Previous studies have shown the importance of donor polymer solution-state aggregation on the morphology and device performance of OSCs. However, little is known of the exact structure of polymer aggregates in the solution-state and how it determines the BHJ morphology and performance of OSCs. In this article, we have demonstrated the donor polymer aggregate structure in solution affects the solid-state morphology and processing temperature resiliency of printed OSCs. By employing SAXS and microscopy measurements, we discovered large fiber-like aggregates in PM7 and polymer network-like aggregates in PM7 D1 and D2 polymer solutions at room temperature. Our DFT studies reveal that PM7 D1 and D2 polymers are more non-planar and flexible than PM7 due to their rotationally flexible ester functionalized terthiophene acceptor moieties which reduce the likelihood of forming large fiber-like aggregates observed in PM7.