The Electrochemical And Photoelectrochemical Properties Of Non Metal Doped Nanostructured TiO₂Electrodes

Due to huge consumption and pollution to environment of fossil energy, development of clean and renewable energy sources has become a very important and crucial task. As one of new energy resources, solar energy is becoming most valuable in development and utilization. At present, solar cells are an important means of efficient use of solar energy. The dye-sensitized solar cells (DSSCs) were first developed in the90th of twenty century and have drawn great attention due to their low cost, high performance and friendliness to environment.Dye sensitized solar cells possesses three major components:dye sensitizer, nanostructured semiconductor electrode, redox. electrolyte and counter electrode. Nanostructured semiconductor electrode has important effect on the photoelectrochemical properties of DSSCs and become the hot topics in this field. In this paper, we focus on this aspect and explored the ways to improve photoelectric conversion efficiency of solar cells.1. It has been reported that there are a large number of surface states in nanocrystalline TiO2electrodes, which are deleterious to the performance of DSSCs since they trap carriers and promote charge recombination. In this paper, we report the synthesis of F-doped TiO2colloids by hydrothermal method and the fabrication of F-doped TiO2electrodes. The doping of F can be beneficial to the improvement of nanoporous TiO2electrodes. The research results showed that the highest conversion efficiency based on N3sensitized F doped nanoporous TiO2electrodes reaches8.07%under irradiation of100mW/cm/white light, obtained with the electrode TiO2(F0.05), a11.3%increase than that of bare TiO2electrodes, which was7.25%. In addition, the band energetics and the trap states at different pHs were investigated with electrochemical and spectroelectrochemical techniques for the first time. The results showed that the flat band edge(Efb) of TiO2(F0.05) is correlated linearly with pH and has the relation of Efb=-0.54-0.04pH vs Ag/AgCl. The time resolved current at different applied potentials clearly indicated a trap-filling process. The results showed that surface state densities are also highly pH dependent and increase with increase in pH. The total trap state densities of1.53×1015,8.46×1015and2.83×1016cm-2were determined at pH3.0,6.8and13.0respectively with maximum located at-0.22V,-0.52V and-0.81V. The results obtained from CVs are in good agreement with that obtained from the measurements of time resolved currents. The size of the peak potentials related to the trap states (0.23,0.57and0.85V for pH3.0,6.8and13.0respectively) in the cyclic voltammograms increases dramatically with increase of pH, indicating that traps are most surface-related.2. The nanoporous TiO2electrode has a remarkable feature 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, BaSO4was adopted as modified layer, The influence of the thickness of BaSO4layer on band energetics and photoelectrochemical properties of F-doped TiO2electrodes was investigated. The flat band edges (Efb) of BaSO4modified F-doped TiO2electrodes have been determined with spectroelectrochemical technique. Compared with bare F-doped TiO2electrodes, the Efb, of F-doped TiO2electrodes modified with BaSO4layer was little moved. On the other hand the total trap densities were remarkably decreased when F-doped TiO2electrodes were modified with BaSO4layers, which effectively suppress the charge recombination. But an excessive BaSO4coating layer on the F-doped TiO2electrode plays a negative role, the overal conversion efficiency would decrease. The total conversion efficiency of the F-doped TiO2electrode modified with two layers of BaSO4reaches8.57%under irradiation of100mW-cm-2white light, about6.6%higher than that obtained with a bare F-doped TiO2electrode.3. N-F-doped TiO2colloids were synthesized with a hydrothermal method, from which nanostructured N-F-doped TiO2electrode was fabricated. Electrochemical and spectroelectrochemical techniques were applied for the determination of the Efb, and the trap states of pure TiO2and N-F-doped TiO2electrodes to investigate the effects of N and F on the band energetics of TiO2electrodes. Furthermore the pure TiO2and N-F-doped electrode were sensitized with dye N3and their photoelectrochemical properties were studied. The result showed that the Efb, of N-F doped TiO2electrode was little moved. On the other hand the total trap densities were remarkably decreased when TiO2electrode was doped with N and F, which effectively suppress the charge recombination, so the overal conversion efficiency has a great improvement. Under irradiation of100mW·cm-2white light, the photoelectric conversion efficiency of the N3sensitized N-F-doped TiO2 electrode was measured to be8.61%, about17.1%higher than that of a N3sensitized pure TiO2electrode.