Bicarbazole-cyanopyridine based bipolar host materials for green and blue phosphorescent OLEDs: influence of the linking style between P- and N-type units

Abstract

Three novel compounds, namely 6-(3-(9H-[3,9′-bicarbazol]-9-yl)phenyl)nicotinonitrile (CNPyPhBCz), 6-(9H-[3,9′-bicarbazol]-9-yl)nicotinonitrile (o-CNPyBCz), and 5-(9H-[3,9′-bicarbazol]-9-yl)nicotinonitrile (m-CNPyBCz), were developed for use as bipolar host materials for organic light-emitting devices (OLEDs). 9H-3,9′-Bicarbazole (BCz) was selected as the hole transporting group and cyanopyridine (CNPy) as the electron transporting group, which were linked either directly or through a phenylene bridge. The molecular conformation, triplet energy and charge transporting abilities strongly depend on the linking style. o-CNPyBCz, with BCz linked at the ortho-position of the pyridyl N atom, possesses the highest triplet energy of 3.05 eV, the non-planar conformation and more balanced charge transporting ability among these analogues. As a result, the o-CNPyBCz hosted sky blue and green phosphorescent OLEDs with bis(4,6-difluorophenylpyridinato-N,C²)-picolinato iridium (FIrpic) and fac-tris(2-phenylpyridine)iridium (Ir(ppy)₃) as doped emitters exhibited the best performance, with low turn-on voltages of 2.7 V and a moderate or high external quantum efficiency (EQE) of 15.9% and 22.6%, respectively.