Fine Tuning of Molecular Energy Level of Low Band Gap Polymer by Introduction of Fluorine for High Performance Organic Photovoltaics

Abstract

Band gap and molecular energy level of alternating conjugated copolymers are the most important parameters to achieve high efficient organic photovoltatcs. Lowering the HOMO energy level of donor polymer is beneficial for achieving high open circuit voltage(Voc). One of the easy and simple methods to lower the HOMO energy level of alternating conjugated copolymer is to introduce the electron-withdrawing group onto the polymer backbone. For this purpose, fluorinated phenyl group is alternatively copolymerized with diketopyrrolo[3,4-c]pyrrole (OPP) which is one of promising building blocks for low band gap alternating copolymer. In this work, we have systematically synthesized low band gap copolymers composed of difluoro or tetrafluorophenylene and DPP. The effect of the number of fluorine substitution of the polymer on their electrochemical and photovoltaic properties will be demonstrated.