With near-trap-free and remarkably balanced bipolar electron and hole transport characteristics, CzDBA is a thermally activated delayed fluorescence (TADF) emitter, converting triplet excitons into singlet excitons via reverse intersystem crossing (RIC). This D-A-D type structure generates TADF with a photoluminescence quantum yield of ~100% and an 84% horizontal dipole ratio in the thin film. A green OLED based on CzDBA exhibits a high external quantum efficiency of 37.8 ± 0.6%, a current efficiency of 139.6 ± 2.8 cd A−1 and a power efficiency of 121.6 ± 3.1 lm W−1 with an efficiency roll-off of only 0.3% at 1,000 cd m−2. Furthermore, the balanced electron and hole transport results in a broad recombination zone, leading to greatly enhanced device stability.