Improvement of the efficiency in polymer electrophosphorescent devices

Abstract

The π-conjugated polymers with semi-conducting properties have recently attracted much attention due to their applicability in the field of optic and opto-electric devices such as light-emitting diodes and lasers. The LEDs based on conjugated polymers have attracted much attention because of their potential application to flat, large area displays, which can be operated at low driving voltage. However, the potential use of polymer-light-emitting diodes is ultimately limited by their low quantum efficiency as well as by their poor stability due to oxygen. In this dissertation, several studies to improve the luminescent efficiency, to improve the luminescent stability and tune the emitting colors have been accomplished by using some different system. In chapter 1, we have fabricated a polymer light-emitting diode heterolayer structure that emits green light. Electroluminescence (EL) and the dynamics of charge carrier recombination in the PVK / tris(2-phenylpyridine)iridium(Ⅲ)$(Ir-(ppy)_3)$ bilayer EL devices were studied. The indium-tin oxide $(ITO) / PEDOT / (PVK/Ir(ppy)_3)$ bilayer / BCP / Ca / Al multilayer devices showed more enhanced color quality compared with those of $PVK: Ir(ppy)_3$ blended device. The current-voltage-luminescence characteristics of the devices, which have a $PVK / Ir(ppy)_3$ bilayer system, are systematically studied and compared with a $PVK:Ir(ppy)_3$ blended device. In chapter 2, We demonstrated efficient green electrophosphorescence using cascade energy transfer from PVK and new host fluorescent polymer to the phosphorescent iridium complex. Although the chemical compatibility between fluorescent polymer and iridium complex, tris[2-phenylpyridine)iridium(III) $(Ir-(ppy)_3)$, is very poor, the efficient energy transfers from poly(N-vinylcarbazole) (PVK) to host polymer and from the host to $Ir(ppy)_3$ were observed in PVK/host $polymer/Ir(ppy)_3$ blend system and blended EL devices.