Considering the HOMO/LUMO levels of the emitter, MesB-DIDOBNA-N (5.73 eV/2.24 eV) and the host, TSPO1 (6.79 eV/2.50 eV), holes and electrons are mainly transported via the emitter and the host molecules, respectively. In other words, the injected holes are mostly trapped on the emitters, which then capture the mobile electrons hopping via the transporting sites of the TSPO1 host, i.e., the trapassisted recombination mechanism. Therefore, at a low cD of 1.5 wt%, it has a certain percentage of electrons that can migrate through the EML without being transferred to the emitters, which then leak into the TCTA layer and induce EL there. Indeed, EL at around 380 nm was observed in MesB-DIDOBNA-N device, which was attributed to the emission from TCTA. When the cD is above 3 wt%, sufficient recombination is achieved, and the EL originates only from MesB-DIDOBNA-N. The EQEmax increased from 9.3 to 11.3 and 13.6% for the devices with emitter cD of 1.5, 3.0 and 5.0 wt%, respectively (Figure ). The MesB-DIDOBNA-N OLED has the highest EQEmax among all the reported fluorescent and TADF emitters with lEL around 400 nm (Figure and Table ). It is noted that the maximum luminance (Lmax) of the device is limited by the severe efficiency roll-off, which is attributed to a combination of carrier imbalance commonly observed in devices with low emitter doping in combination with high-energy band gap hosts and long-lived triplets that lead to an enhanced probability for bimolecular annihilation processes such as singlet-triplet annihilation, triplet-triplet annihilation processes, and triplet-polaron quenching. Low Lmax is endemic in deep blue TADF OLEDs with similar CIEy values, such as devices with tDIDCz (CIEy = 0.02) of 200 cd m -2 at 8 V , 29 DCzBN1 (CIEy = 0.05) of 320 cd m -2 at 10 V, 56 B-O-dpa (CIEy = 0.05) of 180 cd m -2 at 8 V, and tBuOBOtSAc (CIEy = 0.06) of 350 cd m -2 at 12 V, implying that factors other than the nature of the emitter need to be addressed in order to improve device performance, such as the stability of high triplet-energy host materials and the device structure.