Size Effects in the Interface Level Alignment of Dye-Sensitized TiO₂ Clusters

The efficiency of dye-sensitized solar cells (DSCs) depends critically on the electronic structure of the interfaces in the active region. We employ recently developed dispersion-inclusive density functional theory (DFT) and GW methods to study the electronic structure of TiO₂ clusters sensitized with catechol molecules. We show that the energy level alignment at the dye-TiO₂ interface is the result of an intricate interplay of quantum size effects and dynamic screening effects and that it may be manipulated by nanostructuring and functionalizing the TiO₂. We demonstrate that the energy difference between the catechol LUMO and the TiO₂ LUMO, which is associated with the injection loss in DSCs, may be reduced significantly by reducing the dimensions of nanostructured TiO₂ and by functionalizing the TiO₂ with wide-gap moieties, which contribute additional screening but do not interact strongly with the frontier orbitals of the TiO₂ and the dye. Precise control of the electronic structure may be achieved via “interface engineering” in functional nanostructures