Developing a Fluorinated Benzothiadiazole‐Based Polymer Donor as Guest to Construct High‐Performance Ternary Solar Cells

Benzothiadiazole (BT) has shown promising applications in fullerene solar cells. However, few BT‐based polymer donors exhibited a noticeable power conversion efficiency (PCE) for the fused‐ring small molecular acceptor‐based polymer solar cells (PSCs). Herein, we developed a D‐A (D: donor, A: acceptor) polymer donor F‐1 based on fluorinated BT (ffBT) as A unit and chlorinated benzo [1,2‐b:4,5‐b'] dithiophene (BDT‐2Cl) as D unit. Thanks to the strong electron‐withdrawing of ffBT unit, F‐1 not only exhibited low‐lying energy levels to increase open circuit voltage, but also existence noncovalent S···F to enhance molecular planarity and crystallinity. After incorporated F‐1 as guest materials on PM6:L8‐BO‐based system to construct ternary device, the addition of F‐1 can efficient promote the blend films exhibit excellent morphological compatibility, favorable phase separation, and molecular arrangement. As a result, the F‐1 (5%):PM6:L8‐BO‐based device obtained an impressive PCE up to 19.47 %, which is much higher than the counterparts as PCE is 18.35% for PM6:L8‐BO‐based binary device. This study not only provided a practical and promising strategy to develop high‐performance polymer donors by utilizing the advantages of noncovalent conformational locks, but also suggested an efficient strategy to further improve device performance by constructing ternary solar cells for BT unit‐based PSCs.