPbBr₃ Nanocrystals in an Aqueous Medium? Playing with Metal-Ion...

Halide perovskites are game changers in the field of solar energy harvesting and optoelectronics. Yet, the soft and highly ionic nature of halide perovskites renders them water- and moisture-unstable via nonperovskite phase transformation. Introducing nanoconfined aqueous metal halides having different hydration energy using a reverse micellar route, we are able to critically control the perovskite nanocrystals (NCs) to nonperovskite phase transformation. The nanoconfined aqueous monovalent NaCl salt preferentially first phase transforms CsPbBr₃ to CsPb₂Br₅, and then the Br → Cl exchange process starts to form CsPb₂Cl₅ NCs within just ∼2 h of interaction. In contrast, the trivalent AlCl₃ salt first transforms CsPbBr₃ to CsPbCl₃, and this is followed by complete phase transformation to CsPb₂Cl₅ NCs in a ∼7-day duration. Here, the smaller hydration energy of the Na⁺ ion leads to the easy release of water molecules, causing fast CsPb₂Br₅ transformation via CsBr stripping in water. Conversely, the very large hydration energy of the Al³⁺ ion holds the water molecules more firmly, resulting in an extremely slow phase transformation. Overall, we use a very fundamental concept of metal ion hydration energy for efficient prevention of the detrimental nonperovskite phase transformation in the aqueous environment, thus advancing toward sustainable optoelectronic applications of perovskite NCs.