Introducing Asymmetry Induced by Benzene Substitution in a Rigid Fused π Spacer of D-π-A-Type Solar Cells: A Computational Investigation

A rigid fused pi-conjugated segment has a bilaterally symmetric structure for most reported dyes synthesized with the strategy of rigidification in a solar cell. In this work, a series of dyes with an asymmetric or symmetric rigid pi spacer were theoretically investigated through density functional theory and time-dependent density functional theory, to study how asymmetry in a conjugated spacer affects the photophysical properties. The asymmetry is achieved by benzene substitution at different positions in dithienothienodipyrrole. To make a more complete comparison, two more symmetric spacers were also designed by grafting a second benzene. The influence of benzene substitution on the short-circuit current (J(sc)) and open-circuit voltage (V-oc) was expounded through analyzing the geometry, electronic structure, absorption spectra, and other microscopic parameters. Results demonstrated that substituting lowers the light harvest efficiency and maximum photoinduced current. However, the asymmetric configuration of the spacer produced by benzene substitution could result in a narrow energy gap, efficient charge separation, faster interfacial electron injection, energetically promoting dye regeneration, and suppressing recombination. The shift of the conduct band, a key factor affecting V-oc, is increased when substitution occurs from the donor end to the other end of the it spacer. The differences between symmetric and asymmetric rigid spacers originate from not only the nature of the component moiety, but also the substitution position. Taken all together, the asymmetric configuration with a more aromatic benzene on the donor side is more desirable compared to that on the other side and the symmetric spacers. Our work is expected to reveal the structure property relationship and provide guidelines for further dye design.