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

By using cyanopyridine as n-type unit and conjugated linking strategy between n-type and p-type moieties, the novel bipolar host material achieves more balanced charge injection and transportation and finally leads to efficient and stable green phosphorescence organic light-emitting diodes. At extremely high brightness of 10000 cd m−2, the efficiency roll-off is only 6.7% relative to the maximum value.242