Selective efficiency boosting in thermally activated delayed fluorescence emitters by a secondary donor

PLQY and EQE of the TADF devices was improved by introduction of a secondary donor without any color change and delayed lifetime. [Display omitted] •Selective external quantum efficiency improvement by a secondary donor.•Molecular design strategy of a thermally activated delayed fluorescent emitter.•High efficiency and small efficiency roll-off in blue organic light-emitting diodes. The design strategy of thermally activated delayed fluorescence (TADF) emitters for high quantum efficiency in the TADF devices was developed by addition of a secondary donor in an ortho- linkage based donor–acceptor type TADF emitter. The secondary donors were substituted at para- position of the acceptor in the ortho- linkage based TADF compound to improve the absorption and emission properties. The external quantum efficiency of the TADF emitters was increased from 15.8% to 22.7% by the introduction of the secondary donor without changing the emission color of the devices. Based on these results, the molecular design strategy presented in this work can be effective to develop highly efficient blue TADF devices.