Highly efficient deep-blue thermally activated delayed fluorescence (TADF) is observed from a charge-transfer compound bis[4-(3,6-dimethoxycarbazole)phenyl]sulfone (DMOC-DPS). In comparison with the previously reported carbazole/sulfone derivative with tert-butyl substituents on the carbazole donors, DMOC-DPS exhibits a much shorter excited-state lifetime in both an aromatic solution and an organic thin film, because the change of the substituent on the donor affects the molecular energy levels of the first singlet (S1) and triplet (T1) excited states in different ways, decreasing the energy gap between S1 and T1 (ΔEST). An organic light emitting diode (OLED) based on DMOC-DPS achieves a maximum external electroluminescence quantum efficiency (EQE) of 14.5% and reduced efficiency roll-off, with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.16), owing to efficient exciton harvesting that occurs through triplet-to-singlet up-conversion.