Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states

A spin-flip from a triplet to a singlet excited state, that is, reverse intersystem crossing (RISC), is an attractive route for improving light emission in organic light-emitting diodes, as shown by devices using thermally activated delayed fluorescence (TADF). However, device stability and efficiency roll-off remain challenging issues that originate from a slow RISC rate (k_RISC). Here, we report a TADF molecule with multiple donor units that form charge-resonance-type hybrid triplet states leading to a small singlet–triplet energy splitting, large spin–orbit couplings, and a dense manifold of triplet states energetically close to the singlets. The k_RISC in our TADF molecule is as fast as 1.5 × 10⁷ s⁻¹, a value some two orders of magnitude higher than typical TADF emitters. Organic light-emitting diodes based on this molecule exhibit good stability (estimated T₉₀ about 600 h for 1,000 cd m⁻²), high maximum external quantum efficiency ('29.3%) and low efficiency roll-off ('2.3% at 1,000 cd m⁻²).