Abstract Accelerating reverse intersystem crossing (RISC) without sacrificing fast radiative decay is effective in suppressing efficiency roll-off (eRO) in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes. We here report a TADF emitter, CC-TXO-I, combining a bicarbazole donor (CC) and a sulfur-containing acceptor (TXO). The CC is used to accelerate radiative decay via moderate donor-acceptor torsion angle, and the TXO is expected to provide fast RISC by the heavy-atom effect. We realized very large rate constants of RISC (k _RISCs) of ∼10⁷ s⁻¹. Both k _RISCs and rate constants of radiative decay of CC-TXO-I increased with increasing doping concentration, resulting in improved eROs.