Ultrahigh Power Efficiency Thermally Activated Delayed Fluorescent OLEDs by the Strategic Use of Electron‐Transport Materials

Abstract Thermally activated delayed fluorescent (TADF) emitters are one of the most promising candidates for developing low‐cost organic light‐emitting devices (OLEDs) that can achieve an internal quantum efficiency of 100%. However, the power efficiency values are still significantly lower than that of their phosphorescent counterparts. To achieve high power efficiency, both a high external quantum efficiency and low drive voltage are required. Here, TADF OLEDs with an ultrahigh power efficiency of 133 lm W⁻¹ and an unprecedented low turn‐on voltage of 2.22 V are successfully developed via the strategic use of electron‐transport materials (ETMs). 9‐[4‐(4,6‐Diphenyl‐1,3,5‐triazin‐2‐yl)phenyl]‐N,N,N′,N′‐tetraphenyl‐9H‐carbazole‐3,6‐diamine as a TADF emitter is used. A series of green TADF OLEDs are fabricated using two different types of pyrimidine‐based ETMs as an exciplex host partner and electron‐transport layer to minimize the drive voltages. The optimized device shows an external quantum efficiency of 29.2% and maximum power efficiency of 133.2 lm W⁻¹ without any light‐outcoupling enhancement. The device also shows 124.0 lm W⁻¹ at 100 cd m⁻² and 95.2 lm W⁻¹ at a high brightness of 1000 cd m⁻². The drive voltages are 2.48 V at 100 cd m⁻² and 2.88 V at 1000 cd m⁻².