Organic two-layer light-emitting diodes based on high-T_g hole-transporting polymers with different redox potentials

A series of soluble arylamine-based hole-transporting polymers with glass transition temperatures in the range of 130-150°C have been synthesized. The synthetic methodology allows facile 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. These polymers have been used as hole-transport layers (HTLs) in two-layer light-emitting diodes ITO/HTL/Alq/Mg [ITO = indium tin oxide, Alq = tris(8-quinolinato)aluminum]. The maximum external quantum efficiency of the device increases if the redox potential of the HTL is increased to facilitate reduction of the positive charge carriers at the HTL/Alq interface. A fluorinated hole-transport polymer with a relatively large redox potential (390 mV vs ferrocenium/ferrocene) yielded the device with the highest external quantum efficiency of 1.25% photons/e^-. The device stability, however, follows the opposite trend. The device with the most electron-rich HTL exhibited the best performance after prolonged usage.