Achieving slow efficiency roll-off and tunable spectrum in high-performance tandem WOLEDs with mixed hosts

Highly efficient blue and warm white tandem OLEDs were designed and realized by doping blue thermally activated delayed fluorescence (TADF) emitter and red phosphorescent emitter into mixed hosts system together. Based on the optimization of charge generation unit (CGU), blue tandem device with external quantum efficiency of 30.93 % and color coordinates of (0.165, 0.365) was firstly realized. To achieve efficient and high-quality white OLEDs (WOLEDs), red phosphorescent emitter PQ2Ir (dpm) was doped into TCTA:26DCzPPy (1:1) to construct another electroluminescent (EL) unit near the anode. By optimizing the doping concentration of PQ2Ir (dpm), maximizing excitons utilization probability and controlling energy transfer processes, the optimal tandem WOLED with turn-on voltage, maximum brightness, current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 5.2 V, 29,595 cd m- 2, 65.19 cd A-1, 38.94 lm W- 1 and 40.46 %, respectively, was demonstrated. Excitingly, this device can still maintain an EQE up to 25.59 % at the certain brightness of 1000 cd m- 2, which is important for lighting application. The correlated color temperature (CCT) of this device tunes from 4401 K at 1000 cd m- 2 to 2642 K at 20,000 cd m- 2, thereby accommodating various scene requirements.