Efficient multi-component polymer light-emitting diodes with controlled morphology

Abstract

The π-conjugated polymers with semi-conducting properties have recently attracted much attention due to their applicability in the field of optic and optoelectronic devices such as light-emitting diodes (LEDs) and lasers. The LEDs based on conjugated polymer have attracted a lot of interest because of their potential application to flat, large area displays, which can be operated at low driving voltage. Recently, phosphorescent LEDs based on fluorescent polymers doped with phosphorescent dyes, where the singlet and triplet excitons of the molecules contribute to light generation, have been developed to improve the efficiency of LEDs. In this dissertation, several studies to improve the luminescent efficiency of the polymer electroluminescent (EL) devices and the polymer-based electrophosphorescent devices and tune the emitting colors have been accomplished. One approach to improve the luminescent efficiency of the EL devices is to control the morphology in the light-emitting layer. For the preparation of polymer phosphorescent light-emitting devices (PPLEDs), the commonly used concept is to blend a low molecular weight phosphorescent dye into a proper polymer matrix. A polymer host with a triplet state energy above that of the phosphorescent dye is required to guarantee the confinement of the triplet excited state on the quest. In chapter 2.1, we have successfully demonstrated that efficient electrophosphorescence is obtained via cascade energy transfer from poly(N-vinylcarbazole) (PVK) and new host fluorescent polymer to the phosphorescent iridium complex. Efficient energy transfers from PVK to fluorescent polymer and then from this host polymer to iridium complex, fac-tris[2-(2-pyridinyl-kN)(5-(2,5-dimethylphenyl)phenyl)-kC]-iridium(III) $(Ir(PDPP)_3)$, were observed in the PVK/fluorescent $polymer/Ir(PDPP)_3$ blend system, even though the chemical compatibility between this fluorescent polymer and $Ir(PDPP)_3$ is very poor. The phase separation between...