Electrophosphorescent devices based on cationic complexes; control of switch-on voltage and efficiency through modification of charge injection and charge transport

This paper reports an analysis of the properties of polymer light-emitting devices (PLEDs) doped with iridium complexes. Devices based on charged and neutral complexes doped into poly(vinylcarbazole) (PVK) are presented, and the role of the ions and the charge-transport properties of the complexes are discussed. In devices with the charged complexes, the concentration of the complex is found to have a profound effect on both the switch-on voltage and the efficiency. At higher doping concentrations the efficiency is increased and the switch-on voltage decreased. The increase in efficiency and decrease in switch-on voltage at higher dopant concentration are found to be due to an alternative charge transport path via the iridium dopant [Ir(bpy)] + (bis(2-phenylpyridine-C 2 ,N')(2.2'-bipyridine)iridium hexafluorophosphate). However, at lower concentrations the complex becomes an electron trap and the efficiency is reduced. The devices are found to be significantly less efficient than those with neutral complexes. This difference is attributed to the ionic content and the charge trapping properties of the charged complexes. The low efficiency of the charged-complex-based devices could be overcome by utilizing a hole-blocking layer; devices with efficiencies as high as 23 cd A - 1 were obtained.