Creation of Dual Thermally Activated Delayed-Fluorescence Exciplexes in a Bulk Emitting Layer and Its Interface with an Electron Transport Layer for Promoting the Performance of Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes Fabricated by a Solution Process

An exciplex, which is composed of electron donor andacceptor moleculesand formed by intermolecular charge transfer, is an excited-statespecies that is able to emit light or transfer its energy to a lower-energyemitter. In reported exciplex-based organic light-emitting diodes(OLEDs), their working mechanism is to generate exciplexes eitherin the bulk emitting layer (bulk exciplex) or at its interface withan electron transport layer (interface exciplex); both types givepromising device performance. Here, we propose a novel strategy ofcreating both types of exciplexes simultaneously (dual exciplexes)for the generation of more exciplexes for better device performanceas indicated in the improved photoluminescence quantum yield (PLQY).Impressively, the dual exciplex-based device with blue thermally activateddelayed fluorescence (TADF) emitter 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine(DMAC-TRZ) exhibits a record-high maximum external quantum efficiency(EQE(max)) of 26.7% among the solution-processed TADF blueOLEDs. By further doping with the red-emitting phosphor emitter intothe EML, the white device also gives a record-high EQE(max) of 24.1% among the solution-processed TADF-phosphor hybrid whiteOLEDs (T-P WOLEDs) with the Commission Internationale de L'Eclairage(CIE) coordinates of (0.34, 0.42), color rendering index of 70, andcorrelated color temperature of 5198 K. Furthermore, both blue andwhite devices show an ultralow efficiency roll-off with external quantumefficiencies at a practical brightness value of 1000 cd m(-2) (EQE(1000)) of 25.1 and 23.9%, respectively. This is thefirst report of employing a dual exciplex-based OLED with excellentdevice performance.