Imide-modified thermally activated delayed fluorescent materials for pure blue organic light-emitting diodes with low efficiency roll-off

Introduction of a weak electron-donating or electron-withdrawing functional groups with steric hindrance to the molecular skeleton of thermally activated delayed fluorescence (TADF) emitters is necessary to finely tune singlet and triplet energy levels and suppress efficiency roll-off at high brightness without significantly changing their emitting colors. Herein, the weak electron-withdrawing succinimide (SI) is introduced for the first time to carbazole-benzonitrile (CzBN) skeletons as an acceptor through the connection of nitrogen atom of imides to the para-position of the cyano group. The resulting imide-modified TADF emitter SI-DtCzBN exhibits a short delayed fluorescence lifetime (tau d) of 0.85 lis in neat film and 3.4 lis in doped film with a high photoluminescence quantum yield (PLQY) of 80% owing to its moderate singlet-triplet energy gap (Delta EST) of 0.08 eV. OLED devices based on SI-DtCzBN as an emitter show a pure blue emission at 455 nm with EQEmax of 15.9% and extremely low efficiency roll-off. Thermally activated sensitized fluorescence OLEDs (TSF-OLEDs) are also fabricated by the utilization of SI-DtCzBN as a sensitizer and nu-DABNA as an emitter, which display blue electroluminescence with the peak at 472 nm, a narrow FWHM of 21 nm, and EQEmax of 13.6%.