A facile strategy to enhance absorption coefficient and photovoltaic performance of two-dimensional benzo [1,2-b:4,5-b′]dithiophene and thieno[3,4-c]pyrrole-4,6-dione polymers via subtle chemical structure variations

Two novel donor-acceptor (D-A) type conjugated polymers using thiophene and hexyl-thiophene spacers between two-dimensional alkyl-thiophene substituted benzo[1,2-b:4,5-b']dithiophene (BDT-RT) and alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) units are synthesized and characterized. The effects of incorporation of alkyl-thiophene spacers in the polymer backbone on the optical, electrochemical, charge transport and photovoltaic properties are studied. The bulk-heterojunction (BHJ) polymer solar cells (PSCs) based on the polymer with hexyl-thiophene spacers show much better performance (with power conversion efficiency up to 6.08%) than that of polymer without spacers, which is very distinct from alkyl-thiophene spacer effect of other BDT-TPD structured polymers reported previously. The experimental results and theoretical calculations show that a subtle tuning of chemical structure can significantly influence the absorption coefficient by inserting alkyl-thiophene spacers in the polymer backbone. This provides an important route for designing new materials to obtain higher current density (J(sc)) and fill factors (FF). (C) 2013 Published by Elsevier B. V.