Ultra-high-efficiency white organic light-emitting diodes based on TADF material incorporated efficient exciplex hosts

White organic light-emitting diode (WOLED) has been recognized as a healthy light source. However, the device performance is still limited by the hosts or emitters. Herein, multiple-exciplex hosts are developed for WOLEDs, in which a novel exciplex host consisting of a thermally activated delayed fluorescent material, triazine-carbazole (Trz-PhCz), and an electron-transport material, 4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-methylpyrimidine (B4PyMPM), is adopted for green, yellow, orange and red phosphorescent dopants, of which high external quantum efficiencies (EQEs) of 25.0 %, 30.7 %, 32.5 % and 26.2 %, respectively, are achieved. On the other hand, a high-energy exciplex host consisting of 9,9 '-biphenyl-3,3 '-diylbis-9H-carbazole (mCBP) and B4PyMPM is designed for the blue emitter, iridium(III)bis(4,6-(difluorophenyl)-pyridinato-N,C2') picolinate (FIrpic), which guarantees a maximum EQE of 26.3 %. The small exciton energy difference between the mCBP: B4PyMPM and Trz-PhCz:B4PyMPM hosts can facilitate efficient energy transfer between the hosts. As a result, these exciplex hosts facilitate energy-efficient WOLEDs with a maximum EQE, power efficiency and current efficiency of 36.9 %, 137.4 lm W- 1 and 106.7 cd A-1, respectively, without using any optical out-coupling techniques, which provides inspiration for the future design of efficient OLEDs.