The solution-state aggregation of conjugated polymers is critical to the morphology and device performance of bulk heterojunction (BHJ) organic solar cells (OSCs). However, tuning the polymer solution-state aggregation for optimal film morphology remains challenging due to a lack of understanding of polymer aggregates in the solution-state. Herein, we demonstrate that polymer solution-state aggregate structure significantly impacts the BHJ film morphology and processing temperature resiliency of OSCs. Using X-ray scattering and imaging techniques, we ascertain that the donor polymer PM7 forms a combination of large fibrillar aggregates and network-like aggregates comprised of single polymer chains while its derivatives PM7 D1 and D2 are mainly composed of polymer network-like aggregates in solution. Upon increasing the solution temperature, large fibrillar aggregates dissolve while polymer network-like aggregates maintain their network structures. Surprisingly, regardless of the polymer system, we show that OSCs fabricated from polymer network-like aggregates yield favorable BHJ morphology comprised of small domains with face-on preferred molecular orientation while large fibrillar aggregates lead to poor film morphologies consisting of large domains without preferential molecular orientation. Due to their stability with temperature, polymer network-like aggregates are resilient to processing temperature variation whereas fibrillar aggregates are sensitive to processing temperature. Overall, this work highlights the critical role of solution-state aggregation of polymers, especially their aggregate structures, in achieving OSCs with favorable morphology and high processing resiliency.
Supporting Information for "Solution Aggregate Structures of Donor Polymers Determine the Morphology and Processing Resiliency of Non-Fullerene Organic Solar Cells"