Effects of π-spacer and fluorine loading on the optoelectronic and photovoltaic properties of (X-DADAD)n benzodithiophene-based conjugated polymers

Four new conjugated polymers based on benzodithiophene, thiophene and 2,1,3-benzothiadiazole or 5,6-difluoro-2,1,3-benzothiadiazole were synthesized and investigated as absorber materials for organic photovoltaics. The effect of (bi)thiophene π-spacers and fluorine substitution on the physicochemical and optoelectronic properties of the polymers was revealed and correlations were drawn with their electrical characteristics in organic solar cells. In particular, introducing either thiophene spacers or fluorine substituents does not affect much the photovoltaic performance of the polymers, while the combination of both routes was found to be a promising strategy for improving the charge carrier mobilities and morphology of the polymer-fullerene blends as well as light power conversion efficiency in solar cells based on these materials. The bulk heterojunction organic solar cells based on the π-bridged and fluorinated polymer P4 showed the highest short-circuit current density and power conversion efficiency of 7 %, which is an inspiring value for fullerene-based organic photovoltaics. Most importantly, our findings provide important insights into rational design of high-performance conjugated polymers while pursuing a combination of two efficient backbone functionalization strategies based on introduction of fluorine substituents and π-spacers to control the geometry and electronic characteristics of the polymer chains.