Treatment of the ZnO and TiO2 Thin Films by Electric Field in Plasma Sheath to Improve the Metal-Dye Electronic Coupling in Dye-Sensitized Solar Cells

Abstract

In this study, the effect of the electric field of Ar and N2 plasma sheath on the crystalline structure and the optoelectronic properties of ZnO and TiO2 thin films were investigated, in order to improve the electronic coupling of metal-N719 dye in dye-sensitized solar cells. The thin films were prepared via the sol-gel method and then treated in Ar and N2 glow discharge plasma. The plasma effect on the crystalline structure was determined using the XRD spectra and the scanning electron microscope images. The electric field in the Ar plasma sheath is stronger than the N2 plasma, so the crystalline changes in the ZnO/Ar and TiO2/Ar were larger in comparison to ZnO/N2 and TiO2/N2. The variations evaluated via the UV-VIS spectra for the optoelectronic properties coincided with the changes in the crystalline structure. A detailed study of the dispersion parameters and the third-order nonlinear optical susceptibilities represented alterations in the chemical properties of the surface of the thin films.The thin films of ZnO, ZnO/Ar, ZnO/N2, TiO2, TiO2/Ar, and TiO2/N2 were sensitized by the N719 dye via the immersion method, with enough duration of time for the adsorption and the aggregation to take place. According to the measurements of the short circuit photocurrent density, the order of enhancement in the electronic coupling is: ZnO/Ar-N719> ZnO/N2-N719> TiO2/Ar-N719> TiO2/N2-N719. The plasma remarkably enhances the Zn-N719 electronic coupling via the reduction of the Zn2+/dye complexes, in the case of the Ti-N719, increment in the electronic coupling occurs by the reduction of the H-bonds.