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 t ransition temperatures in the range of 130-150 degrees C have been synthesi zed. The synthetic methodology allows facile substitution of the aryl group s on the amine with electron-withdrawing and electron-donating moieties, wh ich permits tuning of the redox potential of the polymer. These polymers ha ve been used as hole-transport layers (HTLs) in two-layer light-emitting di odes ITO/HTL/Alq/Mg [ITO = indium tin oxide, Alq = tris(8-quinolinato)alumi num]. The maximum external quantum efficiency of the device increases if th e redox potential of the HTL is increased to facilitate reduction of the po sitive charge carriers at the HTL/Alq interface. A fluorinated hole-transpo rt polymer with a relatively large redox potential (390 mV vs ferrocenium/f errocene) yielded the device with the highest external quantum efficiency o f 1.25% photons/e(-). The device stability, however, follows the opposite t rend. The device with the most electron-rich HTL exhibited the best perform ance after prolonged usage.