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

•New conjugated polymers based on benzo[1,2-b:4,5-b']dithiophene unit with 4-methylthio substituted thiophene side chains were developed.•4-Methylthio substitution on thiophene side chain leads to deeper-lying highest occupied molecular orbital energy levels for the polymers.•4-Methylthio substitution on thiophene side chain of conjugated polymers is an effective strategy to enhance the open-circuit voltage of polymer solar cells.