Preparing polythiophene derivative with alternating alkyl and thioalkyl side chains via Kumada coupling for efficient organic solar cells

Polythiophene derivatives synthesized by Kumada catalyst-transfer polycondensation (KCTP) have been intensively studied as potential candidates for low-cost organic solar cells (OSCs) due to their capability of scalable production. However, the power conversion efficiency (PCE) is still unsatisfactory so far. In this work, two regioregular polythiophene derivatives, P3SHT and PTST with thioalkyl chains, were prepared by KCTP. Compared to the analogue P3HT with alkyl chains, the new polymers show stronger absorption and downshifted energy levels due to the role of sulfur as a π-acceptor. The non-covalent interaction induced by sulfur also improves the crystallinity of the relevant polymers. However, the photovoltaic properties of P3SHT with thioalkyl chains are inferior to those of P3HT due to the severe phase separation in OSCs. PTST with alternating alkyl and thioalkyl chains exhibits the strongest crystallinity among these polymers since it mitigates the steric hindrance of sulfur atoms while retaining the merit of thioalkyl chains. More importantly, PTST is able to hierarchically pre-aggregate in solution, and then form a favorable morphology in OSCs, consequently yielding a superior PCE to P3HT.