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Ethynylene-Linked Donor–Acceptor Alternating Copolymers

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journal contribution
posted on 14.05.2013 by Wade A. Braunecker, Stefan D. Oosterhout, Zbyslaw R. Owczarczyk, Ross E. Larsen, Bryon W. Larson, David S. Ginley, Olga V. Boltalina, Steven H. Strauss, Nikos Kopidakis, Dana C. Olson
Controlling steric interactions between neighboring repeat units in donor–acceptor (D–A) alternating copolymers can positively impact morphologies and intermolecular electronic interactions necessary to obtain high performances in organic photovoltaic (OPV) devices. Herein, we design and synthesize 12 new conjugated D–A copolymers, employing ethynylene linkages for this control. We explore D–A combinations of fluorene, benzodithiophene, and diketopyrrolopyrrole with analogues of pyromellitic diimide, thienoisoindoledione, isothianaphthene, thienopyrazine, and thienopyrroledione. Computational modeling suggests the ethynylene-containing polymers can adopt virtually planar conformations, while many of the analogous polyarylenes lacking the ethynylene linkage are predicted to have quite twisted backbones (>35°). The introduction of ethynylene linkages into these D–A systems universally results in a significant blue-shift in the absorbance spectra (by as much as 100 nm) and a deeper HOMO value (∼0.1 eV) as compared to the polyarylene analogues. The contactless time-resolved microwave conductivity technique is used to measure the photoconductance of polymer/fullerene blends and is further discussed as a tool for screening potential active layer materials for OPV devices. Finally, we demonstrate that an ethynylene-linked alternating copolymer of diketopyrrolopyrrole and thienopyrroledione, with a rather deep LUMO estimated at −4.2 eV, shows increased photoconductance when blended with a perfluoroalkyl fullerene C60(CF3)2 as compared to the standard PC61BM. We attribute the change in increased free carrier generation to the higher electron affinity of C60(CF3)2 that is more appropriately matched with the deeper LUMO of the polymer.