Preparation And Property Of Dye-sensitized Solar Cells Based On TiO₂ Photoanodes

Nowadays, with the rapid development of world economic and the exhaustion of traditional energy sources, the energy crisis and environmental situation have become urgent for the world. Developing renewable clean energy is an effective way to solve these problems. Solar energy is one kind of renewable energy that will never be exhausted, which is also a clean energy producing no pollution. One of the main methods to utilize solar energy is solar photovoltaic power generation. The essential problem of developing solar nenrgy is how to fabricate the efficient solar cells. As a typical representative of the emerging third generation photovoltaic cells, dye-sensitized solar cell (DSSC) is completely different from traditional photovoltaic cells, which displays several advantages, such as simple preparation, high power conversion efficiency, low cost, no pollution etc. However, the current research and development situation of DSSCs is far from prosperity, therefore a lot of efforts must be continued in-depth until sufficing for industrialized production. TiO2 photoanode is the key factor of DSSCs, which determines the photoelectric conversion efficiency of the cells. By developing variety of preparation technologies, doping and modification on TiO2 photoanodes to improve the effective ultilzation of solar energy, thus further improving the photoelectric conversion efficiency of DSSCs is of great importance. It is definitely that the research topic on solar cell is significantly fundamental importance and potentially practical significance. The main contents are summarized as following:(1) The CdS QDs sensitized anatase TiO2 hierarchical nanostructures were prepared via a facile and controlled hydrothermal process. To adjust the size of CdS QDs, the TiO2 surfaces were first treated with thiolactic acid which provides a binding site for Cd2+ ions. CdS QDs with controllable size are grown onto the TiO2 hierarchical nanostructures. The CdS QDs/TiO2 hierarchical heterojunction nanocomposites displayed a red shift from ultraviolet to visible light region with longer wavelength for the absorption edge. DSSCs based on CdS QDs/TiO2 hierarchical heterojunction nanocomposite with a smaller average size of 7.6 nm for CdS QDs, showed a maximum short circuit current density of 6.09 mA cm-2 and a conversion efficiency of 3.06%, superior to the DSSCs based on the TiO2 hierarchical nanostructures. This improved performance was attributed to the unique microstructure characteristics, and the bandgap energy matching between the CdS QDs and TiO2 played an important role in effective separation of light induced electrons and holes.(2) Using hexadecylamine as the structure-directing agent and hydrazine hydrate as the nitrogen precursor, nitrogen doped anatase TiO2/CuxO core/shell mesoporous hybrids were synthesized through a facile and controlled combined sol-gel and hydrothermal process. DSSCs based on the mesoporous TiO2/CuxO core/shell hybrids show a high short-circuit current density of 9.60 mA cm-2 and a conversion efficiency of 3.86% under one sun illumination. The matching of band edges between CuxO and TiO2, resulting in the formation of a semiconductor heterojunction, played an important role in effective separation of light induced electrons and holes, high electron injection efficiency, and fast electron transference. In comparison with un-doped TiO2/CuxO hybrids, while DSSCs based on the N-doped mesoporous TiO2/CuxO hybrids exhibit the higher short-circuit current density of 13.24 mA cm-2 and a conversion efficiency of 4.57% under one sun illumination. The doping of nitrogen into lattice of TiO2 can extend the light absorption in ultraviolet range to visible light region and effectively decrease the recombination rate of photo-generated electron and holes.(3) A novel type of SrTiO3/TiO2 nanosheet heterostructures was obtained via a facile hydrothermal process, with a tunable microstructure, phase component and surface area by adjusting the molar ratio of Sr and Ti precursors. The synthesized SrTiO3/TiO2 heterostructure nanostructures can provide a high surface area and porous structure for improving dye loading capacity and hence the amount of photogenerated charges contributing to the photocurrent. The formation of heterostructures between SrTiO3 and TiO2 with a uniquely matched bad gap energy structure can efficiently separate photogenerated charge carriers. Photoluminescence emission and electrochemical impedance spectroscopy (EIS), incident photon-to-electron conversion efficiency (IPCE) measurements reveal a lower recombination rate of photogenerated electrons and holes, and a longer electron lifetime for the DSSCs based on the SrTiO3/TiO2 heterostructures. The photoelectric conversion efficiency and short-circuit current density of DSSCs based on SrTiO3/TiO2 heterostructures are greatly enhanced over that of pure TiO2 nanosheets. DSSCs based on the SrTiO3/TiO2 heterostructures shows a highest short-circuit current density of 12.55 mA cm-2 and a maximal photoelectric conversion efficiency of 7.42% under one sun illumination.(4) A novel nanostructure of Au/TiO2 hollow sphere composite was achieved through a facile sol-gel and one-step hydrothermal method, with Au nanoparticles uniformly embedded in TiO2 hollow spheres as the dye carrier to assemble dye-sensitized solar cells (DSSCs) for the enhanced harvest of solar energy. Enhanced conversion efficiency was expected relying on such materials to absorb the incident light as much as possible, which might be ascribed to a large dye capacity and localized surface plasmon resonance (LSPR) between TiO2 and Au nanoparticles. To prepare the composite, TiO2 spheres with a uniform diameter of 500 nm were first prepared by a sol-gel method, and then followed by a respective chemical etching and reduction process to form a hollow structure and give a uniform Au nanoparticle distribution. TEM and SEM proved the hollow feature and XRD confirmed the presence of TiO2 and Au, with the latter homogenously embedded in the body of the former as indicated by EDS measurement. Such sophisticated architecture exhibited a mesoporous structure with a large surface area of～150 m2 g-1, which was beneficial for a large loading capacity of dye molecular. More importantly, the diffuse reflection spectra (DRS) manifested a boosted light absorbance upon the modification of Au nanoparticles as expected. We assembled DSSCs using attained Au/TiO2 hollow sphere composite with different mass ratios and their performances were tested. In accordance with DRS results, the DSSCs based on the composites exhibited evidently enhanced harvest of incident photon as indicated by incident photon conversion efficiency (IPCE), and hence showed higher conversion efficiency than pure TiO2 hollow spheres. Especially, the highest conversion efficiency of 7.57%, a considerable improvement by ca.39%, was obtained at the mass ratio (Au/TiO2) of 5%. In addition, it should be noted that PL and EIS measurements revealed a prolonged lifetime of photogenerated electrons induced by the charge separation between TiO2 and Au, which was also important for improving efficiency. These results highlighted the effects of Au modification in improving the conversion efficiency of DSSCs and proved that proper architecture and constitution design were critical for preparing optimized solar cells.(5) A CdS@Au@TiO2 IOs film was fabricated through the oxidation reduction method with successive ionic layer adsorption and reaction. The TiO2 gel was immersed into photonic crystals template to prepare the 3D TiO2 inverse opal structure film, adding P123 further increases the specific surface area of the film. DSSCs based on all kinds of photoanodes were investigated. DSSCs based on TiO2 IOs photoanode exhibit the short-circuit current density of 9.18 mA cm-2 and a conversion efficiency of 4.89%, while for DSSCs based on the Au@TiO2 IOs and CdS@Au@TiO2 IOs photoanodes, the short-circuit current density are 10.63 and 11.78 mA cm-2, the conversion efficiency are 5.57% and 6.51%, respectively. The enhanced performances are attributed to the unique nanostructures and high specific surface area of P123 modified Au@TiO2 IOs, plasma surface enhancement effect of Au nanoparticles and incorporation of narrow band gap CdS can make light absorption to broaden, electron hole pairs effectively suppress.