Donor and Acceptor Unit Sequences Influence Material Performance in Benzo[1,2-b:4,5-b′]dithiophene–6,7-Difluoroquinoxaline Small Molecule Donors for BHJ Solar Cells

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Advanced Functional Materials, ISSN: 1616-3028, Vol: 26, Issue: 39, Page: 7103-7114

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Wang, Kai; Liang, Ru-Ze; Wolf, Jannic Sebastian; Saleem, Qasim; Babics, Maxime; Wucher, Philipp; Abdelsamie, Maged; Amassian, Aram; Hansen, Michael Ryan; Beaujuge, Pierre
Wiley; Wiley-Blackwell
Materials Science; Physics and Astronomy; Chemistry; 6,7-difluoroquinoxaline; Benzo[1,2-b:4,5-b']dithiophene; Bulk-heterojunction solar cells; Charge transport; Molecular packing; Organic photovoltaics; Small molecule donors
article description
Well-defined small molecule (SM) donors can be used as alternatives to π-conjugated polymers in bulk-heterojunction (BHJ) solar cells with fullerene acceptors (e.g., PC/BM). Taking advantage of their synthetic tunability, combinations of various donor and acceptor motifs can lead to a wide range of optical, electronic, and self-assembling properties that, in turn, may impact material performance in BHJ solar cells. In this report, it is shown that changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline SM donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in BHJ solar cells. In these systems (SM1-3), it is found that 6,7-difluoroquinoxaline ([2F]Q) motifs directly appended to the central benzo[1,2-b:4,5-b′]dithiophene (BDT) unit yield a lower-bandgap analogue (SM1) with favorable molecular packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%. H-H DQ-SQ NMR analyses indicate that SM1 and its counterpart with [2F]Q motifs substituted as end-group SM3 possess distinct self-assembly patterns, correlating with the significant charge transport and BHJ device efficiency differences observed for the two analogous SM donors (avg. 6.3% vs 2.0%, respectively).