In Situ Generation of Long-Term Stable -CsPbI₃ Nanocrystals in the Cs₄PbI₆ Matrix via Ion Migration

Cubic alpha-CsPbI3 is recognized as one of the most promising semiconductors for optoelectronic applications owing to its suitable band gap, excellent optoelectronic performance, and thermal stability. However, its phase instability from alpha to delta poses a significant obstacle for practical applications. How to stabilize alpha-CsPbI3 becomes a paramount issue. Here, we propose a strategy in which alpha-CsPbI3 nanocrystals (NCs) are in situ generated in the Cs4PbI6 single-crystal (SC) matrix through I ion migration, facilitated by its ultralow activation energy (0.104 eV). The existence of the I vacancy can promote ion migration and can increase the content of alpha-CsPbI3. As alpha-CsPbI3 NCs are generated in situ, the light yellow Cs4PbI6 SCs convert to black. The lattice of Cs4PbI6 twines outside of alpha-CsPbI3 NCs without a grain boundary and restricts the migration of I and Cs within the alpha-CsPbI3 lattice. This mechanism effectively stabilizes alpha-CsPbI3 for over 2 years without experiencing a phase transition. Such stable alpha-CsPbI3 NCs exhibit deep red emission (700 nm), a high exciton binding energy (55.5 meV), and a long carrier lifetime (176.9 ns). Furthermore, annealing at 320 degrees C for 12 h did not result in any damage. Additionally, white-light-emitting diodes (WLEDs) were fabricated using black Cs4PbI6 SCs and yellow phosphor YAG:Ce3+ under an atmospheric environment and realized white emission. The WLEDs maintain approximately 86% luminescence after 18 days or under continuous operation for 12 h, demonstrating high stability and eliminating the need for short-wavelength blue chips. Overall, the alpha-CsPbI3 NCs deviating from Cs4PbI6 SCs offer an approach to realize highly efficient light-emitting diode applications.