The Band Energetics And Photoelectrochemical Properties Of Nanostructured Metal Oxide Electrodes

As an alternative energy to the traditional fossil energy, solar energy become the most developmental and utilization prospective resource because of its advantages of abundant reserves, cleanness, renewable. Among the effective devices of utilizing solar energy, the solar cell is the fastest growing and the most active research field in recent years. Since the1990s, dye sensitized nanocrystalline solar cell has been one of the hot topics in the study of the photoelectric conversion device because of its simple preparation, low cost, high photoelectric conversion efficiency and other advantages.As one vital component of dye sensitized solar cells (DSSCs), nanostructured semiconductor electrode has important effects on the photoelectrochemical properties of DSSCs. In this paper, our work focused on the preparation of nanostructured semiconductor film electrode, and the exploration of the effective methods to improve photoelectric conversion efficiency of solar cells by researching the band energetics and photoelectrochemical properties.1. Enhanced energy conversion efficiency of dye sensitized nanocrystalline ZnO films by surface modification. Nanocrystalline ZnO film electrodes were fabricated from ZnO nanocolloids which were synthesized by the solvothermal method. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were used to characterize the crystalline phase, absorption spectra and morphology of the films. The results indicated that ZnO nanoparticles have a high crystalline wurtzite structure with an average diameter of about55nm, and the thickness of the film electrode is about0.38μm. Then, Sr2+and Al3+were chosen to modify the surface of the electrodes, which effectively suppressed the charge recombination and the photocurrent and photovoltage were significantly improved, and as a result the photoelectrical conversion properties of the cells was improved. The photoelectrical conversion efficiency of DSSCs is increased from3.25%to4.04%by Sr2+-modification under the illumination of a white light of100mW·cm-2, and the photoelectrical conversion efficiency is increased from2.32%to3.40%by application of the modified layer of Al3+. The promotion is nearly24%and46%respectively.2. Research the tunability of the band energetics of nanostructured TiO2electrodes in binary solvent system. TiO2colloids were synthesized with a hydrothermal method, and the nanostructured TiO2electrodes were fabricated from the TiO2colloids. XRD and SEM were used to characterize the crystalline phase and morphology of the film electrodes. The results indicated that TiO2nanoparticles have a high crystalline wurtzite structure with an average diameter of about25nm, and the thickness of the film electrode is about0.50μm. Band energetics of the electrodes in two binary solvent systems were measured with spectroelectrochemical measurements and their trap states were investigated with electrochemistry. The flat band edges (Efb) of the nanostructured TiO2electrodes were highly dependent on the composition of the binary solvent system. For the system of water-acetonitrile (MeCN), the Efb were measured to be-1.0,-1.1,-1.3,-1.5and-2.3V vs saturated Ag/AgCl as the volume fraction of MeCN changes from0to0.25,0.5,0.75and1respectively. Accordingly, the total trap state densities were measured to be3.67×1015,5.77×1015,8.94×1015,2.56×1016and4.88×1016cm-2respectively. For the system of acetone-acetonitrile, the Efb, were measured to be-0.5,-1.6,-1.9,-2.1and-2.3V vs saturated Ag/AgCl as the volume fraction of MeCN changes from0to0.25,0.5,0.75and1respectively, and the total trap state densities were measured to be6.15×1015,1.86x1016,3.82×1016,6.96×1016and7.04×1016cm-2respectively. The Efb became more negative and the total trap state became more higher along with the increasing volume fraction of MeCN. The results show that the strong polar or complexing solvents can combine with defect points on the surface of the semiconductor electrode more strongly, so the surface state level of electrode is changed, and the trap state density is decreased.