Triarylamino and Tricyanovinyl End-Capped Oligothiophenes with Reduced Optical Gap for Photovoltaic Applications

Novel low optical gap, p-type semiconducting oligothiophenes asymmetrically end-capped with triarylamino and tricyanovinyl groups, PhN-OFOT(n)-TCN (n = 2, 3), have been synthesized and characterized for photovoltaic applications. With an incorporation of a tricyanovinyl accepting group to the triarylamine, the optical energy gap greatly reduces to 1.46 eV and the LUMO level lowers to 3.9 eV. The initial studies of the bilayer heterojunction PV cells based on the newly developed tricyanovinyl-substituted chromophores as a donor material and C-60 as an acceptor material showed a PCE up to 1.33% with a large open-circuit voltage of 0.82 V in the annealed devices which makes this class of materials promising for further development. Our findings also suggest for the first time that the tricyanovinyl group is highly efficient to lower the LUMO level and reduce the optical energy gap of a p-type semiconducting photosensitizer. It is interesting to find that PhN-OFOT(2)-TCN showed better device performance in bilayer solar cells than PhN-OFOT(3)-TCN although the latter has a slightly narrower optical gap. Since the LUMO energy level of PhN-OFOT(3)TCN (ca. 3.9 eV vs vacuum) is too close to that of C60 (4.0 eV), the yield of photoinduced charge carriers was low, leading to a low power conversion efficiency. Our findings highlight the importance of a large LUMO level offset between electron donors and acceptors to the achievement of high performance organic solar cells.