Computational Study of the Influence of the π-bridge Conjugation Order of Quinodimethane-based Derivatives for Dye Sensitized Solar Cells (DSSCs) Using Density Functional Theory (DFT)

Several series of quinodimethane-based derivatives with different conjugated π-bridge groups (phenyl ring, thiophenyl ring and benzothiadiazoyl ring) were investigated by different conjugation orders using density functional theory (DFT) at B3LYP /6-31g(d) level. Computational results show that, all three π-bridge groups obviously improve the optoelectronic properties of dyes compared with the reference, with more suitable energy levels and wider absorption bands. For the series of dyes with different numbers (n) of phenyl rings, larger order of π-bridge conjugation leads to a larger band gap from 2.501 eV to 2.547 eV, while the other series with thiophenyl rings has a wider absorption band and a red shift of the first absorption peak from 522.86 nm (2.371 eV) to 565.25 nm (2.193 eV) when the conjugation order increases. Benzothiadiazoyl ring was also studied as a part of π-bridge conjugation. The structure of donor - phenyl - benzothiadiazoyl -thiophenyl - anchor shows the lowest LUMO level (-2.929 eV) and a relatively high HOMO level (-5.049) among all the derivatives, but with a wide absorption band since it has two strong absorption peaks (489.24 nm and 400.41 nm with strengths of 0.7883 and 0.6625 respectively). Generally, dyes with thiophenyl rings shows more suitable optoelectronic properties for being candidate sensitizers for dye sensitized solar cells, and a combination of different aromatic rings as π-bridges is also a valuable way for designing new dye molecules in the future's work. (paper)