Preparation Of Titania Photoelectrodes With Controlable Band Gap And Their Photoelectrical Properties

In this thesis, we choose porous nano-TiO₂ as substrate, ammonia as Nitrogen source, autoclave as experimental equipment. In autoclave, we prepared TiO_xN_y nanoparticles with good morphology, uniform size utilizing the reduction properties of ammonia and by changing temperature, pressure and reaction time. Also, we studied the relationships among the doping content, the band gap, the nanoparticles size and the photoelectric conversion efficiency of the N-doped TiO₂ photoelectrodes. By scanning electron microscopy the surface morphology and the cross-section, fresh appearance and structure of nitrogen doped TiO₂ nano-materials were observed. The optical band gap was calculated by the absorption edge of the ultraviolet visible spectrum. Moreover, the optical activity and dynamics of electric charge were investigated by the surface photovoltage spectrum and the transient photovoltage spectrum. Experimental results indicate that an efficient way was discovered to control the composition of N-doped TiO₂ powders using NH₃ as gaseous precursor in autoclave. For instance, the concentration of N-doped TiO₂ was increased with sintering time(12, 24h) or the decrease in reaction temperature(600³00℃) or pressure(0.8⁰.2MPa). And the band gap of semiconductor was narrowed by increasing the concentration of N-doped TiO₂. The study about the photoelectrical properties of N-doped TiO₂ showed that the overall conversion efficiency of solar cell was greatly improved with different concentration. The results indicate that it is helpful for broadening the visible light response range and making it continuous after N dopant. The combination speed between injected carriers and holes becomes slow and the lifetime of charge gets long, so that the photoelectric performance in DSSCs was affected.When we adopt the gas–solid preparation method for the TiO_xN_y semiconductor material, one-dimensional highly oriented TiO_xN_y nanowire arrays were obtained from TiO₂ film. The preferable orientation of nanowires was crystal facet [103]. The groped work provides a facile and promising process for the synthesis of fine and dense NWs with a preferable growth conditions. It was discussed about the oriented growth direction, structure, shape and distribution of microscopic structure of information in samples by transmission electron microscope. The results show that the pressure of NH₃ had a big impact on the length and density of NWs via enhancing the longitudinal growth rate. Therefore, the length of the nanowires can be well controlled by varying the NH₃ pressure in the working environments. The results indicated that nanowire arrays could be fabricated with controllable lengths by adjusting the content of NH₃ under medium pressures, and TiO_xN_y nanowires led to remarkable V_oc values in comparison with TiO₂ nanoparticles.A co-sensitization method was discovered for increasing solar energy conversion efficiency in order to make full use of the sun light with full spectrum while DSSCs only show response to ultraviolet absorption of light and medium wave. The d¹⁰ family of transition metal complexes with N719 dyes was sensitized for co-sensitization TiO₂ anode in solar cells. According to the great surface characteristics of nano-TiO₂ in porous electrode, three transition metal complexes M1(Zn1, Cd1, Hg1) are assembled onto a nanocrystalline TiO₂ film to prepare transition metal complex/N719 co-sensitized photoelectrodes for dye-sensitized solar cell application. Therefore, co-sensitized solar cells based on TiO₂/M1/N719 electrode yield a remarkably high photocurrent density (J_sc), open circuit voltage (V_oc) and energy conversion efficiency under the standard conditions of global AM1.5 solar irradiation due to the different absorption spectral range of dyes, which are relatively higher than that for DSSCs using single organic sensitizers. The redox property was studied by the cyclic voltammetry method. The electrochemical processes of dyes and internal resistance in double composite electrode are simulated by AC impedance equivalent circuit and the proper mathematical physical model. The results show as follows: (1) The compounds performed a reversible process of electrochemical reaction in platinum electrode. All isopolyanions underwent one-step two-electron electrochemical reaction assigned to the transition metal reduction process. It is found that the optimal test condition of the cyclic voltammogram is 1.0 mM M1 in CH₂Cl₂ containing 0.1 M TBAPF₆ solutions with the scan rate of 200mV/s at room temperature. (2) Electrochemical impedance spectroscopy indicates that the arc in the low frequency range of the Nyquist plot with the increase of protons. A physical model was proposed to understand the complex charge-transfer mechanism in DSSCs. Parameters obtained by fitting the impedance spectra of composite solar cells prove that the equivalent circuit is R_s(Q₁R₁)(Q₂(R₂Z_w)). Combining with the electrochemical properties, we discussed the relationships between molecular structures and electrochemical properties. It can be concluded that the electron-donating ability supports the stable single molecule structure and spacious space among molecules. Therefore, the decrease of the bond lengths and deviation distance are advantageous to its electrochemical property, while the decrease of the dihedral angles and hydrogen bonding effect are helpful for obtaining superior cell performance.