Modelling the effects of molecular arrangements in polymer light-emitting diodes.

In order to understand how to enhance the performance of polymer light-emitting diodes (PLEDs), we used a mesoscopic hopping model, taking into account molecular properties and polymer morphology, to investigate the impact of a number of conjugated polymers and molecular arrangements on the functioning of single-layer devices. The model is applied to devices with the active polymer consisting of poly(p-phenylene vinylene) (PPV) and PPV derivatives with stiff conjugated segments having their long axis oriented parallel and perpendicular to the electrode surfaces as well as randomly oriented, which are three of the molecular arrangements that can be obtained experimentally at microscopic scale in solution-processed conjugated polymer thin films. The model provides insight into current efficiency, charge distribution, internal electric field and consequently recombination throughout the polymer layer. We found that the details of molecular arrangement crucially affect the distribution of recombination events far from the electrodes and its field dependence, which has implications for the efficiency of PLEDs. In particular, we found a variation of recombination efficiency in the bulk of uniform ordered polymer films depending on the molecular alignment relative to the electrode surfaces. It turns out that molecular orientation perpendicular to the electrodes increases recombination in the centre of the polymer film as compared to the case of parallel orientation. We conclude that the orientational alignment perpendicular to the electrode surfaces might be a viable strategy towards efficient polymer-based LEDs.