Modulating Chlorination Position on Polymer Donors for Highly Efficient Nonfullerene Organic Solar Cells

Introducing substituent groups has been regarded as an effective method to construct highly efficient polymer donors. However, the correlation between the position of substituent groups and the device performance of polymer donors has rarely been carefully studied and compared. Herein, Cl atoms are introduced into the BDT‐based thienyl side chains and BDD‐based thienyl π ‐bridges to obtain two chlorinated donor−acceptor (D−A) polymer donors H1 and H2, respectively. By systematically comparing the photovoltaic properties of H1 and H2, it is found that the device performance of polymer donors is sensitive to the position of chlorine atoms. The nonfullerene organic solar cells (OSCs) based on H1:IT‐4F and H1:Y6 display a superior power conversion efficiency (PCE) of 12.34 and 15.62%, whereas the PCE of H2:IT‐4F and H2:Y6 is 11.04 and 13.80%. As the H1‐based blend shows more desirable aggregation morphology, more preferential face‐on orientation, and more efficient extraction dissociation occurs. The current work demonstrates that the position of chlorine substitution can be reasonably optimized for state‐of‐the‐art polymer donors in the highly efficient nonfullerene OSCs.