Accelerating Surface Segregation and Changing Interactions Between Conjugated Polymers by Photoexcitation at Elevated Temperatures

Abstract

We have investigated systematically the consequences of photoexcitation of poly(3-hexylthiophene) (P3HT) embedded in a matrix of inert polystyrene (PS) with respect to structure formation and optoelectronic properties. Based on a comparison of observations made simultaneously in illuminated and non-illuminated regions within the same thin-film sample, we showed how changes in spectral and morphological properties induced by photoexcitation depended on annealing temperature. Due to a difference in surface energies, P3HT segregated to the free film surface and formed π–π-stacked aggregates, a process significantly accelerated by photoexcitation. However, at annealing temperatures near but below the melting temperature of P3HT, photoexcitation suppressed the formation of π–π-stacked aggregates, yielding a photoluminescence spectrum characteristic for dispersed individual P3HT molecules. Intriguingly, at even higher annealing temperatures, where P3HT was completely molten, continuous illumination induced a novel morphology of a micron-sized ‘beads-on-a-string’ network, generated by so-far unknown interactions between polymers having experienced photoexcitation. This novel morphology was absent in the nonilluminated regions within the same sample. These various observations before and after photoexcitation with respect to surface energy and the formation of aggregates of P3HT indicate the interplay of illumination-induced changes in intermolecular interactions and appropriate molecular mobility. This mechanism opens up a pathway for controlled modulation of the assembly in conjugated polymers by photoexcitation.