Solution Aggregate Structures of Donor Polymers Determine the Morphology and Processing Resiliency of Non-Fullerene Organic Solar Cells

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.