Thermally activated delayed fluorescence-based tandem OLEDs with very high external quantum efficiency

We fabricated highly efficient thermally activated delayed fluorescence (TADF)-based tandem green- and white-emitting organic light emitting devices (OLEDs). By using 4CzIPN as a green TADF emitter, mCP:TPBI as a co-host, and BPhen:Li/HAT-CN as a connecting unit, we demonstrated the first TADF-based green-emitting tandem OLED with a maximum current efficiency (CE) and external quantum efficiency (EQE) of 117.5cd/A and 32.5%, respectively. This is by far the highest EQE achieved to date for TADF devices. We also explored the first hybrid tandem white-emitting OLED using DMAC-DPS and PO-01 as blue TADF and orange phosphorescent emitters, respectively. The device exhibited a maximum CE and EQE of 78.5cd/A and 28.5%, respectively, with CIE (x, y) of (0.33, 0.45) at a brightness of 1000cdm(-2). From these data, we suggest that blue TADF materials have a great potential to replace the conventional fluorescent blue emitters for highly efficient white OLEDs composed of hybrid tandem structures.