Sterically Shielded Electron Transporting Material with Nearly 100% Internal Quantum Efficiency and Long Lifetime for...

A high triplet energy (T₁) 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-T₁ ETM with a sterically shielding low-T₁ unit. Given the short-range interaction of the Dexter energy transfer, the large steric side groups of the low-T₁ ETM can effectively hinder the T₁ 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-T₁ ETM, with the EQE remaining at 21.2% at 1000 cd/m² and 17.8% at 5000 cd/m². Further, an EQE_max as high as 25.5%, a low turn-on voltage of 2.3 V, as well as a long T₉₀ of over 400 h at an initial luminance of 5000 cd/m² were achieved for green phosphorescent devices. This work highlights a viable strategy for developing high-performance ETMs, paving their way toward practical applications.