Impact of the number of fluorine atoms on crystalline, physicochemical and photovoltaic properties of low bandgap copolymers based on 1,4-dithienylphenylene and diketopyrrolopyrrole

Three low bandgap conjugated copolymers (PB-DPP, PDFB-DPP, and PTFB-DPP) based on 1,4-dithienylphenylene and diketopyrrolopyrrole units were synthesized and characterized as the donor materials for polymer solar cells. The effect of the number of fluorine atoms on thermodynamic, crystalline, physicochemical and photovoltaic properties of the copolymers was comparatively investigated. Results indicate that the absorption peaks of polymer solutions are blue-shifted and optical bandgaps are gradually decreased along with the increase of the number of fluorine atoms. Moreover, PDFB-DPP and PTFB-DPP show better light-harvesting abilities, higher crystallinities and lower-lying HOMO energy levels than those of PB-DPP. Photovoltaic performances of these copolymers were studied and optimized. PDFB-DPP and PTFB-DPP deliver a power conversion efficiency of 5.31% and 4.93%, respectively, both of which are higher than that of PB-DPP (4.13%) due to the increased Jsc and Voc. This work demonstrates that the number of fluorine substitution is crucial to improving optoelectronic properties. (C) 2017 Published by Elsevier Ltd.