Rational Management of Charge Transfer Character of Benzothienopyrimidine-Based N-Type Host for Blue Phosphorescent Organic Light-Emitting Diodes

Two materials, 3-(4-(9H-carbazol-9-yl)benzo[4,5]thieno[3,2-d]pyrimidin-2-yl)-5-(triphenylsilyl)benzonitrile (CzBTPCNmSi) and 9-(2-(3-(triphenylsilyl)phenyl)benzo[4,5]thieno[3,2-d]pyrimidin-4-yl)-9H-carbazole-3-carbonitrile (CNCzBTPmSi), are synthesized as the N-type host (electron-transport type host) of a mixed host, harvesting triplet excitons of the blue phosphor by engineering the charge transfer character using a CN unit. The two host materials are based on benzothienopyrmidine modified using CN, tetraphenylsilyl, and carbazole groups to control the lowest unoccupied molecular orbital level, manage their intermolecular packing, and stabilize the molecule against holes, respectively. The two hosts are mixed with a P-type 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) to offer a mixed host for sky and deep blue-emitting phosphors. In the device applications, CzBTPCNmSi and CNCzBTPmSi show high external quantum efficiency (EQE) (around 24% in the sky-blue devices), whereas only CNCzBTPmSi exhibited high EQE (above 20% in the deep blue devices) due to high triplet energy. Moreover, the CNCzBTPmSi demonstrates a longer device lifetime than the control host without the CN unit. Thus, CN engineering can manage the triplet energy, EQE, and device lifetime of blue phosphorescent devices by controlling the charge transfer character.