Elucidating Roles of Polymer Donor Aggregation in All-Polymer and Non-Fullerene Small-Molecule–Polymer Solar Cells

The aggregation behavior of polymers plays a crucial role in determining the optical, electrical, and morphological properties of donor–acceptor blends in both all-polymer solar cells (all-PSCs) and non-fullerene small-molecule acceptor–polymer solar cells (NFSMA–PSCs). However, direct comparison of the impacts on two different systems has not been reported, although it is important to design universal polymer donors (PD’s). Herein, three PD’s with different side chains (P–EH, P–SEH, and P–Si) are designed to study the PD aggregation effects on the blend morphology and device performance of both all-PSCs and NFSMA–PSCs. It is observed that the aggregation property of PD’s is a critical factor in determining the optimal blend morphologies and ultimately the device performances in both PSC systems. Furthermore, PD aggregation effects on device performance are significantly more impactful in all-PSCs than in NFSMA–PSCs. The P–Si PD exhibiting the strongest aggregation behavior in a processing solvent produces the most severe phase separation in the blend with a polymer acceptor, resulting in the lowest power conversion efficiency (PCE) of all-PSCs. In contrast, when P–Si is used in an NFSMA–PSC, a well-mixed blend morphology is observed, which results in the highest PCE of over 12%. These different roles dependent on PD aggregation mainly originate from the difference in molecular size of the polymer acceptor and small-molecule acceptor, which influences the entropic contribution to the formation of blend morphology. Our work provides a comprehensive understanding of the PD aggregation–blend morphology relationship in different all-PSC and NFSMA–PSC systems, which serves as an important guideline for the design of universal PD’s for both all-PSCs and NFSMA–PSCs.