A ternary phosphine oxide host featuring thermally activated delayed fluorescence for blue PHOLEDs with >20% EQE and extremely low roll-offs

Abstract

Development of high-performance blue organic light-emitting diodes (OLEDs) is one of the main challenges because efficient host-dopant energy transfer is necessary to develop highly efficient OLEDs. In this study, an effective molecular design strategy for blue phosphorescent hosts with high triplet (T₁) energy and the feature of thermally activated delayed fluorescence (TADF) has been demonstrated using a ternary donor–acceptor–acceptor (D–A–A) molecule, DCDPOTZ, in which carbazole, triazine and diphenylphosphine oxide are incorporated as donor and acceptors. DCDPOTZ achieves the typical characteristics of TADF with a high T₁ energy level of 2.93 eV and a small singlet–triplet splitting of 0.27 eV, accompanied by the ambipolar property. Its host characteristics are further improved by its diphenylphosphine oxide group with large steric hindrance for suppressing intermolecular interaction-induced quenching. By adopting bis(4,6-difluorophenylpyridinato)tetrakis((1-pyrazolyl)borate)iridium(III) (FIr6) as a dopant, DCDPOTZ successfully realizes 100% host-dopant energy transfer efficiency. As a consequence, DCDPOTZ endowed its FIr6-based devices with extremely low driving voltages, the state-of-the-art external quantum efficiency (EQE) of up to 22.7%, and 100% internal quantum efficiency. At 1000 nits, the EQE is still beyond 20%, corresponding to a roll-off as low as 11%, which is one of the lowest values reported to date for blue phosphorescent OLEDs.