The Modification And Photoelectrochemical Properties Of Nanostructured TiO₂ Electrodes

With the life of mankind and socio-economic development, mineral energy resources such as coal, oil and natural gas are gradually exhausted. With worsening of the worldwide energy supply, energy will become a major factor constraining the development of national economies. As a replaceable energy sources, solar energy becomes more and more important since it has exploitable and usable. Among the efficient use of solar energy, solar energy photovoltaic utilization is the fastest growing and most dynamic fields of study in recent years. The dye-sensitized solar cells (DSSCs) were first developed in the 90th of twenty century and have drawn great attention due to their simple fabrication processes, low cost and high performance.Dye sensitized solar cells possesses three major components:dye sensitizer, nanostructured semiconductor electrode, redox electrolyte and photocathode. In this paper, we focus on nanostructured semiconductor electrode and redox electrolyte aspects and explored the ways to improve photoelectric conversion efficiency of solar cells.1. The electrolyte plays an important role in transporting electrons and holes in DSSCs. The kind of additives have important effect on the properties of solar cells, the tert-butylpyridine (TBP) and Li+ are the most frequently applied additives. The flat band edges (Efb) of nanostructured TiO2 electrodes in electrolyte solutions with TBP of different concentrations have been determined with spectroelectrochemical technique. TBP played a role in band energetics of nanostructured TiO2 electrodes. The Efb, values of -2.25,-2.46 and-2.62 V were determined in three 0.2 mol·L-1 tetrabutylammonium perchlorate (TBAP) acetonitrile electrolytes which contain 0,0.2 and 0.4 mol-L-1 TBP respectively. Li+ ions shifted Efb positively. The Efb values of -1.12,-1.22 and -1.30 V were determined in three 0.2 mol-L-1 LiClO4 acetonitrile electrolytes which contain 0,0.2 and 0.4 mol-L-1 TBP respectively. The trap state distribution was investigated by the measurements of time resolved current. The total trap state densities of 3.52×1016,3.18×1016 and 3.37×1016 cm-2 were determined in three 0.2 mol-L-1 TBAP acetonitrile electrolytes which contain 0, 0.2 and 0.4 mol-L"1 TBP respectively with trap distribution maximum located at -1.99,-1.89 and -1.85 V. The addition of Li+ ions further reduced the trap state densities. The total trap state densities of 8.39×1015,1.11×1016 and 9.22×1015 cm-2 were determined in three 0.2 mol·L-1 LiC104 acetonitrile electrolytes which contain 0,0.2 and 0.4 mol·L-1 TBP respectively with trap distribution maximum located at -0.72,-0.84 and -0.95 V. Finally the nanostructured TiO2 electrodes were sensitized with dye N3 and their photoelectrochemical properties were studied in electrolytes with TBP of different concentrations. Experiment results showed that as the concentration of TBP increased, the photoelectric conversion efficiency increased due to improved Voc.2. To improve photoelectric conversion efficiency of DSSCs with surface modification. A remarkable feature of the nanoporous electrodes, however, is the lack of a depletion layer at the electrode and electrolyte interface. As a result, the back electron transfer, i.e., the charge recombination between the electrons injected in the conduction band of the semiconductor and the oxidized species in the electrolyte, still remains one of the major limiting factors to the efficiency of the solar cells. In this paper, we demonstrate a facile method of the fabrication of SrCO3 and BaSO4 modified TiO2 electrode. The as-prepared TiO2/SrCo3 or BaSO4 electrodes was characterized by IR spectroscopies, indicating that a SrCO3 or BaSO4 layer was formed on the surface of nanostructured TiO2 electrodes. The influence of the thickness of SrCO3 and BaSO4 layer on band energetics and photoelectrochemical properties of nanostructured TiO2 electrodes were investigated. The results showed that the flat band edges (Efb) of SrCO3 and BaSO4 modified nanostructured TiO2 electrodes have been determined with spectroelectrochemical technique. Compared with bare TiO2 electrodes, the Efb of TiO2 electrodes modified with SrCO3 and BaSO4 layer were little moved. On the other hand the total trap densities were remarkably decreased when TiO2 electrodes were modified with SrCO3 and BaSO4 layers. Experiment results showed that the thickness of SrCO3 and BaSO4 layer should be optimized in order to obtain improved photoelectrochemical properties. The highest conversion efficiency reaches 7.46% under irradiation of 100 mW-cm-2 white light, obtained with the TiO2 electrode modified with one layer of SrCO3. The total conversion efficiency of the TiO2 electrode modified with two layers of BaSO4 reaches 7.56% under irradiation of 100 mW-cm-2 white light, about 6% higher than that obtained with a bare TiO2 electrode.3. A novel and facile preparation of nanostructured TiO2 electrodes modified with BaSO4 layer and their application in dye-sensitized solar cells (DSSCs) were presented. The electrodes were characterized by IR spectroscopies, indicating that a thin BaSO4 layer was formed on the surface of N3 dye-sensitized nanostructured TiO2 electrodes. The influence of the thickness of BaSO4 layer on electrochemical and photoelectrochemical properties of the modification electrodes was investigated. The flat band edges (Efb) of N3 dye-sensitized nanostructured TiO2 electrodes modified with BaSO4 layers has been determined with spectroelectrochemical technique. Compared with bare TiO2 electrodes, the Efb of N3 dye-sensitized nanostructured TiO2 electrodes which were modified with BaSO4 layer was little moved. On the other hand the total trap densities were decreased when N3 dye-sensitized nanostructured TiO2 electrodes were modified with BaSO4 layers. Finally the N3 dye-sensitized nanostructured TiO2 electrodes were modified with BaSO4 layers and their photoelectrochemical properties were studies. Experiment results showed that the highest conversion efficiency reaches 7.40% under irradiation of 100 mW-cm-2 white light, obtained with the TiO2 electrode modified with two layer of BaSO4.