Enhanced Performance in Polymer Light-Emitting Diodes (PLEDs) by Using Water Soluble Conjugation Polymer

Abstract

Organic and polymeric materials have generated interest in recent year for their potential applications in flat panel displays, such as TVs, monitors, and mobile displays, due to their wide viewing angle, fast response time, and low operating voltage with high external quantum efficiency. A well-balanced charge injection leads to an increase in the external quantum efficiency in polymer-based light-emitting diodes (PLEDs). This can be achieved by forming a multilayer structure in device fabrication and considering the charge mobility, the ionization potentials and the electron affinities of the organic materials, and the work functions of the metal electrodes. When taking the hole favor property in most conjugated polymers into consideration, it is important to introduce electron injection and transport layers for more balanced charge injection into the emission zone. The electron injection can be enhanced by using a metal cathode with a low work function, such as Ca or ionic compounds like LiF or CsF. However, active metals like Ca are known to diffuse into the organic layer due to the strong electric field inside the device, which forms quenching sites in the emission layer near the cathode. Brown et al. announced that ionic compounds could also be partially ionized and migrate into the emission layer. In fact, the multilayer structure with selected electron injection and transport layers can enhance the device performance by not only reducing the electron injection barrier and controlling the electron mobility but also preventing the migration of metals or ionic compounds into the emission layer. In general, most conjugated polymers are soluble incommon organic solvents, which cause difficulty for fabrication of multilayer polymer films. During the last decade, considerable research efforts have been carried out on multilayer PLEDs to enhance the device performance to make them suitable for practical use. Lee et al. introduced an ionomer layer next to the emitting polymer for electron injection and a hole blocking layer, which lowered the operating voltage by up to 60%. Niu et al. reported that the use of neutral surfactants blended with poly(ethylene glycol) as an electron injection layer with an Al cathode could improve the current efficiency to almost twice that of Ca/Al cathode. Recently, Tseng et al. showed a general method to solution-process multilayer PLEDs by using an intermediate liquid buffer between polymer layers. Several papers have also reported the enhancement of electron injection by using ion-conducting polymer, cross-linkable inter-layers, and water/ methanolsoluble copolymers. In spite of those improvements in multilayer PLEDs fabrication, however, it is true that we still need more materials for effective hole blocking and electron injection with an easy process. In this study, we synthesized new types of water soluble conjugated polymer and showed enhanced performance on the multi-layered polymer light-emitting diodes (PLEDs) by introducing the water soluble conjugated polymer FPQ as an electron transport layer between the emitting polymer and the cathode metal. The device performances with and without water soluble polymer layer will be compared in terms of the current-voltage characteristics, the luminance, and the external quantum efficiencies.