Enhanced electron injection and efficiency in blended-layer organic light emitting diodes with aluminium cathodes: new 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids incorporating pyridine units

Abstract

This work focuses on the first 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids which incorporate pyridine units within the π-electron system, viz. 2,7-bis{5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}-9,9-dihexylfluorene (6) and 2,7-bis{5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}spirobifluorene (7). The thiophene analogue 2,7-bis{5-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]-thien-2-yl}-9,9-dihexylfluorene 11 was also synthesised and its X-ray crystal structure was obtained. There is a progressive red shift in the UV–Vis absorption and photoluminescence spectra on replacing benzene (8) with pyridine (6) and thiophene (11) consistent with increased planarity of the π-system and reduced HOMO–LUMO gap along the series. Calculations at the DFT (density functional theory) level establish that inclusion of the pyridyl rings in 6 and 7 considerably enhances the electron affinity of the system, compared to phenyl analogues. Single-layer organic light-emitting diodes (OLEDs) have been fabricated by spin-coating blends of poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH–PPV) as the emissive material with added electron transport compounds 6 or 7 to enhance electron injection. The external quantum efficiencies of the devices were greatly enhanced compared to pure MEH–PPV reference devices. ITO/PEDOT ∶ PSS/MEH–PPV : 7 (30 ∶ 70% by weight)/Al devices exhibited an external quantum efficiency (EQE) of 0.5% and a luminous efficiency of 0.93 cd A⁻¹ at 9.5 V and a luminance of 100 cd m⁻². The modest increase in efficiency for the same device when Al was replaced by a Ca/Al cathode (EQE 0.6% and 1.2 cd A⁻¹ at 10.5 V) suggests that the two methods of enhancing electron injection into the MEH–PPV emitter are mutually exclusive. Utilising blended layers is an attractive alternative to using Ca electrodes, which are highly reactive and are unstable in the atmosphere.