Solution‐Processed Thermally Activated Delayed Fluorescent OLED with High EQE as 31% Using High Triplet Energy Crosslinkable Hole Transport Materials

다국어 초록 (Multilingual Abstract)

Solution‐processed organic light‐emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) material as emitter have attracted much attention because of their low cost and high performance. However, exciton quench at the interface between the hole injection layer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and emitting layer (EML) in devices can lead to low device performance. Here, a novel high triplet energy (2.89 eV) and crosslinkable hole‐transporting material grafted with oxetane groups, N,N‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy)hexyloxy)phenyl)‐3,5‐di(9H‐carbazol‐9‐yl)benzenamine (Oxe‐DCDPA)), as crosslinked hole transport layer (HTL) into the interface of PEDOT:PSS layer and EML is proposed for prevention of exciton quenching, and among the reported devices with single HTL in solution‐processed TADF‐OLED, the highest external quantum efficiency (EQE)/luminous efficiency (ηL) of 26.1%/94.8 cd A−1 and 24.0%/74.0 cd A−1 are achieved for green emission (DACT‐II as emitter) and bluish‐green emission (DMAC‐TRZ as emitter), respectively. Further improvement, using double HTLs, composed of N,N′‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy))‐hexylphenyl)‐N,N′‐diphenyl‐4,4′‐diamine with high hole mobility and Oxe‐DCDPA with high triplet energy, leads to the highest EQE/ηL of 30.8%/111.9 cd A−1 and 27.2%/83.8 cd A−1 for green emission and bluish‐green emission, respectively. These two devices show the high maximum brightness of 81 100 and 70 000 cd m−2, respectively.
Double hole transport layers provide an effective prevention of exciton quench by poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) and facilitate hole injection/transporting to a thermally activated delayed fluorescent (TADF) emitting layer, resulting in solution‐processed bluish‐green and green emission TADF device with high maximum brightness and luminous efficiency/external quantum efficiency of 70 000 cd m−2, 83.8 cd A−1/27.2% and 81 100 cd m−2, 111.9 cd A−1/30.8%, respectively.