Effects of chalcogenophene π-bridges on benzodifuran-based polymers for high-performance organic solar cells

The π bridge plays a key role in the photovoltaic performance regulation of donor-π bridge-acceptor (D-π-A) type polymer donors. However, the exploration of suitable π bridges is comparatively understudied compared to the design strategies of the D and A units, especially for benzodifuran (BDF)-based polymers. In this work, we design and synthesize three polymers F10-fu, F10-th, and F10-se with BDF as the donor unit and furan, thiophene, and selenophene as π bridges, respectively. As the π bridge changes from furan to thiophene to selenophene, the corresponding polymers exhibit gradually red-shifted absorption and up-shifted energy levels, and the molecular orientation changes from face-on to a clear coexistence of edge-on and face-on. When blended with a narrow bandgap nonfullerene acceptor (NFA) Y18, the F10-fu, F10-th, and F10-se-based devices display gradually decreased open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). As a result, the power conversion efficiency (PCE) of the F10-fu:Y18-based device is 16.79%, which is superior to that of the F10-th:Y18 (15.68%) and F10-se:Y18 (14.18%). Similarly, the same pattern is shown when combined with a wide bandgap NFA IT-M. It is believed that the results would offer a new strategy to develop polymer donors for high-performance OSCs.