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 t hree 1,3,4-oxadiazole-containing polymers with different solubility an d conjugation length (repeating units of phenylene and oxadiazole). Am ong them, the polymer with the shortest conjugation length (2 phenylen e and one oxadiazole rings) has the widest pi-pi bandgap and is not f luorescent. As an electron-injection material, it has been successfull y used to improve the quantum efficiency of polymer light-emitting dio des (LEDs) using dialkoxy derivatives of poly(1,4-phenylenevinylene) a s 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 fu rther enhances the electron transport property and has lowered the LED operating voltage more than the first polymer. The third 1,3,4-oxadia zole-containing polymer, with an even longer conjugation length, has s trong blue fluorescence. Blue LEDs have been fabricated using this pol ymer as the electroluminescent layer, conducting polyaniline as the ho le-injection layer, calcium as the cathode, and the first 1,3,4-oxadia zole-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% o f external quantum efficiency.