Influence of the fused hetero-aromatic centers on molecular conformation and photovoltaic performance of solution-processed organic solar cells

The novel D–A–π–A–D type molecules, FT1 and FT2, with varied molecular conformation were designed utilizing different fused hetero-aromatic units as the π-bridge building centers. FT1 with a dibenzo-quinquethiophene (DBQT) building center exhibited an eagle-shaped molecular conformation, whereas, FT2 with a dibenzo-sexithiophene (DBST) building center displayed a linear-shaped molecular conformation. In contrast to FT1, FT2 exhibited a much more planar molecular structure and more ordered inter-chain packing. An enhanced power conversion efficiency of 1.71% and short current density of 5.78 mA cm−2 were obtained in the FT2/PC71BM-based OSCs, which were 1.7 times and 1.5 times the corresponding values of the FT1/PC71BM-based cells. Our work demonstrated that the fused hetero-aromatic building centers can significantly affect the molecular conformation and that the photovoltaic performance of small molecules can also be improved by altering the molecular conformation in OSCs.