In this work, single layer, light-emitting devices have been prepared from pristine tris(2,2' bipyridyl) ruthenium(II) hexafluorophosphate ([Ru(bpy)3](PF6)2) and blends of [Ru(bpy)3](PF6)2) with glassy polymers such as poly(methylmethacrylate) (PMMA), polycarbonate (PC), and polystyrene. Due to the electrochemical nature of the device operation, a high external quantum efficiency at a low operating voltage is achieved. For pristine devices fabricated with an Al cathode, external quantum efficiencies in the range of 1.2%–1.5% at 100–1000 cd/m2 have been achieved. Such devices, however, show signs of degradation in time when stored in the off state in inert atmosphere. Blending with glassy polymers such as PMMA results in an improved film quality and a slowing of the device degradation which, in return, decreases the leakage current during device operation. Therefore, external quantum efficiencies of 2%–2.5% at a light output of 200 cd/m2 are observed when the electroluminescent tris(2,2' bipyridyl) ruthenium(II) complex is blended with PMMA or PC. In addition, increased efficiency and lifetime are found when the devices are operated under a 50% duty cycle at 5 V and 1 kHz compared to the operation under constant voltage. With a 50% duty cycle, half lives of around 500–1100 h continuous operation have been achieved at luminance levels in the range of 200–350 cd/m2. When Ag is used as the cathode material, PMMA blend devices exhibit external quantum efficiencies in the range of 2.5%–3.0% at luminance levels of around 50 cd/m2. In addition, devices with a Ag cathode show no signs of degradation when stored in the off state.
Rudmann, H., M. F. Rubner
J. Appl. Phys.