Highly Stable and Efficient Light-Emitting Diodes Based on Orthorhombic γ-CsPbI 3 Nanocrystals.

Orthorhombic γ-CsPbI 3 possesses the highest structural stability among the optically active (light-emissive) CsPbI 3 perovskites. Here, we make use of a seed-assisted heteroepitaxial growth to fabricate seed/core/shell CaI x /γ-CsPbI 3 /CaI 2 nanocrystals. Ultrasmall CaI x nanoparticles serve as seeds to template the Pb-centered octahedral arrangement which enables the formation of the γ-CsPbI 3 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, CaI 2 , isolates the formed γ-CsPbI 3 nanocrystals from the environment, which also significantly improves their stability under ambient conditions. Optical and electrical studies indicate that the seed/core/shell CaI x /γ-CsPbI 3 /CaI 2 structure possesses a shallower set of trap states as compared to cubic α-CsPbI 3 nanocrystals. Light-emitting diodes utilizing these γ-CsPbI 3 nanocrystals show a record high external quantum efficiency of 25.3%, high brightness of over 13600 cd/m 2 , 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/m 2 with no decline of brightness, which indicates their great commercial potential.