Mesoscopic Modelling of Conducting and Semiconducting Polymers

We present generalised Monte-Carlo calculations to assess the effects of texture and related key factors on the properties of polymer-based light emitting diodes. We describe one class of mesoscopic model giving specific realisations of the polymer network. The model, with simple physically-based rules, shows the effects of polymer structural order on current flow, trapping, and radiative and non-radiative charge recombination within the polymer layer. Interactions between charges are included explicitly, as are image interactions with the electrodes. It is important that these Coulomb interactions are not simplified to an averaged space charge, since the local interactions can lead to effective trapping of charge, even in the absence of defective chains or impurity trapping. There proves to be an important role for trapping, in which charges are localised for times long compared with transit times. The competition between current flow, trapping, and radiative and non-radiative charge recombination means that some of the trends are not intuitively obvious. For example, if radiative recombination occurs only on short polymer chains, as is the case for certain polymer systems, the internal efficiency appears to saturate for a concentration of these shorter luminescent chains about 20-30%. As the proportion of shorter chains increases, trapping increases, whereas current efficiency decreases. Our approach provides a natural link between atomistic models of individual polymer molecules and the macroscopic descriptions of device modelling. Such mesoscopic models provide a means to design better film structures, and hence to optimise the effectiveness of new organic materials in a range of applications. Physics Abstracts Classification 78.60, 73.61 Ph, 78.66 Qn