Highly efficient white-electrophosphorescent devices based on polyfluorene copolymers containing charge-transporting pendent units

Abstract

We have developed an efficient green-emitting polymer (FTO-BT5) through the incorporation of low-bandgap 2,1,3-benzothiadiazole (BT) moieties into the backbone of a blue-light-emitting polyfluorene copolymer (PF-TPA-OXD), which contains hole-transporting triphenylamine (TPA) and electron-transporting oxadiazole (OXD) pendent groups. A polymer light-emitting device based on this BT-containing copolymer exhibited very bright green emission having Commission Internationale d'Énclairage (CIE) chromaticity coordinates (0.32, 0.61); the maximum brightness was 10232 cd m⁻² and the maximum luminance efficiency was 8.9 cd A⁻¹. In addition, a highly efficient three-band white-light-emitting diode was prepared using PF-TPA-OXD as a blue-light-emitting host doped with this green-light-emitting FTO-BT5 copolymer and a red-light-emitting Os phosphor Os(fppz). Without co-doping any other electron- and/or hole-transporting molecules, this white-light-emitting device reached a maximum external quantum efficiency of 4.1% (8.3 cd A⁻¹) at a luminance of 402 cd m⁻² and a current density of 4.8 mA cm⁻². When the luminance increased from 26 cd m⁻² (0.36, 0.32) to 5258 cd m⁻² (0.33, 0.31), the corresponding CIE chromaticity coordinates remain almost unaltered and very close to the emission of pure white light; the value of η_ext remained above 3% in this high-luminance region.