High-Power-Efficiency Blue Electrophosphorescence Enabled by the Synergistic Combination of Phosphine-Oxide-Based Host and Electron-Transporting Materials

A new host material of (4-{1-[4-(diphenylphosphoryl)phenyl]-cyclohexyl}phenyl)bis(4-methylphenyl)amine (POPCPA) is designed and synthesized by integrating electron-donating 4,4'-dimethyldiphenylamine unit and electron-accepting diphenylphosphine oxide group into the cyclohexane skeleton. The design strategy endows the host material with a high triplet energy of 2.93 eV, a shallow HOMO level of -5.24 eV, and a bipolar charge transporting feature. In addition, a new electron-transporting (ET) material of 1,3,5-tri[3-(diphenylphosphoryl)phenyl]benzene (TP3PO), which possesses a high triplet of 2.78 eV, a deep HOMO level of -6.40 eV, and a good ET ability, is constructed by the suitable combination of three diphenylphosphine oxide groups and the triphenylbenzene skeleton. These features render these phosphine-oxide-based functional materials ideal for blue phosphorescent organic light-emitting diodes (PhOLEDs). By employing these functional materials, a blue device exhibits low driving voltages of 2.6, 3.6, and 5.4 V at the luminance of 1, 100, and 1000 cd m(-2), respectively, and the highest power efficiency (up to 45.3 lm W-1) to date for iridium(III) bis(4',6'-difluorophenylpyridinato)-tetrakis(1-pyrazoly)borate (FIr6)-based blue PhOLEDs, which is significantly higher than those of the FIr6-based PhOLEDs with n-doped electron-transporting layer or p-i-n structure. These results suggest that the high-power-efficiency blue PhOLEDs can be achieved by elaborate designing of host and electron-transporting materials systematically to suit the blue emitter and emission zone structure.