Decoration of Dibenzofuran Using Cyanocarbazole via 6-Position as a Molecular Design Approach for High-Triplet-Energy Bipolar Host Materials

Abstract

In this study, two new dibenzofuran derivatives featuring one or two cyanocarbazole units, 6-(dibenzo[b,d]furan-4-yl)-9-phenyl-9H-carbazole-3-carbonitrile (mBFCzCN) and 6,6'-(dibenzo[b,d]furan-4,6-diyl)bis(9-phenyl-9H-carbazole-3-carbonitrile) (dBFCzCN), were developed as host materials for phosphorescent organic light emitting diodes (PhOLEDs). A new molecular design connecting the cyanocarbazole to the dibenzofuran using the cyanocarbazole 6-position instead of its 9-position was created, and the effects of number of cyanocarbazole units in the dibenzofuran building block on the photophysical and electroluminescence properties were investigated in detail. The mBFCzCN compound revealed high triplet energy (2.78 eV) than that of dBFCzCN (2.68 eV) and good bipolar charge transporting properties. The potential of these materials as hosts for blue and green PhOLEDs was investigated using bis(4,6-(difluorophenyl)pyridinato-N,C^2' )picolinate iridium(III) (FIrpic) and tris(2-phenylpyridinato)iridium(III) (Ir(ppy)₃ ) dopants, respectively. The results indicated that the mBFCzCN with one cyanocarbazole unit showed better device performance than the dBFCzCN with two cyanocarbazole units in the blue and green devices. High external quantum efficiencies of 19.0 and 21.2 % were demonstrated in the blue and green PhOLEDs with the mBFCzCN host due to its high triplet energy and good bipolar charge transporting characteristics.

Keywords: carbazoles; charge transport; electroluminescence; host materials; phosphorescent organic light-emitting diodes.