Magnetoresistance and Doping Effects in Conjugated Polymer-Based Organic Light Emitting Diodes.
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Magnetoresistance (MR) and doping effects have been investigated in a poly(3-hexylthiophene-2,5-diyl) (P3HT) based organic light emitting diodes. In single device of fixed composition (Au/P3HT/Al as spun and processed in air), the measured MR strongly depends on the drive conditions. The magnetoconductance (MC) varies from negative to positive (-0.4% ≤ MC ≤ 0.4%) with increasing current density, depending on which microscopic mechanism dominates. The negative MC is due to bipolaron based interactions and the positive MC to triplet-polaron based interactions (as confirmed by light emission). Oxygen doping is prevalent in P3HT devices processed in air and the effect of de-doping (by annealing above the glass transition temperature) is investigated on the MC of an Au/P3HT/Al diode. De-doping reduces the current through the device under forward bias by ~3 orders of magnitude, but increases the negative (low current) MC from a maximum of -0.5% pre-annealing to -3% post-annealing. This increased negative MC is consistent with bipolaron theory predictions based on Fermi level shifts and density of states (DoS) changes due to de-doping. The decrease in current density is explained by increased injection barriers at both electrodes also resulting from de-doping. Deliberate chemical doping of P3HT is carried out using pentacene as a hole trap centre. The trapping effect of pentacene is confirmed by reproducible and significant hole mobility-pentacene concentration behaviour, as measured by dark injection (DI) transient measurements. The enhanced carrier injection resulting from the pentacene doping also leads to increased electroluminescence (EL). The resultant MC in pentacene doped devices is strongly dependent on carrier injection and can be significantly enhanced by doping, for example from -0.2% to -0.6% depending on device and drive conditions. Throughout this thesis Lorentzian and non-Lorentzian function fitting is carried out on the measured MC, although the underlying microscopic mechanisms cannot always be discerned.
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