Carbazole (Cz) has been widely used as an electron-donating group in optoelectronic materials because of its moderate donor strength and the ease with which chemical derivatization can occur. Carbazole is also a fully aromatic structure, with no bonds to a sp 3 -centre as exists in acridine, phenoxazine and phenothiazine, bonds that are prone to scission and routes to degradation in the device. First, we compare the optoelectronic properties of Cz-TRZ derivatives where the Cz is either ortho, meta or para disposed with respect to the diphenyltriazine, as shown in Figure . The para-analogue CzTRZ (here, it is renamed as p-CzTRZ), first reported by Lee et al., is not a TADF emitter due to its too strong conjugation between Cz and TRZ, leading to a large DEST of 0.36 eV; the compound is a blue emitter with a λPL of 449 nm and a photoluminescence quantum yield, FPL, of 71% (10 wt% DPEPO). A blue OLED with CIE coordinates of (0.17, 0.11) showed only an EQEmax of 4.2%. A slightly improved OLED efficiency (EQEmax of 5.8%) was later reported by our group, which provided further confirmation that in the device p-CzTRZ acts as a fluorescent emitter.