Density functional study of mono-branched and di-branched di-anchoring triphenylamine cyanoacrylic dyes for dye-sensitized solar cells

Abstract

Electronic structures and absorption spectra of mono-branched and di-branched di-anchoring triphenylamine cyanoacrylic dyes were calculated by density functional theory. Our result suggested that one can develop organic dyes focusing on the preferred higher absorptivity or the wider absorption range, based on the symmetric or asymmetric di-branched architectures, respectively. The calculated absorption spectra for mono-branched dyes are in good agreement with the experiment. Different types of aromatic rings in the π-conjugated backbone of mono-branched dyes have varying effect on the absorption wavelength in the order of pyrrole < furan < thiophene. Among the mono-branched dyes studied, dye 6 with the extended π-conjugated structure of a thiophene–pentacyclodithiophene–thiophene showed the highest molar absorption coefficient, also at the longest absorption wavelength (∼570 nm). Our study gave an insight into how the absorption properties of di-branched dyes relate to their mono-branched analogs. The symmetric di-branched di-anchoring dye can take an advantage of a higher molar absorptivity than its mono-branched analog due to the near degeneracy of LUMO and LUMO+1 orbitals, which facilitates two transitions at the similar energy. For the asymmetric di-branched dianchoring dyes, the combination of two π-spacer units that absorb light at different wavelength regions leads to a wider absorption wavelength range, which can benefit DSSCs differently from the previous symmetric di-branched dyes. Asymmetric di-branched dyes need to be explored further for dye-sensitized solar cell applications.