<title>Photo-crosslinkable polymers as hole transport materials for organic light-emitting diodes</title>

Abstract

A series of soluble arylamine-based hole transporting polymers with glass transition temperatures in the range of 97-108 degree(s)C have been synthesized. The synthetic methodology allows substitution of the aryl groups on the amine with electron-withdrawing and electron-donating moieties, which permits tuning of the redox potential of the polymer. The TPD-based monomers have been copolymerized with cinnamate-based moieties to obtain photo-crosslinkable polymers. These polymers have been used as hole-transport layers (HTLs) in multi-layer light-emitting diodes ITO/HTL/AlQ<SUB>3</SUB>/Mg:Ag [ITO=indium tin oxide, AlQ<SUB>3</SUB>=tris(8-hydroxyquinolinato)aluminum]. The maximum external quantum efficiency of the device increases as the redox potential of the HTL is increased. A fluorinated hole- transport polymer with a relatively high oxidation potential (390 mV vs ferrocenium/ferrocene) yielded the device with the highest external quantum efficiency and the longest lifetime under constant current operation. UV cross-linking was optimized to obtain an insoluble hole-transport layer with stable performance. Processing of these materials is compatible with a standard mask aligner used for photolithography. Electroluminescent devices have also been fabricated by spinning a blend of polystyrene and AlQ<SUB>3</SUB> on top of the crosslinked hole-transport layer.