4-Methylthio substitution on benzodithiophene-based conjugated polymers for high open-circuit voltage polymer solar cells

Two new two-dimensional conjugated polymers PBDTT-4S-TT and PBDTT-4S-BDD based on benzo [1,2-b:4,5-b') dithiophene unit with 4-methylthio substituted thiophene side chains were designed and synthesized. The two polymers exhibit slightly blue-shifted absorptions and deeper-lying highest occupied molecular orbital energy level compared with their analogues without 4-methylthio substituents. Polymer solar cells of the two polymers employing 6,6-phenyl-C-71-butyric acid methyl ester as the electron acceptor afford an open-circuit voltage of up to 0.98 V, which is much higher than that of their parent polymers without 4-methylthio substituents. The highest power conversion efficiencies of 6.6% and 7.8% were realized by PBDTT-4S-TT and PBDTT-4S-BDD, respectively. These results suggest that 4-methylthio substitution on thiophene side chain of two-dimensional polymers is an effective strategy to enhance the V-oc of polymer solar cells.