Modulation of n-Type Units in Bipolar Host Materials toward High-Performance Phosphorescent OLEDs

9'-Pyridinyl-9'H-9,3':6',9 ''-tercarbazole (PyCz) is a bipolar host material in phosphorescent organic light-emitting diodes (PhOLEDs). A second n-type unit, either pyridine or diphenylphosphine dioxide (DPPO), is introduced onto the pyridine ring of PyCz at para- or metasite to design and prepare four novel dual n-type unit bipolar host materials m-BPyCz, p-BPyCz, m-POPyCz, and p-POPyCz. The incorporation of the second n-type unit pulls down the lowest unoccupied molecular orbitals and facilitates electron injection and transportation, resulting in better charge-balancing ability. As a result, these dual n-type unit bipolar hosts exhibit higher efficiencies and slower efficiency roll-off in their blue and green PhOLEDs. In particular, m-POPyCz containing a bulky DPPO as the second n-type unit with a metalinking possesses the best charge-balancing state and generates a maximum external quantum efficiency (eta(ext)) of 27.0% (corresponding to a current efficiency of 51.9 cd A(-1) and a power efficiency of 46.5 Im W-1) in its sky-blue device and still remained at a high eta(ext), of 23.6% even at the practical brightness of 1000 cd m(-2). These results clearly demonstrate that the dual n-type unit bipolar hosts with an optimized substitution position and steric effect is a new and effective type of host materials for high-performance OLEDs.