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.