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

Cubic α-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 α to δ poses a significant obstacle for practical applications. How to stabilize α-CsPbI3 becomes a paramount issue. Here, we propose a strategy in which α-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 α-CsPbI3. As α-CsPbI3 NCs are generated in situ, the light yellow Cs4PbI6 SCs convert to black. The lattice of Cs4PbI6 twines outside of α-CsPbI3 NCs without a grain boundary and restricts the migration of I and Cs within the α-CsPbI3 lattice. This mechanism effectively stabilizes α-CsPbI3 for over 2 years without experiencing a phase transition. Such stable α-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 °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 α-CsPbI3 NCs deviating from Cs4PbI6 SCs offer an approach to realize highly efficient light-emitting diode applications.