Dissolution-Dictated Recrystallization in Cesium Lead Halide Perovskites and Size Engineering in δ-CsPbI₃ Nanostructures

Crystal strain, crystal size, and reactivity with surrounding species underlie the thermal stability of the photoactive γ-CsPbI₃ in theoretical and practical perspectives. The spontaneous transformation of optically γ-CsPbI₃ nanocrystals (NCs) to inactive δ-CsPbI₃ nanostructures (NSs) hinders the development of the fabrication of efficient photovoltaic devices. To understand this process, we conducted a comprehensive investigation on the phase transformation kinetics and the nucleation and growth of δ-CsPbI₃ NSs from γ-CsPbI₃ NCs (∼8 nm) in a stepwise manner. The reaction scheme involved independently carrying out reactions where the γ-CsPbI₃ NCs reacted with optimized amounts of aprotic polar solvent (acetone), which leads to dissolution followed by a recrystallization process at solvent interfaces (acetone/hexane) to observe even the early changes in this process. Interestingly, the γ-CsPbI₃ NCs during dissolution in acetone enable the release of PbI₂ NCs, eventually leading to changes in crystal phase, size, and shape of the NCs. As a result, we observed unique absorption spectra and multiple emission features that enable white light emission. In contrast to the previously explored phase transformation process (γ-CsPbI₃ to δ-CsPbI₃ NSs) observed in larger-sized γ-CsPbI₃ NCs (∼18 nm), which occurs through an oriented self-assembly process when the NCs come in contact with polar solvents, in our two-step solvent introduction procedure, the γ-CsPbI₃ NCs first transform into zero-dimensional Cs₄PbI₆ NCs by their dissolution in acetone. Depending on the rate of dissolution which is proportional to the amounts of supplied acetone, the reaction solution can result in Cs₄PbI₆ NCs, γ-CsPbI₃ NCs, or δ-CsPbI₃ NSs during the recrystallization process. Furthermore, our investigations provided insights into this phase transition mechanism governed by the seeded growth phenomenon. This research facilitates enhanced control over undesired transitions, thereby promoting the development of refined and uncomplicated methodologies for recycling stable γ-CsPbI₃ NCs.