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

Abstract

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_maxs 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.