Importance of Terminal Group Pairing of Polymer Donor and Small‐Molecule Acceptor in Optimizing Blend Morphology and Voltage Loss of High‐Performance Solar Cells

Abstract As a variety of non‐fullerene small molecule acceptors (SMAs) have been developed to improve power conversion efficiency (PCE) of organic solar cells (OSCs), the pairing of the SMAs with optimal polymer donors (P_Ds) is an important issue. Herein, a systematic investigation is conducted with the development of the SMA series, named C6OB‐H, C6OB‐Me, and C6OB‐F, which contain distinctive terminal substituents –H, –CH₃, and –F, respectively. These SMAs are paired with two P_Ds, PBDT‐H and PBDT‐F. Interestingly, the P_D/SMA pairs with similar terminal groups yield enhanced molecular compatibility and energetic interactions, which suppress voltage loss while improving blend morphology to enhance simultaneously the open–circuit voltage, short–circuit current, and fill factor of the OSCs. In particular, the OSC based on the PBDT‐F:C6OB‐F blend sharing fluorine terminal groups achieves the highest PCE of 15.2%, which outperforms those of PBDT‐H:C6OB‐F (10.1%) and PBDB‐F:C6OB‐H OSCs (11.2%). Furthermore, the PBDT‐F:C6OB‐F OSC maintains high PCEs with active layer thicknesses between 85 and 310 nm. In contrast, the PCE of PBDT‐H:C6OB‐F‐based OSC already drops by 80% from 10.1% to 2.1% when the active layer thickness increases from 100 to 200 nm. This study establishes an important P_D/SMA pairing rule in terms of terminal functional groups for achieving high‐performance OSC.