| Nanometer titanium dioxide is a common broadband gap semiconductor material，among its three crystalline phases, the anatase TiO2has a3.2eV forbidden band width,with larger redox potential, higher electron mobility and diffusion coefficient, andshows excellent electronic transmission performance. Compared with other materialssuch as ZnO, SnO2and CdS, titanium dioxide’s better electron transport propertiesmakes it the first choice of the anode material in the dye-sensitized solar cells(DSSC).It’s easy to cause point defects, line defects and layer defects in semiconductoritself and foreign material defects, introducing the defect energy levels in the band gap,and greatly affect its strength, plasticity, the resistivity, change the surface energy state,and have deep influence on the physical and chemical properties of the crystal. Themorphology characteristics, degree of crystallization, integrity, and defects densityvaries because of different experimental methods and processing technology. Each ofthe minute differences may bring great impact on the material properties.TiO2is used as photoelectrode in dye sensitized solar cell (DSSC) due to itsbroadband gap semiconductor characteristics, and its main function is to receive theelectrons stimulated by dye photosensitizer, and then transport to the external circuitthrough conduction band. This requires the crystal defects in titanium as little aspossible, to keep semiconductor in its intrinsic state. If there are defects exist in TiO2itself, trap energy level will be formed between semiconductor conduction band andvalence band. This kind of defect level can capture or release the photoelectron, makesit goes through several transition to enter the semiconductor conduction band, it willdelayed the photoelectron injection time, and increase the odds of photoelectron becompound by oxidation state electrolyte, at the same time. Thereby causing loss to theelectron，and affecting the photoelectric conversion efficiency of the battery.Nano-titanium dioxide films are prepared by three methods as microemulsion,hydrothermal method and commercial P25coating method. The films demonstrategreat difference in there crystallization degree and the concentration of oxygen vacancydefects by phase analysis and surface elements energy state analysis. Low crystallinity andcomplete crystallinity films corresponding to less oxygen vacancy, while crystallization in theintermediate state contains more oxygen vacancies. Atomic rearrangement in crystallization processled to the breakage of Ti-O bond, the more broken bond in crystallization of intermediates led to higher oxygen vacancy. Therefore, the crystallization difference directly led to the oxygen vacancydifference. From the absorption spectrum, the visible light absorption of three types of titaniumdioxide are corresponding to the content of oxygen vacancy, that is the light absorption intensityincrease with the increase of the oxygen vacancy. Particle granularity and microstructure of threefilms are approximate, specific surface area and porosity difference is not enough to cause thechange of energy band of semiconductor. So the major differences of three kinds of films, in terms ofelectron transfer, mainly depends on oxygen vacancy.When applying to DSSC, it is found through photoelectric properties test that thedye utilization and photovoltage are inversely proportional with oxygen vacancy content intitanium dioxide. So it is determined that oxygen vacancy is the cause of battery performancechanges.Combined with electrochemical impedance spectroscopy and intensity modulationspectroscopy to test the dynamics process of electronic transmission and composite. Thefilm with low oxygen vacancy shows a small electronic positive transfer resistance, bigelectronic diffusion coefficient and short transmission time; on the contrary, the reversemigration resistance is big and diffusion coefficient is small, so the compound rate islow, thus cause less electronic loss and high utilization rate. Therefore, oxygen vacancyblocking the positive transfer of electrons, increasing electronic composite losses, reduce theeffective electronic and eventually affect the photoelectric performance. |
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