A sky-blue thermally activated delayed fluorescence emitter based on multimodified carbazole donor for efficient organic light-emitting diodes

A new sky-blue thermally activated delayed fluorescent (TADF) emitter TRZ-DCF, has been developed based on 2-(9H-carbazol-9-yl)-7,7-dimethyl-5,7-dihydroindeno[2,1-b]carbazole (DCF) as donor and 2,4-diphenyl-1,3,5-triazine as acceptor. Due to the fusion of fluorene and carbazole moieties into the single carbazole donor, the multimodified emitter exhibits obvious TADF characteristics with a smaller singlet-triplet splitting (ΔEST) value of 0.22 eV and relatively high photoluminance quantum yield (PLQY) of 84%. As a result, a maximum external quantum efficiency (EQE) of 18.7%, a current efficiency (CE) of 39.0 cd A−1 and a power efficiency (PE) of 30.5 lm W−1 with a Commission Internationale de L’ Eclairage (CIE) coordinates of (0.18, 0.34), significantly surpassing the performance of the reference emitter 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (TRTZ), are achieved in optimal device construction. It is found that the EQE roll-off (20.8%) of TRZ-DCF is acceptable, that the efficiency drops to 14.8% at 1000 cd m−2 in optimal device construction. These results demonstrate the multiple modifications of weak electron-donating carbazole donor is an effective way of constructing high-efficiency blue TADF emitters with small roll-off.