Regulating the conformation of phenoselenazine enable efficient TADF emitter with rapid reverse intersystem crossing rate

Triplet exciton utilization plays a crucial role in determining the performance of organic light-emitting diodes (OLEDs). Pure organic thermally activated delayed fluorescence (TADF) emitters based on donor–acceptor architecture are promising emitters for their low cost and high efficiency. However, many TADF emitters exhibit slow reverse intersystem crossing process and therefore suffer from large efficiency roll-off in the electroluminescence device, which hinders their practical application for the advanced OLEDs operating at high brightness. Herein, through regulating the conformation of the heavy-atom containing phenoselenazine moiety in a donor–acceptor molecular framework with a methyl group, we have constructed a TADF emitter with both small singlet–triplet splitting energy and pronounced heavy-effect. The designed emitter MCPPSeZ shows decent photoluminescence quantum yield of 67 % together with rapid reverse intersystem crossing rate of 4.2 × 106 s−1. Consequently, the corresponding yellow light-emitting electroluminescence device realize good external quantum efficiency (EQE) of 21.4 % and significantly efficiency roll-off, with EQEs remain 19.5 % and 13 % at the brightness of 1000 and 10 000 cd m−2, respectively. Moreover, using MCPPSeZ as the sensitizer for a red multi-resonance emitter, the narrowband emissive hyperfluorescent OLEDs are fabricated, which exhibit an EQE over 20 % accompanied by suppressed efficiency roll-off.