All-fluorescent white organic light-emitting diodes with EQE exceeding theoretical limit of 5% by incorporating a novel yellow fluorophor in co-doping forming blue exciplex

Abstract

Abstract For all-fluorescent white organic light-emitting diodes (WOLEDs), it is a current challenge to achieve comparable device efficiency to phosphorescent devices. A novel yellow fluorophor (Y), synthesized by our group, realizes ultra-high electroluminescence performance with maximum luminance and external quantum efficiency (EQE) reaching 74820 cd/m2 and 5.65%, respectively, indicating a huge potential applications in developing high-performance all-fluorescent WOLEDs. Herein, we employ above yellow emitter Y to demonstrate a series of all-fluorescent WOLEDs, where the devices are fabricated by doping yellow emitter Y in blue-emitting thermally activated delayed fluorescent (TADF) host of mCP:PO-T2T-forming exciplex. And white emission are realized by the combination of blue and yellow emissions from mCP:PO-T2T host and yellow emitter Y through the incomplete energy transfer. By simply adjusting the doping ratio of yellow emitter Y in mCP:PO-T2T host, the optimized all-fluorescent white device achieves high device performance with maximum current efficiency, power efficiency, and EQE reaching 20.78 cd/A, 24.25 lm/W, and 8.22%, respectively. The EQE obviously exceed the theoretical limit value of 5% for all-fluorescent WOLEDs, and is comparable to some fluorescent/phosphorescent or all-phosphorescent WOLEDs in reported literatures. The achievement of such high performance is mainly ascribed to the highly efficient exciton utilization derived from excellent yellow fluorescent emitter Y and effective reverse intersystem crossing (RISC) from triplet level to singlet level in mCP:PO-T2T exciplex host.