High face-on ratio isoindigo copolymers with extended nano-fibrillar networks in fullerene-based thick (>300 nm) photovoltaics achieving a high efficiency of 10.7%

Abstract

Three D–A π-conjugated copolymers PnTIF (where n = 2, 4, or 6), composed of isoindigo electron acceptor units and difluorinated bithiophene (BTF) electron donor units, were synthesized and characterized. The HOMO and LUMO energy levels, solubility (and miscibility with PCBM), crystallinity (and crystalline orientation) and PnTIF:PCBM thin film morphology (nano-fibrillar network and phase separation) of the copolymers could be adjusted by the number (0 for P2TIF, 1 for P4TIF, 2 for P6TIF) of flexible 3-dodecylthiophene spacers between the isoindigo and BTF units. From the tested polymer solar cells fabricated with PnTIF:PCBM bulk heterojunctions, P4TIF:PC₆₁BM solar cells exhibited a remarkably high short-circuit current density of ∼22 mA cm⁻² together with a fill factor of ∼61% and an open-circuit voltage >0.80 volt, resulting in a power conversion efficiency of 10.7%, making it one of the highest among PC₆₁BM-based polymer solar cells.