Sterically Shielded Electron Transporting Material with Nearly 100% Internal Quantum Efficiency and Long Lifetime for Thermally Activated Delayed Fluorescent and Phosphorescent OLEDs

A high triplet energy ( T 1 ) is usually taken as the prerequisite of the good exciton confinement of electron transporting materials (ETMs); however, there is usually a tradeoff with large mobility and stability. Here, we demonstrated that good exciton confinement can also be realized utilizing a low- T 1 ETM with a sterically shielding low- T 1 unit. Given the short-range interaction of the Dexter energy transfer, the large steric side groups of the low- T 1 ETM can effectively hinder the T 1 of the emitters from being quenched by increasing the intermolecular distance. Based on this concept, a maximum external quantum efficiency (EQE max ) as high as 21.3% was observed in the sky-blue thermally activated delayed fluorescence device using a low- T 1 ETM, with the EQE remaining at 21.2% at 1000 cd/m 2 and 17.8% at 5000 cd/m 2 . Further, an EQE max as high as 25.5%, a low turn-on voltage of 2.3 V, as well as a long T 90 of over 400 h at an initial luminance of 5000 cd/m 2 were achieved for green phosphorescent devices. This work highlights a viable strategy for developing high-performance ETMs, paving their way toward practical applications.