W-shaped bipolar derivatives of carbazole and oxadiazole with high triplet energies for electroluminescent devices

Abstract

Four new compounds containing accepting oxadiazole and donating carbazole moieties were designed, synthesized and characterized by different techniques ranging from theoretical calculation to testing in electroluminescent devices. The derivatives show high thermal stability with 5% weigh loss temperature ranging from 359 to 383 °C and glass formation capability with glass transition temperatures being in the range of 107–147 °C. The solutions in non-polar solvents and solid neat films of the compounds or emit light in blue region with ca. 15% of photoluminescence quantum yields for non-doped films. The values of triplet energy (2.97–3.04 eV) of the synthesized oxadiazole and carbazole compounds were found to be higher than the energy of visible region excitons. Well-balanced hole and electron mobilities reaching 1 × 10−4 cm2/V at high electric fields were estimated for the layers of the compounds. Two derivatives of oxadiazole and carbazole were applied as hosts in blue electrophosphorescent devices. The best fabricated device demonstrated pure emission of a phosphorescent emitter and showed external quantum efficiency of 8.8% and brightness of 8700 cd/m2 at applied voltage of 10 V.