Multichloro-Substitution Strategy: Facing Low Photon Energy Loss in Nonfullerene Solar Cells

The large photon energy loss (Eloss) in the conversion from photons to electrons is still one of the critical factors in limiting the performance of polymer solar cells (PSCs). In order to simultaneously obtain the decreased Eloss and expanded spectral response, the reasonable design of wide band gap conjugated polymer with a low-lying HOMO level is still a great challenge in nonfullerene PSCs. In this work, four D–A type wide band gap conjugated polymers with introduced chlorine atoms on the conjugated thienyl-side chains of the BDT units, namely, PBT1Cl-Bz, PBT2Clas-Bz, PBT2Cls-Bz, and PBT4Cl-Bz, were synthesized and characterized. Because of the large dipole moment of carbon–chlorine (C–Cl) and the strong noncovalent interaction of Cl···S and Cl···π, the tetrachloro-substituted PBT4Cl-Bz exhibits a much deeper HOMO level (−5.64 eV) than the other three polymers. Therefore, a high open-circuit voltage (Voc) of 0.96 V was achieved while keeping a decent Jsc of 16.04 mA cm–2 when matched with nonfullerene acceptor IT-4F, and a quite small Eloss of 0.54 eV was achieved which was close to the values of some inorganic and hybrid solar cells. In addition, the chlorination of conjugated side chain demonstrated that the amount and position of chlorine atoms contributed to enhanced molecular packing and morphology control. Benefits from the stronger intermolecular π–π interaction and the suitable phase separation, the highest hole mobility, and reduced bimolecular recombination are obtained from PBT4Cl-Bz. As a result, the photovoltaic performance of the PBT4Cl-Bz:IT-4F-based device achieved a power conversion efficiency of 9.25% without any solvent additive. These results indicate that the multiple chlorination of conjugated thiophene side chain on BDT units will be an extremely promising method to modify their molecular energy levels with small energy loss for applications in nonfullerene PSCs.