Controlling hole-transport in aluminum tris (8-hydroxyquinoline), Alq(3)-based organic light emitting diodes to improve the device lifetime by an oxidized transport layer

Abstract

A salt containing polymer, called an oxidized transport layer (OTL), was investigated for hole injection and transport into Alq(3)-based multilayered organic light emitting diode (OLED) devices. The OTL comprises an aryldiamine containing hole transport polymer binder with a corresponding low molecular weight radical cation salt. We demonstrate herein that the OTL behaves like a tunable resistor for holes, and its hole-transport properties can be controlled by the salt concentration and thickness of the OTL. Based on a careful analysis of the current density-voltage (J-V) device characteristics as a function of the above parameters, electron/hole currents were balanced to minimize oxidative degradation of Alq(3). It was found that an OLED device (ITO/OTL/NPB/Alq(3)/CsF/Al) with a 5000 Angstrom thick OTL at 5% salt concentration operated with a half-life exceeding 1000 h at a constant current of 10 mA/cm(2). Similar devices with 2.5% and 10% salt doping showed an order of magnitude lower half-life attributed to unbalanced carrier concentrations. Moreover, by demonstrating that the majority of the field drops across the Alq(3) layer, the doping level as opposed to OTL thickness was established as the primary contributor controlling hole transport in these OLEDs. This beneficial behavior is, however, observed only above a certain OTL thickness in the vicinity of 5000 Angstrom. (C) 2004 American Institute of Physics.