Halogen-Free Wide Band Gap Polymer Donors Based on Dicyanobithiophene for Efficient Organic Solar Cells

Conjugated polymer donors have always been one of the important components of organic solar cells (OSCs), particularly those featuring simple synthetic routes, proper energy levels, and appropriate aggregation behavior. In this work, we employed a nonfused electron-deficient building block, dicyanobithiophene (2CT), for constructing high-performance donors. Combining this with side-chain engineering, two novel halogen-free polymer donors, PB2CT-BO and PB2CT-HD, were reported. PB2CT-BO with shorter alkyl chains on the thiophene π bridges exhibited enhanced packing ordering and improved polymer crystallinity. When paired with BTP-CN-HD as the electron acceptor, the PB2CT-BO-based OSC attained an impressive power conversion efficiency (PCE) of 14.2% within a bulk-heterojunction (BHJ) configuration. Additionally, the active layers were refined through a layer-by-layer (LbL) approach, leading to a more organized molecular packing and an improved fibrillar network. Consequently, the OSC employing PB2CT-BO processed with the LbL approach achieved a notable PCE of 15.3%. This enhancement is credited to a reduced energy loss (E_loss) of 0.514 eV and the formation of a favorable morphology. This study highlights the considerable promise of the 2CT unit in the progression of high-efficiency polymer donors with a reduced E_loss.