Towards improved efficiency of polymer solar cells via chlorination of a benzo[1,2-b:4,5-b′]dithiophene based polymer donor

A chlorine substituted benzo[1,2-b: 4,5-b'] dithiophene unit based conjugated polymer, PBT-Cl, is designed and synthesized as a donor material for high-performance polymer solar cells (PSCs). Relative to its fluorine substituted counterpart PBT-F, PBT-Cl has a simple synthetic procedure, slightly blue-shifted absorption spectrum, and lower-lying HOMO level. Importantly, the PSC based on PBT-Cl : IT-4F showed higher charge carrier mobility with a more balanced mu(h)/mu(e) ratio, weaker bimolecular recombination, and suppressed trap-assisted recombination than the PBT-F : IT-4F based device, leading to a significantly higher power conversion efficiency (PCE) of 11.60%. Notably, the high PCE of 11.60% was obtained from the PBT-Cl based device processed using m-xylene solvent. What is more, a PBT-Cl based semitransparent PSC demonstrated a promising PCE of 8.18%, with a high AVT of 31.7%. These results demonstrate that the replacement of the fluorine atom by the chlorine atom should be an efficient strategy for designing high-performance polymer donor materials.