Direct vs Indirect Mechanisms for Electron Injection in Dye-Sensitized Solar Cells

Dye-sensitized solar cells based on organic dyes have attracted considerable attention in recent years. These dyes present important differences in geometry and electronic structure, and diverse mechanisms for electronic injection into semiconductor conduction band have been reported. In this work, the electronic structure and the optical response of several dyes, free and bound to TiO2, have been analyzed and compared using time-dependent density functional theory (TDDFT) performing calculations both in real time and frequency domains. The aim is to illustrate which aspects of each sensitizer produce different electron injection mechanisms. From the analysis it is concluded that for adsorbed NKX-2311, a coumarin based dye, the photoexcitation occurs toward an orbital mainly localized on the molecule, so this molecule is classified as a type I sensitizer, which follows an indirect mechanism for electron injection. In contrast, catechol and 2,3-naphthalenediol are type II sensitizers and follow a direct mechanism for injecting electrons into the TiO2 conduction band. In this case, a new band appears in the spectrum, which corresponds to a photoexcitation from states fully localized on the dye to states mainly localized on the cluster but with appreciable dye contribution. NKX-2311 and catechol show limit behaviors. Nevertheless, intermediate ones are also possible. As we move in the sequence catechol, 2,3-naphthalenediol, C343, alizarin, and NKX-2311, we pass progressively from a purely direct injection mechanism to a purely indirect injection mechanism. This is related to the relative position of the molecule LUMO energy to the edge of the semiconductor conduction band.