Overall, our film morphology and device performance results indicate that the initial solution-state aggregate structures of donor polymers largely determine the final film morphology and device performance of OSCs. We found that the large fiber-like aggregates formed in the solution-state led to large domains without preferential molecular orientation while network-like aggregates resulted in small domains with predominantly face-on orientation. Since the small molecule acceptor, ITIC-4F, is amorphous based on our GIWAXS results (Fig. ), we predict that the donor polymer aggregation in solution drives the film morphology which has also been demonstrated in other studies.