Cation‐Mediated Low‐Frequency Phonon Suppression in Lead‐free Manganese Halides for High‐efficiency Green Light‐emitting Diodes

Lead halide perovskites (LHPs) have emerged as promising materials in optoelectronics, yet concerns over lead toxicity drive the search for lead‐free alternatives with efficient electroluminescence, especially in green‐emitting applications. Here, the photophysical functions of A‐site cations in manganese bromides are revealed, and design dimethylamino‐functionalized A‐site cations to modulate both phonon dynamics, film morphology, and energy level alignment, enabling unprecedented efficiency in solution‐processed green‐emitting lead‐free metal halide devices. Appropriately attaching of dimethylamino groups to benzene rings not only builds p–π conjugation that increases the rigidity of PPh4+ A‐site cations, but also weakens hazardous van der Waals interaction, which suppresses A‐site related nonradiative recombination. Importantly, methyl groups in dimethylamino groups enhance the flexibility of the A‐site cation, which suppresses the formation of grain boundaries. Moreover, dimethylamino groups regulate the energy levels of PPh4+, reducing charge injection barriers. Notably, electroluminescent devices are achieved with a maximum external quantum efficiency (EQEmax) of 12.0% and large‐area emission of 4 × 4 cm2, underscoring their potential for next‐generation display technologies.