Fabrication And Characterization Of Photoanode Materials Used In Inorganic-sensitized TiO₂ Solar Cells

TiO₂ nanocrystalline solar cell is one of hotspots in the field of materials for photo-electric energy conversion and nanotechnology. Abroad, the research group of M. Gratzel has made a rapid progress in the pilotscale experiment of dye-sensitized TiO₂ nanocrystalline solar cells (DSSCs). Many researchers are optimistic in the industrialization of DSSCs. Even now, several problems, such as dye degradation, high price of dye, and stability of cells, still lie in practical applications. Based on the development of DSSCs in the world, this dissertation presents a new type of CuInS₂ nanocrystalline sensitized TiO₂ solar cell. Combined with mature technologies of preparing thin films and nanoparticals, the main studies of this dissertation are listed as following:(1)Compact anatase TiO₂ nanacrystalline thin films and M_xO_y-TiO₂(M=In³⁺, Al³⁺, Zn²⁺, Zr⁴⁺, Cu²⁺) composite thin films were prepared and In₂O₃-TiO₂ composite thin film was maily studied based on the mature technology of sol-gel. In the case of M_xO_y-TiO₂ composite thin films, with the increment of M ions, the mean grain size of anatase TiO₂ decreases. In₂O₃ crystals are found to exist in the as-prepared In₂O₃-TiO₂ composite films, which resuls in the increase of optical indirect bandgap (E_gi), conductivity, concentration and mobility of carriers. In the other cases of Al³⁺, Zn²⁺, Zr⁴⁺, Cu²⁺-incorporated TiO₂ films, these ions are considered to be in existence of amorphous oxides. E_gi of the as-prepared Al³⁺, Zr⁴⁺ incorporated TiO₂ film increases. On the contrary, E_gi of Cu²⁺-incorporated TiO₂ film decreases. And the presence of Zn²⁺ can't influence E_gi obviously.(2)Stable blue-white transparent TiO₂ sols were fabricated at 120℃and short-carbon chain organic alkali played a role of a structural template. Large quantities of layered Ti-compound were proved to be in the as-prepared TiO₂ sols. A novel anatase TiO₂ nanosquares array and regular nanorods were repectively synthesized at different autoclaved temperatures for different hours. The results reveal that the as-prepared anatase nanosquares with the edge length of～13nm are mono-dispersed, uniform, and quasi-singe crystalline. Aspect ratio of the as-fabricated anatase nanorods is 3～4. Compared with the nanosquares, the nanorods grow up slightly in radial direction and elongate into rods along b-axial direction due to the selectively adsorption of template molecules. With the increase of carbon chain of the organic alkali templates, the as-prepared nanoparticles are inclined to the formation of rod-like nanoparticles.(3)TiO₂ nanoparticles with the mean grain size of 8.8～16.6nm and M_xO_y-TiO₂(M=In³⁺, Al³⁺, Zn²⁺, Zr⁴⁺) composite nanopowders were fabricated at pH=1.5 by peptizing and hydrothermal treatment. In the case of M_xO_y-TiO₂ composite nanopowders, anatase TiO₂ is the main crystal phase, and M_xO_y exists as a minor phase. The growth of anatase TiO₂ nanocrystals is suppressed due to the presence of M_xO_y. It is noted that In₂TiO₅ is formed in the In₂O₃-TiO₂ composite nanopowders. Generally, In₂TiO₅ occurs in the systems which must be heat-treated at 1000℃. In the case of Zr-incorporated TiO₂ composite powders, ZrTiO₄ is formed.(4)Wet-chemical methods were employed to fabricate CuInS₂ (CIS) and CdS light -absorbed layers. The as-fabricated CIS thin films with the thickness of 100～200nm have high absorption coefficient (α) of 10⁵cm^-1, and narrow direct bandgaps (E_gi) of 1.28～1.62eV. Keeping Cu/In ratio constant in the precursor solutions, the absorption coefficient increases and E_gi decreases with the increment of S; In addition, the absorption coefficient increases and E_gi decreases with the addition of In³⁺ when keeping S content constant. Cubic CdS crystal with the mean grain size of～24nm is the main phase in the CdS thin films fabricated by CBD method. And the optical bandgap of the as-prepared CdS thin film is 2.37～2.38eV.(5)Solar cells were assembled according to "sandwich" structure using the materials refered above and characterized with open-circuit voltage (V_oc) and short-circuit current density (J_sc). The testing results show that natural dye-sensitized P25 nanocrystailine solar cell with the thickness of 10μm has the max energy conversion efficiency (η). In the case of CIS semiconductor-sensitized P25 solar cells, the photovoltaic efficiency of cells with thinner sensitizer is a little higher than that of cells with thicker sensitizer when P25 films are kept constant. Among all of the as-fabricated solar cells, CIS-sensitized solar cell has the minimum Voc and Jsc, but the filling factor (FF) of it is the biggest. M_xO_y-TiO₂ composite nanopowders were also used to fabricate photoanodes of DSSC by "doctor blade" approach. The solar cell based on In₂O₃-TiO₂ composite anode has bad photovoltaic properties because of the formation of In₂TiO₅. The solar cells with amorphous Al₂O₃-TiO₂ or ZrO₂-TiO₂ composite anode have relatively higher open-circuit voltage. In addition, The solar cell with ZnO-TiO₂ composite anode has relatively higher short-circuit current density.