Highly Stable and Efficient Light-Emitting Diodes Based on Orthorhombic γ-CsPbI3 Nanocrystals

Orthorhombic γ-CsPbI3 possesses the highest structural stability among the optically active (light-emissive) CsPbI3 perovskites. Here, we make use of a seed-assisted heteroepitaxial growth to fabricate seed/core/shell CaIx/γ-CsPbI3/CaI2 nanocrystals. Ultrasmall CaIx nanoparticles serve as seeds to template the Pb-centered octahedral arrangement which enables the formation of the γ-CsPbI3 phase and at the same time inhibit lattice strain by blocking the force transfer that otherwise leads to an octahedral twist and so improve the structural stability of the resulting nanocrystals. An outer shell composed from the same material, CaI2, isolates the formed γ-CsPbI3 nanocrystals from the environment, which also significantly improves their stability under ambient conditions. Optical and electrical studies indicate that the seed/core/shell CaIx/γ-CsPbI3/CaI2 structure possesses a shallower set of trap states as compared to cubic α-CsPbI3 nanocrystals. Light-emitting diodes utilizing these γ-CsPbI3 nanocrystals show a record high external quantum efficiency of 25.3%, high brightness of over 13600 cd/m2, and an operational lifetime of ∼14 h before reaching 50% of their initial luminance. These devices can repeatedly be illuminated over 650 times at ∼500 cd/m2 with no decline of brightness, which indicates their great commercial potential.