High efficiency polymer light-emitting diodes and lasers

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 polymers have attracted much attention because of their potential application to flat, large area displays, which can be operated at low driving voltage. To make more efficient electroluminescent (EL) device, it is necessary to balance the injection rate of charge carriers. In this dissertation, several studies to improve the luminescent efficiency, to enhance the environmental stability and to tune the emitting colors have been accomplished. One approach to improve the luminescent efficiency of the EL devices is to employ several different kinds of charge injecting materials such as ionomers, single-ion conductors, and organic-salt doped polymer blends. First, ionomers have been employed as an electron injecting and hole blocking material. The effect of ion concentration, the neutralization level and the metal counter-ions in ionomers was systematically studied to obtain the optimal EL characteristics in the polymer light-emitting diodes using poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an emissive layer and sulfonated polystyrene ionomers as an electron-injecting layer. Second, single-ion conductors are employed as charge injection materials. A single-cationic conductor was used for the electron injection and a single-anionic conductor for the hole injection. The charge injection can be greatly promoted due to the ionic space charges near both electrodes. A light-emitting single-energy-well device was fabricated by employing both the single cationic- and anionic- conductor. It can be a further advanced polymer light-emitting device than the polymer light-emitting electrochemical cell (LEC) using bi-ionic conductors. Third, to reform the LEC devices, we fabricated multi-layer E...