Revisit on the role of oxadiazole hole blocking layer in organic multilayer electroluminescent devices

Hole-blocking function of an oxadiazole derivative layer in a triple-layer organic electroluminescent device was revisited. Triple-layer devices with the structure of ITO/4,4′-bis[(3-methylphenyl)phenylamino]biphenyl/9,10-bis(4-cyanostyryl)anthracene (BSA-l)/1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (OXD-7)/MgAg were fabricated The OXD-7 layers were partially doped with a fluorescent squarilium dye, 2,4-bis[4-(diethylamino)-2-hydroxyphenyl]cycrobutenediylium-1,3-dioxide (Sq) or wholly doped with a dye which quenches fluorescence from OXD-7, 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (TIN). The emission from Sq was observed only in the device doped with Sq at the BSA-1/OXD-7 boundary region. The emission intensity from the emitter BSA-1 was not reduced by doping with the quencher TIN within the OXD-7 layer. These results gave direct evidence for the absence of injection of holes into the OXD-7 layer, in other words, the effective hole-blocking capability of the OXD-7 layer.