Abstract
Abstract In this study, the dewetting-induced photoluminescent (PL) enhancement in thin films of conjugated polymers of poly[2-methoxy-5- ((2'-ethylhexyl)oxy)-1,4-phenylenvinylene] (MEH-PPV) and its blends with polystyrene (PS) was investigated. For thermal dewetting of MEH-PPV films on silicon wafer, the PL enhancement depended on film thickness and the dewetting process, but reached as large as more than 19 folds increase of the apparent luminescence. The enhancement was resulted from the formation of an ultrathin layer of residual MEH-PPV left behind on the retreated areas by film dewetting fronts. The thickness of the residual layer was about 1~5 nm, approximately equal or less than one tenth of the unperturbed dimensions of the macromolecules. Accompanied by the large PL enhancement, a blue shift around 20 nm, attributed to molecular separations in the residual layer, was observed. During the dewetting process, the large-scale frictional movements on the substrate of the film extended the sheared macromolecules and formed strong adhesion bonding to the substrate. Molecularly constrained, the conjugated polymer chains within the residual layer were restricted from exciton-phonon interactions and the non-radiative decay of the photoluminescence processes were effectively suppressed to yield the large PL enhancements. MEH-PPV films also underwent dewetting when imbibed in a suitable solvent vapor at ambient temperature. The solvent molecules plasticized the polymer chains and mobilized the molecular mechanisms of releasing the residual stress to cause thin film dewetting. Depending on how the stress was released and how the polymer chains were dragged along the substrate, the formation of the residual polymer layer and the photoluminescent enhancement varied. In addition, since the interactions between the polymer film and the substrate played important role in the formation of the residual layer, the thickness of silicone treatment on the substrate was found to dominate the enhancement. A light-emitting diode based on the dewetted conjugated polymer film was also successfully demonstrated, illustrating the great potential of using thin film dewetting as a novel material process method for making conjugated polymers for electroluminescence applications.