How to Optimize the Interface between Photosensitizers and TiO₂ Nanocrystals with Molecular Engineering to Enhance Performances of Dye-Sensitized Solar Cells?

In this work, the interfacial properties of a series of metal-free organic naphthodithienothiophene (NDTT)-based photosensitizers adsorbed on TiO₂ surfaces were investigated by a combination of ab initio calculations and experimental measurements. The calculations and experiments reveal that because of the efficient charge transfer from the adsorbed dyes to TiO₂ nanocrystal surface there is an upward shift for the energy levels of dyes and a downward shift for the conduction band of surface TiO₂ and that the band gaps for both of them are also reduced. Such electronic level alignments at the interface would lead to increased light absorption range by adsorbed dyes and increased driving force for charge injection but reduced open-circuit potential (V_oc). More interestingly, we found that molecule engineering of the donor group and introducing additional electron-withdrawing unit have little effect on the electronic level alignments at the interface (because band gaps of the dyes adsorbed on TiO₂ surfaces become approximately identical when compared with those of the dyes measured in solution) but that they can affect the steric effect and the charge separation at the interface to tune V_oc and the short-circuit current density (J_sc) effectively. All these findings suggest that optimizing the interfacial properties of dyes adsorbed on TiO₂ surfaces by synchronously modifying steric effects of dye molecules anchored on TiO₂ and charge-transfer and separation properties at the interfaces is important to construct efficient dye-sensitized solar cells.