α-BaF₂ Nanoparticle Substrate-Enabled γ-CsPbI₃ Heteroepitaxial Growth for Efficient and Bright Deep-Red Light-Emitting Diodes

All-inorganic CsPbI₃ 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 CsPbI₃ black phase hinders the fabrication of efficient and bright LEDs. Here, we report a novel α-BaF₂ nanoparticle substrate-promoted solution-processable heteroepitaxial growth to overcome this hindrance and obtain high-quality optically active γ-CsPbI₃ thin films, achieving efficient and bright deep-red LEDs. We unravel that the highly exposed planes on the α-BaF₂ nanoparticle-based heteroepitaxial growth substrate have a 99.5% lattice matching degree with the (110) planes of γ-CsPbI₃. This ultrahigh lattice matching degree initiates solution-processed interfacial strain-free epitaxial growth of low-defect and highly oriented γ-CsPbI₃ thin films on the substrate. The obtained γ-CsPbI₃ 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^-2.