A universal thermally activated delayed fluorescent host with short triplet lifetime for highly efficient phosphorescent OLEDs with extremely low efficiency roll-off

Triplet-involved quenching processes lead to significant efficiency roll-off in phosphorescent organic light-emitting diodes (OLEDs). Thermally activated delayed fluorescent (TADF) materials can reduce the triplet exciton density in the emitting layer (EML) via the up-conversion of triplet into singlet exciton. However, the effect of the triplet lifetime of TADF hosts on efficiency roll-off is still under-researched. Inspired by the fast spin-flip process in TADF with a small singlet-triplet energy gap, we herein reported a triazine-carbazole hybrid (Trz-PhCz) with a short triplet lifetime, and extremely low efficiency roll-offs were achieved for green, yellow, orange and red OLEDs with external quantum efficiencies (EQEs) over 20% at 10 000 cd m −2 . Particularly, the optimized orange device showed a record-high maximum EQE of 31.4%, and 25.5% at 10 000 cd m −2 . Furthermore, unprecedented high PE max s of 111.8 lm W −1 and 98.9 lm W −1 were obtained for Ir(ppy) 2 (acac)- and PO-01-based devices, respectively. Notably, a two-color white OLED based on a single host Trz-PhCz also showed a remarkably high maximum power efficiency (PE max ) of 102.5 lm W −1 with the Commission Internationale de l'Eclairage (CIE) of (0.42, 0.48). So, the Trz-PhCz-hosted phosphorescent OLEDs showed state-of-the-art device performance, and this work provided insight into the design of efficient TADF host materials via controlling their triplet lifetime for practical applications. A universal TADF host material with a small singlet-triplet energy gap was developed for phosphorescent OLEDs.