Novel oxadiazole-containing polyfluorene with efficient blue electroluminescence

A novel oxdiazole-containing polyfluorene copolymer (PF-OXD) was synthesized by attaching two electron-deficient, 4-tert-butylphenyl-1,3,4-oxadiazole groups onto the C-9 position of the alternating fluorene unit to form a 3-D cardo-structure. This polymer possesses a very high glass transition temperature (T_g: 213 °C) and very good thermal stability due to its rigid cardo-linkage. The results from the photoluminescence measurements of the isothermally heated PF-OXD thin film (150 °C for 20 h) show that the commonly observed aggregate/excimer formation in polyfluorenes is very effectively suppressed in this polymer due to its 3-D structure and high T_g. A double-layer LED device using this polymer as the emitting layer shows a bright blue emission with a low turn-on voltage at 5.3 V and a high brightness of 2770 cd/m² at a drive voltage of 10.8 V. The maximum external quantum efficiency is 0.52% at 537 cd/m² with a bias of 7.4 V. The improved device performance over that of poly(9,9-dioctylfluorene) (POF) may be due to better electron injection and transport in PF-OXD and the efficient energy transfer from the OXD side chain to the polyfluorene main chains.