New organic materials for light emitting devices based on dihexylfluorene-co-ethylenedioxythiophene copolymers exhibiting improved hole-injecting properties

We report the synthesis, optical and electrochemical characterisations of 9,9-di-n-hexylfluorene-co-3,4-ethylenedioxythiophene (DHF-co-EDOT) copolymers obtained from a mixture, in various ratios, of the two corresponding dibrominated monomers by dehalogenative polycondensation. Elemental analysis, IR studies coupled with 1H NMR clearly indicate, as expected, that the amount of each monomer unit in the final materials is strongly correlated with the feed composition even though the reactivity of the dibrominated EDOT seems lower than that of dibrominated DHF. It appears that even a low EDOT content, i.e. 11–15 mol%, within a PDHF main chain, leading to the copolymer COPO1, caused significant changes in the electronic properties of the material when compared to PDHF homopolymer. Higher EDOT contents lead to less soluble copolymers which are not suitable for investigation of their use as luminescent semiconducting π-conjugated materials in organic light emitting diodes (OLEDs). Green organic light emitting devices based on COPO1 have been investigated and showed improved hole injection properties when compared to devices based on PDHF homopolymer. The origin of the emitted light has been attributed to the concomitant emission of aggregates PDHF segments with EDOT short oligomeric segments. The use of an additional poly(3,4-ethylenedioxythiophene-2,5-diyl) (PEDOT)–polystyrene sulfonate (PSS) layer on the indium tin oxide (ITO) anode has also been investigated and leads to improved operating lifetime.