High Efficiency Deep-Blue Phosphorescent Organic Light-Emitting Diodes with CIE x, y (<= 0.15) and Low Efficiency Roll-Off by Employing a High Triplet Energy Bipolar Host Material

Recently, bipolar host materials are the most promising candidates for achieving high performance phosphorescent organic light-emitting diodes (PHOLEDs) in order to maximize recombination efficiency. However, the development of host material with high triplet energy (E-T) is still a great challenge to date to overcome the limitations associated with the present PHOLEDs. Herein, a highly efficient donor-pi-acceptor (D-pi-A) type bipolar host (4'-(9H-carbazol-9-yl)-2,2'-dimethyl-[1,1'-biphenyl]-4-yl)diphenylphosphine oxide (m-CBPPO) comprising of carbazole, 2,2'-dimethylbiphenyl and diphenylphosphoryl as D-pi-A unit, respectively, is developed. Interestingly, a high E-T of 3.02 eV is observed for m-CBPPO due to highly twisted conformation. Furthermore, the new host material is incorporated in PHOLEDs as emissive layer with a new carbene type Ir(cb)(3) material as a deep-blue emitter. The optimized devices show an excellent external quantum efficiency (EQE) of 24.8% with a notable Commission internationale de l'eclairage (x, y) <= 0.15, (0.136, 0.138) and high electroluminescence performance with extremely low efficiency roll-off. Overall, the above EQE is the highest reported for deep-blue PHOLEDs with very low efficiency roll-off and also indicate the importance of appropriate host for the development of high performance deep-blue PHOLEDs.