Zinc Oxide-based Nanostructure Films And Application To Dye-sensitized Solar Cells

Owing to their low cost and high efficiency, the dye sensitized solar cells (DSSCs) as light harvesting energy conversion devices have been proven to have a good chance to become a notable competitor for solar cells based on today technology of p-n junction. Moreover, the application of ZnO in dye sensitized solar cells has been rising over the few years due to its excellent physical and chemical properties in addition to its similarity with the primary photovoltaic material for DSSCs, TiO2. Furthermore, ZnO, being chemically more reactive than TiO2, can be synthesized in various nanostructure forms even under mild conditions. The major part of this thesis deals with the synthesis of ZnO nanostructure films onto FTO (fluorine-doped tin oxide) glass substrates and their incorporation into DSSCs as photoanode. The as-fabricated ZnO films were characterized by the X-ray diffraction (XRD) measurements, the field-emission scanning electron microscopy (FESEM) and the transmission electron microscopy (TEM). The DSSCs based on ZnO nanostructure films were assembled and characterized using optical and electrical measurements.There are many conventional methods to fabricate photoanode with ZnO nanostructures film. In chapter2, Zinc oxide (ZnO) films were prepared successfully by simple thermal evaporation of zinc acetate dihydrate at low temperature onto FTO glass substrates coated with thin ZnO seed layer. The reported method is very simple, catalyst free, low cost, easily controllable, and led to the synthesis of different morphologies of ZnO which give the possibility to enhance the performances of DSSCs through structural and morphology modification of the photoanode. This two-steps approach also offers advantages such as excellent surface adhesion and large area deposition. Moreover, the synthetic parameter such as temperature was found to determine the morphology of nanostructures. ZnO nanorod (NR) and nanoparticle (NP) films have been synthesized at245and350℃, respectively, for6h. The DSSCs were fabricated using the ZnO nanostructure films as photosensitized electrodes. The ZnO NP-based DSSC achieved a maximum photoelectric conversion efficiency (PCE) of1.56%, a short-circuit photocurrent density (Jsc) of5.12 mA/cm2and an open circuit voltage of0.46V (AM1.5100mW cm-2). The PCE of ZnO NP-based DSSC was slightly higher than that of DSSC based NRs. This increase was ascribed to the reduced recombination loss and prolonged electron lifetime according to electrochemical impedance spectroscopy (EIS).Furthermore, Zinc oxide Nanorods film was also prepared successfully by spreading ZnO binder-free paste onto FTO glass substrates. The ZnO paste with suitable viscosity was obtained by ball milling treatment of a mixture of ZnO NRs powder, ethanol and acetic acid. The DSSC based on that ZnO NRs film achieved an efficiency of only0.77%(AM1.5100mW cm-2), and had a short-circuit photocurrent density of3.61mA/cm2and an open circuit voltage of0.42V.In order to obtain a better performance of ZnO-based DSSC, the paste was made of a blend of nanorods and nanoparticles, and was applied onto FTO glass by using the doctor blade method. It was found that the blend of nanorods and nanoparticles can improve the performance of ZnO-based DSSC. The DSSC based on ZnO nanorods/nanoparticles composite film achieved an efficiency of1.90%. It was also found that, the ZnO nanorods/nanoparticles composite film coated with a layer of TiO2, can further improve the photoelectric conversion efficiency of the cell, reaching2.26%.