Electronic and Optical Properties of Dye-Sensitized TiO2 Interfaces

We start with a discussion of isolated dyes in solution, focusing on the dye's atomic structure, ground and excited state oxidation potentials, and optical absorption spectra. We examine both Ru(II)-polypyridyl complexes and organic " push-pull" dyes with a D-p-A structure, where the donor group (D) is an electron-rich unit, linked through a conjugated linker (pi) to the electron-acceptor group (A). We show that a properly calibrated computational approach based on Density Functional Theory (DFT) combined with Time Dependent DFT (TD-DFT) can provide a good description of both the absorption spectra and ground and excited state oxidation potential values of the Ru(II) complexes. On the other hand, organic push-pull dyes are not well described by the standard DFT/TD-DFT approach. For these dyes, an excellent description of the electronic structure in gas phase can be obtained by the many body perturbation theory GW method, which has, however, a much higher computational cost.

Dye-sensitized solar cells (DSCs) represent a promising approach to the direct conversion of sunlight to electrical energy at low cost and high efficiency. DSCs are based on a film of anatase TiO2 nanoparticles covered by adsorbed molecular dyes and immersed in a liquid redox electrolyte. Upon photoexcitation of the chemisorbed dye, electrons are injected into the TiO2 conduction band and can travel across the nanostructured film to reach the counter-electrode, while the oxidized dye is regenerated by the redox electrolyte. In this review we present a summary of recent computational studies of the electronic and optical properties of dye-sensitized TiO2 interfaces, with the aim of providing the basic understanding of the operation principles of DSCs and establishing the conceptual basis for their design and optimization.