Low bandgap copolymers based on monofluorinated isoindigo towards efficient polymer solar cellsElectronic supplementary information (ESI) available: Solar cell optimization tables, 1H NMR spectra of monomers and polymers, and gel permeation chromatograms and data. See DOI: 10.1039/c5py00693g

To explore the effectiveness of monofluorinated isoindigo as an electron-deficient building block in push-pull conjugated polymers for organic solar cell applications, four low bandgap copolymers are effectively synthesized and characterized. The effects of fluorine introduction, thiophene spacer length and polymer molar mass on the general electro-optical polymer characteristics, thin film blend microstructure and electronic performance are investigated. Isoindigo monofluorination effectively improves the power conversion efficiency from 2.8 up to 5.0% upon molar mass optimization, without using any processing additives or post-treatments. Low bandgap copolymers based on fluorinated isoindigo afford 5.0% efficiency in polymer solar cells.