Influence of the nature of the anchoring group on the interfacial energy level alignment in dye-sensitized solar cells: A theoretical perspective

Abstract

We investigate at the theoretical (time-dependent) density functional theory level the influence of the nature of the anchoring group on the electronic and optical properties of oligothiophene dyes when adsorbed on ${\mathrm{Ti}\mathrm{O}}_{2}$ surface. The computed electronic structures point to a strong orbital hybridization between the dye and the substrate in the presence of the carboxylic acid and thiocarboxylic acid group, leading to a pronounced pinning effect of the lowest unoccupied molecular orbital (LUMO) level and faster electron injection. In contrast, phosphonic acid and catechol promote a weak electronic coupling between the two components and hence slower injection times. The simulated absorption spectra demonstrate that carboxylic and thiocarboxylic anchoring groups can induce a large redshift of the lowest optical transition of the dye upon adsorption due to a strong stabilization of the LUMO level triggered by the pinning effect while a small redshift prevails for phosphonic and catechol dyes. When pinning is active, the chain-size evolution of the lowest optical transition is also less sensitive to the conjugation length compared to the free dyes.