Nanosecond-time-scale delayed fluorescence towards fast triplet-singlet spin conversion for efficient orange-red OLEDs with negligible efficiency roll-off

Efficient thermally activated delayed fluorescence (TADF) relies on fast reverse intersystem crossing (RISC) that promotes triplet-singlet spin conversion to shorter excited state lifetime, which is a decisive factor to achieve high electroluminescence performance. In this study, two new TADF emitters BenCN-PXZ and BDCN-PXZ employing an asymmetric donor-acceptor framework are designed and synthesized by combining different CNmodified acceptor units with phenoxazine (PXZ) as a strong electron donor unit. The influence of CN-group with different substituting sites is systematically compared to highlight the structure-property relationship. BenCNPXZ and BDCN-PXZ exhibit red emission with emission peak over 600 nm and short delayed lifetime within 0.6 mu s in the neat film. Surprisingly, the delayed lifetime of BDCN-PXZ is still shorter than 1 mu s even being doped into CBP host, implying their promising application in organic light-emitting devices (OLEDs). Both Non-doped and doped OLEDs are fabricated for comparison, among which, doped device based on BDCN-PXZ: CBP presented superior device performances and negligible efficiency roll-off (2.8%) at 1000 cd m(-2), which is almost the best results for orange-red TADF-based OLEDs.