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Closed‐shell light‐emitting diodes (LEDs) suffer from the internal quantum efficiency (IQE) limitation imposed by optically inactive triplet excitons. Here, an unrevealed emission mechanism of lead halide perovskites (LHPs) APbX 3  (A = Cs/CN 2 H 5 ; X = Cl/Br/I) that circumvents the efficiency limit of closed‐shell LEDs is explored. Though efficient emission is prohibited by optically inactive  J  = 0 in inversion symmetric LHPs, the anharmonicity arising from stereochemistry of Pb and resonant orbital‐bonding network along the imaginary A +… X − (T 1u ) transverse optical (TO) modes, breaks inversion symmetry, introducing disorder and Rashba–Dresselhaus spin‐orbit coupling (RD‐SOC). This results in bright cohelical and dark antihelical excitons. Many‐body theory and first‐principles calculations show that the optically active cohelical exciton is the lowest excited state in organic/inorganic LHPs. Thus, RD‐SOC can drive to achieve the ideal 50% IQE by utilizing anharmonicity, much over the 25% IQE limitation for closed‐shell LEDs.

Anharmonicity‐Driven Rashba Cohelical Excitons Break Quantum Efficiency Limitation