Synthesis and Characterization of Zinc Oxide Nanoparticles for Dye-Sensitized Solar Cells

Stable ZnO nanoparticles suitable for further derivitization of their surface were synthesized in the liquid phase from homogeneous ethanolic solutions of the precursors, lithium hydroxide and zinc acetate. The particle growth was monitored by in situ absorption spectroscopy and dynamic light scattering (DLS). It was found that the growth of the particles was governed by temperature as well as the presence of the reaction byproduct lithium acetate during the ripening process. In particular, the reaction could be almost completely arrested by removal of this byproduct. The “washing” consisted of repeated precipitation of the ZnO particles by addition of alkanes such as heptane, removal of the supernatant, and redispersion in ethanol. The ZnO nanoparticles, before and after “washing”, were characterized by absorption spectroscopy, DLS, thermogravimetric analysis (TGA), X-ray powder diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). We will also describe the photosensitization of the ZnO nanoparticles. Firstly, the surface of the colloidal ZnO nanoparticles was successfully modified by dye molecules, i.e., 5-(N-(3,4-dihydroxyphenethyl)-2-phenoxyacetamide)- 10, 15, 20-(p-tert-butyl-triphenyl)-porphyrinato-zinc and 5-(tris-ethoxy-silane-propyl-amide-acetato-phenoxy)- 10, 15, 20-(p-tert-butyl-triphenyl)-porphyrinato-zinc, for photochemical characterization. Steady-state absorption and emission studies confirmed an interaction between the ZnO and dye molecules. Next, mesoporous films of the ZnO nanoparticles suitable for use as electrodes in dye-sensitized solar cells (DSSCs) were prepared by the doctor blade technique. Following sintering the films were sensitized by the dye molecules mentioned above. The morphology of the films was characterized by using scanning electron microscopy (SEM). Finally, the current generated during photoelectrochemical measurements of these films was ascribed to an efficient photoinduced electron transfer from the photoexcited porphyrin to the ZnO moiety. The performance of the films was compared with that from existing nanoporous films of ZnO and TiO₂.