Synergistic effects of chlorination and a fully two-dimensional side-chain design on molecular energy level modulation toward non-fullerene photovoltaics

Two chlorinated medium-band-gap polymer donors with fully two-dimensional (2D) conjugated thiophene side chains, named 2D-PBTCl and 2D-PBTCl2, have been designed and successfully synthesized for application in non-fullerene solar cells to elevate the open-circuit voltage and optimize the power conversion efficiency (PCE). Chlorine atoms were directly attached to the backbones of the polymers, which induced non-ignorable steric hindrance because of the relatively large size of chlorine. To overcome this influence, a structural design of extended conjugated thiophene units was further introduced to increase the p-p interactions. In this way, two new chlorinated D-A polymers were developed that combined two-dimensional thiophene conjugated side chains for enhanced intermolecular interactions with chlorine atoms for an elevated open-circuit voltage in photovoltaic devices. As a result of chlorination, which substantially changed the charge distribution along the polymer backbone through its electron-withdrawing nature and resonance effect, the two polymers exhibited deep-lying HOMO levels and blueshifted absorbances for more complementary light absorption with a non-fullerene acceptor, such as ITIC. By taking advantage of the synergistic chlorination of the backbone and the introduction of conjugated thiophene side chains, the optimized bulk heterojunction device based on 2D-PBTCl and ITIC showed a PCE of 8.81% compared to the PCE of PTB7-Th of 6.91%, which corresponds to an approximately 28% improvement in solar energy conversion. These results demonstrated that the combination of chlorination and the design of fully 2D conjugated side chains is a promising strategy for fine-tuning the energy levels and film morphologies of conjugated polymers.