α-BaF Nanoparticle Substrate-Enabled γ-CsPbI Heteroepitaxial Growth for Efficient and Bright Deep-Red Light-Emitting Diodes.

All-inorganic CsPbI perovskite is attractive for deep-red light-emitting diodes (LEDs) because of its excellent carrier mobility, high color purity, and solution processability. However, the high phase transition energy barrier of optically active CsPbI black phase hinders the fabrication of efficient and bright LEDs. Here, we report a novel α-BaF nanoparticle substrate-promoted solution-processable heteroepitaxial growth to overcome this hindrance and obtain high-quality optically active γ-CsPbI thin films, achieving efficient and bright deep-red LEDs. We unravel that the highly exposed planes on the α-BaF nanoparticle-based heteroepitaxial growth substrate have a 99.5% lattice matching degree with the (110) planes of γ-CsPbI. This ultrahigh lattice matching degree initiates solution-processed interfacial strain-free epitaxial growth of low-defect and highly oriented γ-CsPbI thin films on the substrate. The obtained γ-CsPbI thin films are uniform, smooth, and highly luminescent, based on which we fabricate efficient and bright deep-red LEDs with a high peak external quantum efficiency of 14.1% and a record luminance of 1325 cd m.