Dual‐B‐Site Strategy Underpins Stable Unimodal Deep‐Blue Emitting Perovskites

Abstract The operational stability of mixed‐halide blue perovskite light‐emitting diodes (PeLEDs) is fundamentally constrained by intrinsic halide ion migration. Employing single‐halide perovskite systems represents a viable route to circumvent this issue, yet solution‐processed pure‐bromide perovskites typically exhibit undesirable green emission, impeding attainment of high‐color‐purity blue electroluminescence. Herein, this challenge is addressed through the strategic incorporation of Mg 2+ into the B‐site of the pure bromine‐based perovskite lattice. This substitution induces a substantial spectral blueshift and promotes phase homogenization by facilitating efficient energy transfer from small‐n to large‐n phases. Notably, the first systematic investigation and mechanistic elucidation of the origin of bimodal emission in blue PeLEDs is presented. Consequently, the champion PeLED delivers stable deep‐blue emission at 464 nm with a maximum external quantum efficiency of 4.76%. This study presents a definitive paradigm for achieving spectrally stable and efficient deep‐blue emission from single‐halide perovskites. The elucidated mechanism underlying emission bimodality provides critical guidance for future development of blue PeLEDs.