A New Electron-Rich Unit for Polymer Electron Acceptors: 4,4-Difluoro-4H-cyclopenta[2,1-b:3,4-b']dithiophene.

We report 4,4-difluoro-4H-cyclopenta[2,1-b:3,4-b']dithiophene (fCDT) as a new electron-rich unit to design polymer electron acceptors. Owing to the fluoro substitutes, fCDT unit exhibits downshifted LUMO energy level, diminished steric hindrance effect and strong intermolecular interaction. The resulting polymer electron acceptor exhibits low-lying LUMO energy level and high electron mobility, as well as good all-polymer solar cell device performance.