Study On The Dye-sensitized Nanocrystalline TiO₂ Solar Cells

The conversion of sunlight to electrical power is an irreversible trend in the field of energy generation. Photovoltaic is one of the fastest growing industries at present. In the last five years, the production of photovoltaic cells has increased steadily at an average rate of 40% per year in the world, which exceeds the development of IT industry. However, how to reduce its cost and improve its energy conversion efficiency is the most important issue in PV industry.The dye-sensitized nanocrystalline solar cell is one of hot-spot in the field of materials for photoelectric conversion and nano technology. Based on low-cost, simple technique and stable character, it can offer a better method to use solar energy in cheaper and more convenient way. However, how to further improve its stability and energy conversion efficiency is also a major problem for its application prospects. Based on the development of the dye-sensitized nanocrystalline solar cells and the matured technology of preparing nanoparticles in our country, this dissertation deals with the dye-sensitized nanocrystalline solar cells through reducing cost by preparing nanocrystalline films using home-made nanoparticles and synthesizing a novel metal-phthalocyanine derivatives with good photo-response, improving charge transport properties of nanocrystalline films by the method of magnetron sputtering deposition, and improving the stability with solid-state medium replacing the liquid electrolyte.The main contents and results of this dissertation are listed as following: 1. Based on the trend in solar cells and PV status in our country, a high-surface-area nanocrystalline TiO₂ films were successfully prepared from two kinds of easy-reaggregation primary nanoparticles with the mean size of 26 nm in ethanol solution by a novel technique of quickly volatilizing solvent to fix the nanoparticles. Structure and properties of the films were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) andX-ray photoelectron spectroscopy (XPS). The results show that the mean size of nanoparticles doesn't change with heat-treatment at 450°C and the roughness factor about 86 and 80 was obtained for 1 ^m thickness films. A roughness factor of 82 was found for the commercial P25 TiO2 films with the mean size of 36 nm and the same thickness. The electrochemical properties of the interfaces of the TiC"2 I film electrodes in propylene carbonate (PC) containing 0.05 M tetrabutyl ammonium bromide (TBAB) was also measured and show that the electroactive of the T1O2 I film is similar to P25. Moreover, this method is adapted to prepare nanostructural films using other materials and smaller primary nanoparticles for dye-sensitized nanocrystalline solar cells;2. To reduce the electron recombination rate on nanocrystalline films, a novel hybrid Ti(>2 electrode containing a sputter deposited layer and a nanocrystalline layer was fabricated on a conducting glass by DC magnetron sputtering and doctor blade technique, respectively. The sputter deposited layer (20 40nm) fabricated at 100 250°C is found to be outstanding in improving the performance of the dye-sensitized solar cells. In the case of P25, a high solar-to-electric energy conversion efficiency of 8.1%, larger short-circuit photocurrent of 20.8 mA-cm"2 and open-circuit voltage of 682 mV were obtained under irradiation of white light (98mW-cm"2). Comparing with the pure nanocrystalline electrodes, this hybrid electrode has doubled the short-circuit current density and raised the open-circuit voltage over 30%. At the same time, two kinds of cheap nanometer TiC>2 powders hydrolyzing from titanyl nitrate were used in the hybrid nanocrystalline TiC?2 electrode and fabricated dye-sensitized nanocrystalline solar cells. These suggest that this hybrid electrode provides a potential in improving performance of the dye-sensitized nanocrystalline solar cells;3. We synthesized a functional metal-phthalocyanine derivative VOPc. It can dissolve easily in DMF and has large photo-response in a broad wavelength from 650nm to 700nm. When coated in nanocrystalline titanium dioxide electrode, its photo-response is extended. We fabricated the solar cells after sensitizing nanocrystalline electrode by VOPc. The photo-electric conversion efficiency about0.16% (98mW-cm"2) and the largest monochromatic current yeild about 10% were obtained;4. We attempt to use Cul, a common p-type semiconductor material, to fabricate solid-state dye-sensitized solar cells by the method of magnetron sputtering deposition. The results show that the component of y-CuI can be controlled in this method. A high effective transmittance and lager conductivity were obtained. The conductivity of Cul has been improved about three magnitude orders than the films prepared by solution. At the same time, we obtained conversion efficiency about 0.14% under the irradiation of white light (65.2 mW-cm"2);5. An all solid-state dye-sensitized nanocrystalline solar cells have been successfully fabricated with the PEO/PVDF polymer redox electrolyte hybrid with titanium dioxide nanoparticles. The dependences of the glass transition temperature, crystallinity, morphology and conductivity of the blend polymer redox electrolyte on mixture ratio of polymer and hybrid were characterized by differential scanning calorimetry (DSC), scanning electron microscope (SEM) and electrical conductivity measurements. The results show that the introduction of PVDF containing fluorine element 'F' having strong negativity and the titanium dioxide nanoparticles in the PEO electrolyte reduces the crystallinity of the blend polymer, increases the ionic conductivity and effectually overcomes the recombination rate at the interface of TiO2 / solid-state electrolyte. Moreover, the blend polymer redox electrolyte PEO/PVDF with the ratio of 4 to 6 hybridized with TiO2 nanoparticles showed the largest ionic conductivity and more outstanding overall energy conversion efficiency (about 4.8 %, 65.2 mW-cm'2) than the plain polymer redox electrolyte in the dye-sensitized nanocrystalline solar cells. Base on this method, it is hopeful to carry out a practicality device.