4,4'-Bis(9-carbazolyl)-2,2'-dimethylbiphenyl (CDBP) has a large bandgap (E = 3.4 eV) and a high triplet energy level (T1 = 3.0 eV). It is used as a fluorescent and phosphorescent host material in blue, green, red and white OLED and TADF devices.
The high triplet energy level of CDBP is caused by the steric twisting from the methyl groups (in the 2 and 2′ position) of the biphenyl core. Compared to CBP and mCBP, CDBP has higher tendency to form excimers due to the localisation of the of the excitation onto the both ends of the carbazole units.
CDBP can also function as an exciton-blocking layer - it can block excitons migrating from the emitting layer into the hole-transport layer.
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