Fine-Tuning of Fluorinated Thieno[3,4-b]thiophene Copolymer for Efficient Polymer Solar Cells

Novel low band gap fluorinated copolymers have been synthesized and characterized. The performance of inverted polymer solar cells made by these fluorinated copolymers blended with fullerene derivatives was also investigated. The studies of the effect of fluorine units on the electronic properties and the thin film morphologies of fluorinated copolymers demonstrated that an optimal window of 20-40% fluorine units coupled with thieno[3,4-b]thiopehene exists for a maximum hole mobility that correlated well with a dispersed interconnected morphology of the fluorinated copolymer, thus enhancing the performance of polymer solar cells. More fluorine units coupled with thieno[3,4-b]thiopehene adversely affected the phase morphology to coarsen that in turn reduced polymer solar cells performance. Nevertheless, the polymer solar cell performance of the highest fluorinated content polymer was twice as much as the nonfluorinated copolymer. These results indicate that the electronic properties as well as the thin film morphologies, not only in nanoscale but also in microscale, of electroactive and photoactive polymers are collectively very important for organic photovoltaic devices.