Effects of auxiliary electron-withdrawing moieties on the photovoltaic properties of D-π-A’-π-A phosphonic acid-based DSSCs

Abstract

Dye-Sensitized Solar Cells (DSSCs) that are based on a donor-bridge-acceptor-bridge-donor (D-π-A’-π-A) oligothiophene dye-containing benzothiadiazole mono-functionalized by a phosphonic acid were theoretically studied during the changing of the auxiliary acceptor (A’) by six other auxiliary acceptors. To further understand the molecular structure of these series of dyes, a density functional theory (DFT) calculation was performed to study the molecular geometries, frontier molecular orbitals (FMO) by employing the method B3PW91 with the basis set 6-311G(d) for the soft atoms (H, C, N, O, P, Si, S) and LANL2DZ for the heavy atoms (Se, Te). The optical properties of the free and the adsorbed dyes were modelled using TD-DFT simulations employing the functional BHandH. The calculated absorption spectra of the designed dyes indicate that the change of electron-withdrawing groups increases the absorption wavelength significantly. Our results showed that the BTD, BXD, BTED, BSED, BIA, and BSD dyes exhibit a narrower HOMO–LUMO energy gap than BTZ dye. This led to a broader optical absorption and lower oscillator strength in dyes containing Benzimidazole, Benzosiladiazole, Benzoselenodiazole, and Benzotellurodiazole moieties that were verified to be better auxiliary acceptor units. The simulated absorption spectra of the adsorbed dyes on (TiO2)9 are red-shifted compared to the free dye. The adsorption energies of all dyes are similar which reveals that the adsorption energy is not depending on the auxiliary moiety but rather on the anchor group. Therefore, it is expected that the results obtained from this investigation will have a substantial impact on the development of DSSC of phosphonic acid-based dyes.