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

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 Mg²⁺ 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.