Interplay Between π‐Conjugated Polymer Donors and Acceptors Determines Crystalline Order of Their Blends and Photovoltaic Performance

Crystallinity of π‐conjugated polymer donors as well as acceptors is a crucial factor for boosting the performance of organic photovoltaic (OPV) cells. However, in the state‐of‐the‐art OPVs based on nonfullerene acceptors (NFAs), π‐conjugated polymers with low crystallinity are typically used and provide high efficiencies. Here, π‐conjugated polymers, named PTzBT and PTzBTE based on thiazolothiazole (TzTz), having similar crystallinity but different aggregation properties are blended with a number of acceptor materials such as PC71BM, IT‐4F, Y6, and Y12 having different aggregation property. Interestingly, whereas PTzBTE with higher aggregation properties forms crystalline structures in the blends regardless of the acceptor material, PTzBT with lower aggregation properties exhibits crystalline structures only when blended with acceptors having higher aggregation properties. Notably, the crystalline order of the materials in the blend films is correlated closely with the OPV performance. This study proposes that the interplay between the polymer donor and acceptor will play a crucial role in determining the crystalline order in the bulk heterojunction systems and thereby high photovoltaic performance. This work prepares two π‐conjugated polymers, PTzBT and PTzBTE, based on the thiazolothiazole building unit, as donors for organic photovoltaic cells. It is found that the aggregation properties of both the donor and acceptor materials play a crucial role in determining their crystalline order in the blends, which directly correlates to the performance of the photovoltaic cells.