1,3,4-OXADIAZOLE-CONTAINING POLYMERS AS ELECTRON-INJECTION AND BLUE ELECTROLUMINESCENT MATERIALS IN POLYMER LIGHT-EMITTING-DIODES

We report the synthesis, characterization, and optical properties of three 1,3,4-oxadiazole-containing polymers with different solubility and conjugation length (repeating units of phenylene and oxadiazole). Among them, the polymer with the shortest conjugation length (2 phenylene and one oxadiazole rings) has the widest pi-pi* bandgap and is not fluorescent. As an electron-injection material, it has been successfully used to improve the quantum efficiency of polymer light-emitting diodes (LEDs) using dialkoxy derivatives of poly(1,4-phenylenevinylene) as the electroluminescent layer and aluminum as the cathode. The second polymer, with an additional oxadiazole ring in the conjugated segment, is also an electron-injection polymer. This extra oxadiazole ring further enhances the electron transport property and has lowered the LED operating voltage more than the first polymer. The third 1,3,4-oxadiazole-containing polymer, with an even longer conjugation length, has strong blue fluorescence. Blue LEDs have been fabricated using this polymer as the electroluminescent layer, conducting polyaniline as the hole-injection layer, calcium as the cathode, and the first 1,3,4-oxadiazole-containing polymer as the electron-injection layer. These devices emitted a bright blue light, with 4.5 V of turn-on voltage and 0.1% of external quantum efficiency.