We study the degree of pi-pi interactions in two copolymers composed of 2-octylbenzotriazole and dialkyldithienosiloles with linear hexyl (P1) and branched 2-ethylhexyl groups (P2). Although the branched ethyl group can be positioned above the p-plane of dithienosilole to bring disadvantage to stack among the polymers, the solid-state absorption spectra of the polymers reveal that P2 possesses stronger interchain interactions than P1. We also evaluate the photovoltaic properties of each polymer with [6,6]-phenyl-C-71-butyric acid methyl ester. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 4829-4839, 2012