We analyze the linear absorption spectrum of regioregular poly(3-hexylthiophene) films spun from a variety of solvents to probe directly the film microstructure and how it depends on processing conditions. We estimate the exciton bandwidth and the percentage of the film composed of aggregates quantitatively using a weakly interacting H-aggregate model. This provides a description of the degree and quality of crystallites within the film and is in turn correlated with thin-film field-effect transistor characteristics. ; Comment: Applied Physics Letters (in press); 9 pages, three figures