Asymmetric-triazine-cored triads as thermally activated delayed fluorescence emitters for high-efficiency yellow OLEDs with slow efficiency roll-off†
Two yellow-emissive triads, DPXZ–as-TAZ and TPXZ–as-TAZ, featuring an asymmetric triazine (1,2,4-triazine) core as an electron acceptor, phenoxazine units as electron donors and phenyl groups as π-linkages are constructed. Both emitters possess high thermal stability, suitable HOMO/LUMO levels, relatively high PLQY, near-zero singlet–triplet energy splitting, apparent TADF features and short delayed lifetimes. As a result, a yellow OLED utilizing TPXZ–as-TAZ as an emitter reaches a maximum external quantum efficiency (EQE) of 13.0%, which is on a par with a similar device (13.3%) based on a symmetric-triazine (1,3,5-triazine) cored analogue. Surprisingly, a TPXZ–as-TAZ-based device exhibits a high EQE of 11.5% and an ultra-slow efficiency roll-off of 11.5% at a high luminance of 1000 cd m−2. For the first time, this work reports the huge potential of asymmetric triazine compounds as TADF emitters for high-performance OLEDs.