Reducing the side-chain influences of isoindigo-based polymer donors by backbone fluorination in photovoltaic applications

The effect of synergistic structural engineering (asymmetric side chains and backbone fluorination) for polymer donors in photovoltaic applications is investigated in this study. Six isoindigo (IID)-based polymer donors with slightly different side-chain and backbone designs are composed of a bithiophene (BT) or fluorinated bithiophene moiety to form two series of polymers [PII2T (P1–P3) and PII2TF (P4–P6)] while symmetric/asymmetric side-chain combinations are composed of decyltetradecane branched alkyl side chain (DT) and siloxane-terminated side chain (SiO–C8). The results indicate the different side chain design and backbone fluorination modulate the energy levels, crystalline properties, and molecular packing behaviors of the polymers. P2 and P5 with asymmetric side chains are showed to deliver the highest power conversion efficiency (PCE) in the respective series of PII2T and PII2TF. The morphological analyses reveals that the mismatched side-chain geometry results in a face-on dominated packing to yield superior device performance. Meanwhile, the fluorinated polymers (P4–P6) show superior performance than the corresponding non-fluorinated polymers (P1–P3). Moreover, backbone fluorination largely reduces the performance difference between the polymers with symmetric side chains and asymmetric sides chains. This work demonstrates that the synergistic effect of asymmetric side chains and backbone fluorination can be a rational design towards high-performance conjugated polymers.