Although the efficiency of metal halide perovskite light emitting diodes (PeLEDs) has been improved to an attractive level, the poor stability of perovskite emitting layers is a major concern for the application of PeLEDs. Herein, we report a facile ligand-assisted precipitation synthesis of stable dual-phase CsPbBr₃-CsPb₂Br₅ nanocrystals (NCs) for improving the stability of PeLEDs. In our synthetic process, the bromide-rich circumstance is beneficial to generate high quality dual-phase perovskite NCs with PLQY as high as 92% and a narrow emission linewidth (19 nm). More importantly, as-synthesized dual phase perovskite NCs exhibit extremely high thermal stability in heating tests in air with a considerable humidity of 30%-55% in comparison with previously reported single phase CsPbBr₃ NCs. The aforementioned advantages of our synthesized dual-phase CsPbBr₃-CsPb₂Br₅ NCs allow for the fabrication of light emitting layers of PeLEDs under ambient conditions. The fabricated green PeLED based on CsPbBr₃-CsPb₂Br₅ NCs shows a low turn-on voltage of 2.5 V and a high brightness of 8383 cd m^-2 at 8 V. Owing to the high stability of dual-phase CsPbBr₃-CsPb₂Br₅ NCs, the fabricated PeLED also exhibits better operational stability in comparison with those PeLEDs based on single phase CsPbBr₃ NCs. Our work presents a new route to fabricate stable perovskite light-emitting diodes using room temperature precipitated dual-phase CsPbBr₃-CsPb₂Br₅ NCs as emitting layer materials.

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