Synthetic Metals 161 (2011) 628–631 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Modelling of organic magnetoresistance as a function of temperature using the triplet polaron interaction Sijie Zhang ∗ , A.J. Drew, T. Kreouzis, W.P. Gillin Department of Physics, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom article info Article history: Received 11 October 2010 Received in revised form 14 November 2010 Accepted 19 November 2010 Available online 18 December 2010 Keywords: Organic magnetoresistance (OMR) Exciton trapping Triplet polaron interaction (TPI) abstract The organic magnetoresistance (OMR) of 50 nm aluminium tris(8-hydroxyquinoline) organic light emit- ting diodes (OLEDs) has been measured over a range of temperatures and operating voltages. The OMR data for this device, the current change through the device with applied field, have been fitted using the triplet polaron interaction (TPI) model that includes two independent processes, namely the exciton trapping and the triplet polaron interaction. Two Lorentzian functions were used to fit the data, and the prefactors for the two processes were found to scale linearly with the triplet population. Over the whole temperature range the data appears to fall on a single line which covers nearly six orders of mag- nitude of exciton concentration. This work demonstrates that the magnitude and shape of the OMR can be predicted and will be useful for understanding the fundamental mechanism behind the OMR. © 2010 Elsevier B.V. All rights reserved. 1. Introduction There is an increasing interest in the effects of magnetic fields on the current transport and efficiency in the organic devices. In 2003 Kalinowski et al. observed that the photoconductivity in organic devices can be perturbed by a magnetic field [1], and then showed that a weak magnetic field can affect the current and light emission from an organic light emitting diodes (OLEDs) and hence its effi- ciency [2]. Since then the study of these phenomena has increased dramatically [3–6], but there has not yet been a successful model that can fully explain organic magnetoresistance (OMR) and predict the trends observed in the magnetic field effects as the operating conditions of the devices are changed. Such a model will be essen- tial for understanding the fundamental mechanism of the OMR. There are two contrasting approaches to explain the OMR. One group proposed a bipolaron based model for the OMR which pre- dicts that the effect can be seen in unipolar structures [7]. However, the majority of the current models are primarily based on the effect of magnetic fields on excitons or the pair states prior to exciton for- mation [2–4]. This is because the majority of experiments suggest that OMR can only be seen in devices above turn-on (an applied voltage above the built in potential of the device). The exception to this is for devices that contain a poly(3,4,-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) hole transport layer [8,9], in which the OMR can be seen before the devices turn-on. ∗ Corresponding author. Tel.: +44 020 7882 5043. E-mail address: sijie.zhang@qmul.ac.uk (S. Zhang). The triplet polaron interaction model (TPI), which has been proposed in our previous work [6,10–13], is based on the effect of excitons (primarily the long lived triplets) on charge transport [6]. We recently demonstrated that for aluminium tris(8-hydroxyquinoline) (Alq 3 ) based devices with different layer thicknesses and cathodes the OMR can be accurately modelled using two Lorentzian processes that both scale linearly with exci- ton concentration over nearly six orders of magnitude [14]. In this paper, we use this model to fit the OMR data as a function of temper- atures and operating voltage for Alq 3 based organic light emitting diodes (OLEDs). 2. Experimental The basic device structure consists of an indium tin oxide (ITO) coated glass substrate (purchased from Merck) with a sheet resistivity of (∼13 /), 50 nm of N,N ′ -diphenyl-N,N ′ -bis(3- methylphenyl)-(1,1 ′ -biphenyl)-4,4 ′ -diamine (TPD) as the hole transport layer (HTL) and 50 nm of Alq 3 as an emissive/electron transport layer. Onto these devices, a cathode was deposited con- sisting of a 1 nm LiF layer followed by 100 nm of aluminum. The TPD and Alq 3 were purchased from Aldrich and purified using train sublimation prior to use. The ITO substrate was patterned using photolithography and cleaned by ultrasonicating in detergent solu- tion, water, acetone, and chloroform. Following this, the ITO was treated in an oxygen plasma for 3 min at 30 W and 2.5 mbar pres- sure using a Diener electronic femto plasma system. The plasma treated substrate was immediately transferred to the deposition chamber for device fabrication. The deposition of the organic lay- 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.11.027