A major attraction of polymeric semiconductors is that their optoelectronic properties can be
tuned by controlling local microstructure, such as chain arrangement and conformation, in principle allowing functionality to be tailored to specific device applications. This local microstructure is influenced by intrinsic factors, such as chemical structure and molecular weight,but also by film formation conditions, such as solvent and casting temperature. Here, the effect of blending conjugated with insulating polymers, an additional tool to control structural and optoelectronic properties, is extensively explored. Focusing predominantly on widely studied poly(3-hexylthiophene-2,5-diyl) (P3HT) as a model system, the addition of a polar insulator poly(ethylene oxide) (PEO) is shown to affect both the nature of aggregation (in terms of excitonic coupling), its temperature dependence and evolution during film formation. Spatially restricting evaporation of a P3HT:PEO blend leads to periodic variation in aggregation type, attributed to self-induced compositional changes, demonstrating that insulators can be used to achieve a measure of spatial control. The origin of a temperature dependent reduction in effective vibronic peak spacing of photoluminescence spectra P3HT:PEO is investigated by considering self-absorption, coupling to insulator vibration and additional Raman modes, and correlated with increased non-radiative decay. Finally, the two chemical functionalities, namely poly(thiophene) and oligo(ethylene oxide), are combined as a graft polymer, a block co-polymer, and blends with both, demonstrating that matching side
chain and insulator polarity can promote translational order, aggregation and planarity. Together, this work demonstrates how the addition of insulating to conjugated polymers influences local microstructure and hence enables control of optoelectronic characteristics, with potential applications to a wide range of conjugated polymers and devices.