Impedance spectroscopy study of N719-sensitized ZnO-based solar cells

ZnO porous films prepared by electrodeposition and by sol-gel techniques have been sensitized by the N719 dye and used as photoelectrode in dye-sensitized solar cells. Similar stationary current-voltage characteristics have been measured for films having two drastically different morphologies and nanostructures. The solar cells have been studied by impedance spectroscopy over large frequency and applied voltage ranges in order to elucidate the inherent correlation between the photoelectrode structure and properties. The electrical response has been analyzed using an ad-hoc electrical model to extrapolate the electronic structure and charge carrier kinetic properties of the photoelectrodes. The two films exhibit very different density distributions of their trap states below the conduction band edge. Moreover, their electron lifetimes and transport times vary differently with the DOS due to different surface and conduction properties. The charge collection efficiency has been calculated by two different approaches, one being derived from first principles. Very high values are reported in all cases. Globally, the limitation of N719/ZnO solar cell performances is related to their open circuit voltage and their short circuit current. The latter point is discussed in the light of our impedance results and photoelectrode light absorption data.