The nanoimprinting of polymer solar cells via soft lithography is an attractive approach for enhancing light absorption in the active layer. Many efficient polymer solar cells utilize a high boiling point solvent additive such as 1,8-diiodooctane (DIO) during active layer deposition to optimize morphology. By studying active layer films based on the PTB7-Th:PC71BM system prepared with different amounts of the solvent additive DIO, it is shown that the soft imprinting of such blends critically relies upon the presence of residual solvent additive that plasticizes the film. In particular, a minimum of ∼2 volume % of DIO in the casting solution is found to be necessary to enable effective imprinting. The microstructure of imprinted layers is also comprehensively characterized using atomic force microscopy, grazing incidence wide-angle X-ray scattering and resonant soft X-ray scattering, demonstrating that soft lithography can be used to effectively impart structure on the a photonic length scale without changing the nanoscale morphology and microstructure.