Cyanopyridine based bipolar host materials for phosphorescent light-emitting diodes with low efficiency roll-off: Importance of charge balance

Abstract

Two novel bipolar isomers, namely 3,5-bis(3-(9H-carbazol-9-yl)phenyl)isonicotinonitrile (m-DCzPyCN), and 3,5-bis(9-phenyl-9H-carbazol-3-yl)isonicotinonitrile (3-DCzPyCN), are designed and synthesized by selecting cyanopyridine as n-type unit and N-phenylcarbazole as p-type unit. Two peripheral carbazoles are conjugated through their C-3 atom with the central cyanopyridine in 3-DCzPyCN but are indirectly linked through the N-9 atom via meta-phenylene bridges in m-DCzPyCN. The linking strategy had significant effect on the physical properties of the isomers. The single-carrier device study reveals that 3-DCzPyCN has much better positive and negative charge balancing ability than m-DCzPyCN. The 3-DCzPyCN hosted green phosphorescent organic light-emitting diode exhibits superior performance over m-DCzPyCN, with the maximum current efficiency of 84.1 cd A−1 and external quantum efficiencies of 24.2% and low efficiency roll-off. At an extremely high brightness of 10000 cd m−2, the efficiency still remained at 78.4 cd A−1, which corresponds to a roll-off of 6.7% from the maximum value. Even at an ultra-high brightness of 100000 cd m−2, the efficiency is reduced by 47.0%. The superior performance of 3-DCzPyCN is ascribed to its remarkable charge balancing capability, which is the combined result of the cyanopyridine as n-type unit and the conjugated linking strategy between p- and n-type moieties.