New 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids as electron transporting materials for blended-layer organic light emitting diodes

Abstract

We describe the synthesis of 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrid molecules, e.g. 2,7-bis[2-(4-tert-butylphenyl-1,3,4-oxadiazol-5-yl]-9,9-dihexylfluorene 6, 2,7-bis{4-[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene 10, 2,7-bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene 11, 2,7-bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-spirobifluorene 13 and analogue 16, comprising the 9,9-dihexylfluorene or spirobifluorene core units to which are attached aryl- or diaryl-oxadiazole units to provide linearly extended π-conjugated systems. The X-ray crystal structure is reported for compound 11. We have fabricated single-layer organic light-emitting diodes (OLEDs) using blends of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as the emissive material with the electron transport (ET) compounds 6, 10, 11, 13 and 16 added to enhance electron injection. For all the devices studied electroluminescence originates exclusively from the MEH-PPV material. The external quantum efficiencies of the devices increased with increasing concentration of the ET compound up to 95% by weight, and are greatly enhanced (>two orders of magnitude) compared to pure MEH-PPV reference devices. Further improvements have been achieved by adding a layer of PEDOT : PSS and efficiencies reach ca. 0.4% at 30 mA cm⁻² for devices in the configuration ITO/PEDOT : PSS/MEH-PPV–13 (5 : 95% by weight)/Al.