Modelling of organic magnetoresistance in aluminium tris(8-hydroxyquinolinate) light emitting diodes

Abstract

This thesis concerns itself with the scientific study of the modelling of organic magnetoresitance (OMR). This can be divided into two parts: the magnetic field effects on the intersystem crossing (ISC) in organic semiconductors, and the modelling of OMR, including triplet polaron interactions (TPI). In my studies of the magnetic field effect on photoluminescence (PL), the ISC rate, kISC, is estimated by modelling the dependence of the PL under high excitation intensity. Using a modified rate model, a kISC of 2.3 x 104s-1 is derived at a temperature of 80K in Alq3. An excited state absorption (ESA) mechanism was also proposed to help understand how the ISC can occur from higher excited triplet states to the singlet state, rather than just from the singlet to a lower lying triplet state. This is necessary as the measured activation energy from the transfer from T1 to S1 is only 15±5meV. In addition, the effect of a magnetic field on photoluminescence intensity for Alq3 is reported, in order to explain the change in the kISC caused by an applied magnetic field. The magnetic field may affect the mixing of a pair state prior to exciton formation as well as the exciton itself. I then present the modelling of OMR as a function of device thickness. Here, a TPI model is proposed to fit the OMR data. For all Alq3 devices of any thickness, the OMR data can be modelled using just three processes: triplet dissociation, polaron trapping and TPI. Both the sum of prefactors for dissociation and trapping (ad+ at), and the prefactor for TPI, ai, are proportional to the exciton concentration within the device, over the full range of operating conditions. This is the first time that a predictive model of OMR has been developed. This model is then extended to fit the OMR data as a function of temperature. In addition, I discuss some surprising phenomena at low temperature, such as a delay between the onset of light emission and the onset of OMR, and the decrease in the percentage efficiency change with the effect of a magnetic field.