Anharmonicity-Driven Rashba Cohelical Excitons Break Quantum Efficiency Limitation

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) APbX₃ (A = Cs/CN₂H₅; 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.