Within an extended Su-Schrieffer-Heeger model including impurity interactions,the dynamical process of exciton dissociation in the presence of an exteal electric field is investigated by using a non-adiabatic evolution method Under the action of impurities,the stability as well as the effective mass of the exciton is reduced.Our results show that the field required to dissociate the excitons depends sensitively on the strength of the impurity potential.As the impurity potential strength increases,the dissociation field decreases effectively.The theoretical results are expected to provide useful predictions conceing which polymers with properly impurity-assisted interactions are likely to be more suitable for use in organic solar cells.