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

Abstract

The aggregation behavior of polymers plays a crucial role in determining the optical, electrical, and morpho- logical properties of donor-acceptor blends in both all-polymer 12 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 (P-D's). Herein, three P-D's with different side chains (P-EH, P-SEH, and P-Si) are designed to study the P-D aggregation effects on the blend morphology and device performance of both all-PSCs and NFSMA-PSCs. It is observed that the aggregation property of P-D's is a critical factor in determining the optimal blend morphologies and ultimately the device performances in both PSC systems. Furthermore, P(D )aggregation effects on device performance are significantly more impactful in all-PSCs than in NFSMA-PSCs. The P-Si P-D 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 P-D 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 P-D aggregation- blend morphology relationship in different all-PSC and NFSMA-PSC systems, which serves as an important guideline for the design of universal P-D's for both all-PSCs and NFSMA-PSCs.