Fluorinated and Alkylthiolated Polymeric Donors Enable both Efficient Fullerene and Nonfullerene Polymer Solar Cells

In this work, four donor (D)-acceptor (A) copolymers based on benzodithiophene (BDT) and benzothiadiazole (BT) with different alkylthiolated and/or fluorinated side chains are developed for efficient fullerene and nonfullerene polymer solar cells (PSCs). The synergistic effect of sulfuration and fluorination on the optical absorption, energy level, crystallinity, carrier mobility, blend morphology, and photovoltaic performance is investigated systematically. By incorporating sulfur atoms onto the side chains, a little blueshifted but significantly increased absorption can be obtained for PBDTS-FBT compared to PBDT-FBT. On the other side, a little more blueshifted but much stronger absorption and much lower-lying highest occupied molecular orbital (HOMO) level can be realized for PBDTF-FBT when introducing fluorine atoms instead of sulfur atoms. With the combination of both fluorination and sulfuration strategies, PBDTS-FBT exhibits the best absorption ability, lowest HOMO energy level, and highest crystallinity, which make PBDTSF-FBT devices show the highest power conversion efficiency (PCE) of 10.69% in fullerene PSCs and 11.66% in nonfullerene PSCs. The PCE of 11.66% is the best value for PSCs based on BT-containing copolymer donors reported so far. The results indicate that fluorination and sulfuration have a synergistically positive effect on the performance of D-A photovoltaic copolymers and their solar cell devices.