N-Phenylcarbazole substituted bis(hexylthiophen-2-yl)-benzothiadiazoles as deep red emitters for hole-transporting layer free solution-processed OLEDs

Two structural isomers of donor-π-acceptor-π-donor (D-π-A-π-D) type red light-emitting molecules (C3TBz and C4TBz), comprising N-phenylcarbazole as a donor, 2,1,3-benzothiadiazole as an acceptor, and either 3-hexylthiophene or 4-hexylthiophene as π-linkages, respectively, were designed and synthesized as hole-transporting emissive materials for OLEDs. C3TBz and C4TBz were chemically characterized, and their photoelectric properties were investigated by spectroscopy, electrochemical and theoretical studies. They showed strong D-A characteristics with intense deep red fluorescence in solutions, decent hole-transporting ability, good thermal and electrochemical stabilities. Both materials were successfully employed as emitters in the solution-processed double-layered OLEDs, which resulted in a pure red emission with promising device performance. Notably, the C3TBz-based device reached the best EL results with a maximum brightness of 2557 cd m−2, a maximum EQE of 3.08%, a maximum luminance efficiency of 4.87 cd A−1, and a low turn-on voltage of 3.2 V.