The Investigation of Phosphorescent Dopants and Novel Blue Fluorescent Polymer Hosts for PLED Devices

Abstract

This thesis has focused on using experimental and simulation based techniques in an attempt to understand the interactions between polymer hosts and phosphorescent dopants in Organic Light Emitting Devices (OLEDs).

The viability of the SEmiconducting Thin Film Optics Simulation (SETFOS) software as a modelling tool has been established using the well documented material poly(3,4-ethylenedioxythiophene) (PEDOT). Parameters including resistivity and work function were extracted using SETFOS and the trends observed compared favourably to the commercially provided values, despite some limitations.

SETFOS was then used, along with steady state and transient electroluminescence characterisation, to investigate the effects of both phosphorescent dopant colour and concentration on device performance and extract important device parameters, such as the density of states and carrier mobilities. Different device behaviours were observed depending upon the dopant colour and concentration, highlighting the importance of both to device performance. SETFOS was again found to be able to produce quantitative values for a number of device parameters, but several more limitations within the models were identified, which makes further analysis and investigation necessary.

Having gained an understanding of host and dopant interactions in OLED devices, the information gathered was used in the characterization of novel high triplet host polymers for OLED applications. Seven polyfluorene based copolymers were investigated in devices with a range of different coloured phosphorescent dopants and charge transport molecules. Unfortunately, they were found to be unsuitable for use as host materials in OLEDs, acting instead as charge traps.

These polymers, along with four others, were alternatively assessed on their ability to perform as deep blue, or violet, fluorescent materials in undoped Polymer LED (PLED) devices. These devices were found to have some of the highest device characteristics currently detailed in the literature, and represent a variety of new ways of achieving efficient deep blue emission using PLED devices.