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 °C) and glass transition temperatures (T_g > 361 °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 P1host 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 cm²/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 P1host:emitter layers.