The intensity of the delayed PL increases with increasing temperature, evidencing the TADF character of this compound (Figure ). All these results indicate that DOBDiKTa has an improved TADF character compared to DiKTa. Firstly, we fabricated devices using mCP as the host, device 1. The electroluminescence (EL) spectrum shows narrowband blue emission (FWHM = 36 nm) at 457 nm (Figure ), which was consistent with the thin film PL spectrum (Figure ). The maximum external quantum efficiency (EQEmax) was 11.3% (Figure ), much lower than the theoretical prediction of 15% according to EQE = IQE×hout, where IQE is 75% and the light out-coupling efficiency (hout) is assumed to be 20%. To exclude the possible exciton loss due to the insufficient exciton confinement of the mCP host, a higher triplet energy level host, PPT [2,8bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene, ET = 2.96 eV] was employed. The FPL of the 1.5 wt% doped DOBDiKTa films in PPT increased to 79% compared to 75% in mCP at the same doping concentration. Nevertheless, the device (device 2) with the same device structure but using PPT as the host showed EQEmax as high as 16.7%, which is almost a 50% improvement compared to device 1. The significant improvement cannot be explained by the photoluminescence quantum yield alone, but instead should be related to the host material and bipolar carrier balance in the emitting layer (EML). Although a high EQEmax was observed in device 2, this device showed a 1 V larger turn-on voltage (Von), lower current density, lower luminance, and larger efficiency roll-off than that of device 1 (Figure ). In addition, the EL spectrum of device 2 was slightly red-shifted (lEL = 459 nm) and broadened (FWHM = 41 nm), undermining the color purity (Table ). The higher Von, lower current density and luminance of device 2 than those of device 1 were attributed to the low carrier mobility of the electron transport type PPT host, which, however, helped enhance the recombination probability of injected electrons and holes, leading to an improved EQE at low current density. As the current density increased, the unipolar PPT host showed more severe carrier imbalance, resulting in a larger efficiency roll-off than the device with the mCP host, which is a hole-dominated bipolar host. The larger permanent electronic dipole moment of PPT than mCP is responsible for the spectral red-shift and broadening.