Effective bipolar hosts prepared via dipole moment engineering for phosphorescent emitters and white OLEDs

Abstract

Efficient host materials with a bipolar charge transport character have attracted much attention of both academic and industrial communities. In this contribution, dipole moment engineering is proposed to transform a classical hole-type host material (o-CBP) into a bipolar organic light-emitting diode (OLED) host material via introducing nitrogen (N) heteroatom atoms. Thus, 2,2′-di(9H-carbazol-9-yl)-3,3′-bipyridine (o-CBPy) and 9-(3-(2-(9H-carbazol-9-yl-)phenyl)pyridin-2-yl)-9H-carbazole (o-CPPy) are designed and prepared. After introduction of N atom(s), both novel host materials possess a large charge-transfer dipole moment, which is favorable for bipolar transport. Relatively high thermal stability and triplet energy states are obtained for compounds o-CBPy and o-CPPy, and o-CBPy possesses a charmingly balanced charge-transport property as well. Impressively, both sky-blue and green phosphorescent OLEDs employing o-CBPy as the host matrix show very a high efficiency (>30%) and small efficiency roll-off. The white OLEDs based on o-CBPy also exhibit an EQE_max of 25.5% and a color rendering index (CRI) of 76. This research demonstrates that dipole moment engineering is an effective strategy for designing high efficiency bipolar host materials for OLEDs.