Computational-chemistry Research Projects

Dye-sensitized solar cells


A dye molecule on a TiO2 surface in a dye-sensitized solar cell A novel efficient and environmentally-friendly solar cell based on nanostructured metal oxide materials is the dye-sensitized solar cell (DSSC) or Grätzel solar cell. Ordinary metal oxides have too wide band gaps to absorb visible light efficiently, but in dye-sensitized solar cells the longer wavelengths can be harvested by coating the nanocrystalline metal oxide (e.g. TiO2) by a molecular dye (usually a Ru2+-bipyridine complex). The excitation energy is lower in the dye, and the molecular excitation is followed by an electron injection from the dye to the metal oxide's conduction band. Although much experimental information has been gathered on these systems, more in-depth data about the electron injection process is needed to better understand their function.

Perylene carboxylic acid with CH2-CH2 linker at TiO2 HOMO and LUMO(ads) of perylene carboxylic acid at a TiO2 surface We study the dye adsorption process and electron-injection mechanism by calculations on model dyes adsorbed on TiO2 and ZnO surfaces. In the solar cell the dye binds to the surface through anchor groups, and we have looked for alternative anchor candidates by computational means. It is for example found that phosphonic acid provides a significantly stronger bond than the commonly used carboxylic acid. Visualization of the LUMO orbitals and inspection of DOS spectra reveal strong electronic coupling between adsorbate and metal-oxide substrate. Computed injection times are in the 10-40 fs regime, where the carboxylic anchor enables twice as fast electron transfer compared to phosphonic acid. Such information can be used to aid the construction of new types of solar cells.

Lars Ojamäe, Annika Lenz, Maria Lundqvist

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This work was supported by the Swedish Research Council and by CeNano at Linköping University.