Vinyl-type polynorbornene with 9,9 -(1,1 -biphenyl)-4,4 -diylbis-9H-carbazole side groups as a host material for highly efficient green phosphorescent organic light-emitting diodes

Abstract

The soluble polynorbornene (P1) bearing 9,9'-(1,1'-biphenyl)-4,4'-diylbis-9H-carbazole (CBP) side groups was investigated as a host material for green emitters in phosphorescent OLED devices. The vinyl addition polymerization of norbornene monomers using Pd(II) catalyst efficiently produces P1 in combination with 1-octene chain transfer agent. P1 exhibits high thermal stability with high decomposition (T(d5) > 451 degrees C) and glass transition temperatures (T(g) > 361 degrees C). The HOMO (ca. -5.5 eV) and LUMO (ca. -2.1 eV) levels with the triplet energy of ca. 2.60 eV suggest that P1 is suitable for a host material for green emitters. The solution-processed devices based on the emissive layers containing P1 host doped with various concentration of fac-Ir(ppy)(3) (1-6 wt%) display stable green emission of fac-Ir(ppy)(3) with high device performances. The external quantum efficiency and power efficiency reach 7.2% and 11 lm/ W, respectively, at the optimum doping concentration of fac-Ir(ppy)(3) (2 wt%). The device performances are found to be slightly lower than those of PhOLED with molecular CBP host but higher than those of a PVK-based device. It is shown that in conjunction with the good processability of polynorbornene backbones, the high levels of the effective hole and electron mobilities of P1 (ca. 10(-3) and 10(-5) cm(2)/Vs, respectively) as well as large triplet energy inherited from CBP side groups are mainly responsible for the high performance of the phosphorescent OLEDs with solution-processed P1 host: emitter layers.