Influence of the solvent on the aggregation of a poly(3-hexylthiophene)-uinquethiophene-S,S-dioxide blend at surfaces: an SFM study

We describe a study on the morphology of thin films processed from equiweight solution blends of the electron donor regioregular poly(3-hexylthiophene) (P3HT) with the electron acceptor 3 ''-methyl,4 ''-hexyl-2,2 ':5 '',2 '':5 '',2 ''':5 ''',2 ' ''' -quinquethiophene-1 '',1 ''-dioxide (T5HOM). Spin-coated films supported on a glass/ITO/PEDOT : PSS substrate have been prepared with systematic variation of the type of solvent. Scanning Force Microscopy investigations revealed that, depending on the solvent type, different kinds of complex phase segregated morphologies on the sub-micrometer scale can be obtained. The formation of either lateral or vertical phase segregated structures is primarily governed by the different tendency of the two molecular systems to self-aggregate in the given solvent. A minor role can also be ascribed to an interfacial recognition phenomenon. Among the different architectures obtained, phase separated T5OHM crystals with sizes of hundreds of nanometers embedded in a P3HT grainy amorphous matrix seems to be the most suitable candidate for photovoltaic applications.