Cyanopyridine Based Bipolar Host Materials for Green Electrophosphorescence with Extremely Low Turn-On Voltages and High Power Efficiencies

Low driving voltage and high power efficiency are basic requirements when practical applications of organic light emitting diodes (OLEDs) in displays and lighting are considered. Two novel host materials m-PyCNmCP and 3-PyCNmCP incorporating cyanopyridine moiety as electron-transporting unit are developed for use in fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) based green phosphorescent OLEDs (PhOLEDs). Extremely low turn-on voltages of 2.01 and 2.27 V are realized, which are even lower than the theoretical limit of the emitted photon energy (hv)/electron charge (e) (2.37 V) of Ir(ppy)3. High power efficiency of 101.4 lm/W (corresponding to a maximum external quantum efficiency of 18.4%) and 119.3 lm/W (24.7%) are achieved for m-PyCNmCP and 3-PyCNmCP based green PhOLEDs. The excellent EL performance benefits from the ideal parameters of host materials by combining cyano and pyridine to enhance the n-type feature. The energetic favorable alignment of HOMO/LUMO levels of hosts with adjacent layers and the dopant for easy charge injections and direct charge trapping by dopant, their bipolar feature to balance charge transportations, sufficiently high triplet energy and small singlet/triplet energy difference (0.38 and 0.43 eV) combine to be responsible for the extremely low driving voltages and high power efficiencies of the green PhOLEDs.