Polyfluorene-based light-emitting diodes with an azide photocross-linked poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid)hole-injecting layer

We used a water-soluble bis(fluorinated phenyl azide) to cross-link a poly(ethylene dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS), hole-injection layer, with a view to its future use with water-soluble emitters. To enable direct comparison with conventional PEDOT:PSS, we studied the cross-linked films in diodes incorporating the organic-solvent soluble polymer poly( 9 , 9 ′ -dioctylfluorene-alt-benzothiadiazole). Kelvin probe characterization of the PEDOT:PSS and electroabsorption measurements of the devices consistently show a 0.2 eV increase of the PEDOT:PSS work function upon cross-linking. We also observe a 70-fold reduction in resistivity, an increase of the current above threshold and a decrease of the "leakage" current below threshold.