The Electrochemical And Photoelectrochemical Properties Of Nanostructured Semiconducted Electrodes

With the socio-economic development and the improvement of living level, people are in the pursuit of a more comfortable living environment, which would consume more and more energy. With worsening of the worldwide energy supply, energy will become a major factor constraining the development of national economies. As a clean and renewable energy sources, solar energy becomes more and more important. Among the efficient use of solar energy, solar energy photovoltaic utilization is the fastest growing and most dynamic fields of study in recent yeas. Among various solar cells, 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 and redox electrolyte. Nanostructured semiconductor electrode and redox electrolyte have important effect on the photoelectrochemical properties of DSSCs and become the hot topics in this field. In this paper, we focus on these two aspects and explored the ways to improve photoelectric conversion efficiency of solar cells.1. To improve photoelectric conversion efficiency of DSSCs with surface modification. The nanoporous TiO2 electrodes possess large surface to volume ratio, and as a result, the dye molecules adsorbed on nanoporous film are tremendously increased, leading to improved efficiency of solar cells. 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, MgTiO3 and CaTiO3 were adopted as modified layer, which conduction band stand higher than that of TiO2, so the MgTiO3 and CaTiO3 layer can be beneficial to the improvement of nanoporous TiO2 electrodes. The research results showed that the highest conversion efficiency based on N3 sensitized CaTiO3 modified nanoporous TiO2 electrodes reaches 9.23% under irradiation of 100 mW/cm2 white light, obtained with the electrode TiO2/CaTiO3(45min), a 34% increase than that of bare TiO2 electrodes. The conversion efficiency of N3 sensitized MgTiO3 modified nanoporous TiO2 electrodes was increased from 6.12% to 8.75% under the illumination of a white light of 100 mW/cm2.2. The electrolyte plays an important role in transporting electrons and holes in DSSCs. Redox species, organic solvents, additives and the kind of metal ions have important effect on the properties of solar cells. It is reported that small metal ions, such as Li+ and Zn2+ ions, can be inserted in TiO2 lattices and combine with electrons in conduction band to form dipole, which facilitates recombination between the dipole and I3- in the electrolyte and influence the photoelectrochemical properties of TiO2 nanoporous electrodes. In this paper a new Schiff base iodide complex (N, N'-Bis (salicylidene) ethylenediamine zinc (Ⅱ) iodide) was synthesized and for the first time applied as the electrolyte of DSSCs. The research results showed that the formation of this metal complex cation minimized the negative effects of the insertion of small metal ions in TiO2 lattices and improved the photoelectrochemical properties of TiO2 nanoporous electrodes. The best results were obtained with optimized electrolyte, and the short-circuit photocurrent density, open-circuit photovoltage, fill factor and the photoelectric conversion efficiency are 18.34 mA·cm-2,0.64 V,0.66 and 7.75% respectively, demonstrating that (N, N'-Bis (salicylidene) ethylenediamine zinc (Ⅱ) iodide) is a very efficient redox electrolyte.3. Studies were undertaken on band energetics of nanostructured SrTiO3 film electrodes. The nanostructured semiconductor electrode is the core of DSSCs and the carrier of charge separation and transportation. SrTiO3 with a perovskite structure shares more structural and band gap similarities with anatase TiO2 and is applied in DSSC with great potential, but there has rarely been report on thorough investigation of the band energetics of nanostructured SrTiO3 films so far. In this paper, SrTiO3 nanoparticles with narrow size distribution were synthesized and used to fabricate transparent nanostructured SrTiO3 film electrodes. 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 SrTiO3 is correlated linearly with pH and has the relation of Efb=-0.59-0.04pH vs. Ag/AgCl. The time resolved current at different applied potentials clearly indicated a trap-filling process. The results showed that trap state densities are also highly pH dependent and increase with increase in pH. Furthermore the correlation between the band energetics of nanostructured SrTiO3 electrodes and the photoelectrochemistry of the N3 sensitized electrodes were also investigated. The flat band edges of nanostructured SrTiO3 electrodes have been determined to be-1.48,-0.80 and-0.80 V in three electrolytes of 0.2 mol·L-1 tetrabutylammonium perchloride (TBAP) and 0.5 mol·L-1 LiClO4 with propionitrile (PN), acetylacetone (Acac) and PN/Acac as solvents. The total trap state densities were calculated to be 4.19×1016,2.38×1016 and 1.19×1016 cm-2 in three electrolytes of 0.2 mol·L-1 TBAP and 0.5 mol·L-1 LiClO4 with PN, Acac and PN/Acac as solvents. The N3 sensitized SrTiO3 electrodes showed the highest open-circuit voltage and short-circuit current density in Acac, in good agreement with the electrochemical measurements that the nanostructured SrTiO3 electrode has the flat band edge well matching with the excited state of N3 and the smallest trap state densities in Acac.