Breakthrough in Efficiency of Exciplex Organic Light-Emitting Diodes Using 9-Phenylfluorene Based Electron Acceptors

Exciplex-thermally activated delayed fluorescence (exciplex-TADF) system is an excellent candidate for fabrication of high-efficiency organic light-emitting diodes (OLEDs) because of its more easily achieved small singlet–triplet energy splitting (Δ E ST ) and doping-control. However, exciplex-TADF is still faced with problems of low external quantum efficiency ( η ext ) and unclear effect of the structure-modification in electron acceptors. Herein, we provide a steric hindrance-increase strategy to obtain high-efficiency exciplex-emissions. Through introducing steric hindrance group (9-phenylfluorene) into N -ethylcarbazole of the dicyano-substituted 9-phenylfluorene, more noticeable electron–hole separation is obtained in the resultant acceptor-based exciplex system, which helps the exciplex harvest smaller Δ E ST , larger driving force and higher emission efficiencies. Encouragingly, the corresponding OLED displays a maximum η ext of 25.8%, which is one of the best efficiency values among reported exciplex-OLEDs, simultaneously possessing excellent current efficiency of 83.6 cd A –1 and power efficiency of 93.7 lm W –1 . It is expected that this work will offer a new avenue for designing electron acceptors for highly efficient exciplex-emissions