Dye Sensitized Solar Cells Based On ZnO/TiO₂ Photo-anode:Architecture Design And Reaction Mechanism Study

The environmental problems and energy crisis have become increasingly serious. How to solve them efficiently has aroused widely attentions. As a rich and clean energy, solar energy is considered to be one of the most promising ways to solve these problems. So far, the most widely applied solar cells are silicon-based solar cells. Although the silicon-based solar cells have high photoelectric conversion efficiency, it is complicated and expensive in fabrication process. Compared to silicon-based solar cells, dye sensitized solar cells (DSSCs) are promising due to their relatively low manufacturing cost, high photoelectric conversion efficiency, and good durability. In order to further improve the DSSCs photoelectric conversion efficiency, scientists from all over the world have made great efforts.The structure of photoelectrodes has great influences on the photoelectric conversion efficiency of DSSCs. The development of nanotechnology opens door for design of various nanostructures. Nanostructures will endow materials new properties, and thus will improve the photoelectric conversion efficiency of DSSCs effectively. Photoelectric conversion efficiency is highly related to the specific surface area, electronic transmission performance and light scattering properties of photoelectrodes. Reasonable design of electrode structure will improve the photoelectric conversion performance. This thesis aims to develop simple and versatile methods to realize photoelectrodes with a variety of structures in order to enhance the efficiency of DSSCs. The electrodes with different structures based on ZnO and TiO2 compound semiconductors, such as ultra-long nanowires, core-shell structures, and composite structures, were synthesized on FTO conductive substrate. The research is on the controllable process of synthesis, reaction mechanism and related advantages as photoelectrodes. This work provides an experimental and theoretic basis for design of novel photoelectrodes structures. Moreover, our work successfully demonstrated the importance of photoelectrodes structures on the photoelectric conversion efficiency of DSSCs.The main contents are included as follow:(1) Preparation for long ZnO nanowire arrays (NAs) and their applications for DSSCsLong ZnO NAs grown on fluorine-doped tin oxide (FTO) conductive substrates have been synthesized via a facile hydrothermal method. By adjusting growth conditions, ZnO NAs with tunable lengths can be achieved. The morphology of NAs can be affected by the concentration of ammonia, and refreshing the reaction solution. It is worthy to be noticed that, the long NAs with length of 24 μm been got by refreshing the reaction solution one time. It is found that NAs length play important roles in the enhancement of photoelectric conversion efficiency of DSSCs. With the length increasing, the photoelectric conversion efficiency improved due to the increasing surface area of photoelectrodes.(2)Synthesis of ZnO@TiO2 core-shell long nanowire arrays and their applications for DSSCsBased on the above research, long ZnO NAs grown on FTO conductive substrates applied as the precursors. A nanocomposite made of ZnO nanowires core and TiO2 shell was further realized by a novel "fast-dip-coating" method conducted in the Ti(OC4H9)4-dissolved ethanol solutions. The core-shell structures successfully resolve issues existed in ZnO-based cell like the instability in N719 system. The formed ZnO@TiO2 core-shell NAs on FTO substrates were applied as electrodes for DSSCs. It is found that both the TiO2 coating and NAs length play important roles in the enhancement of photoelectric conversion efficiency of DSSCs. When the length of ZnO@TiO2 NAs reaches up to 14 μm, the electrode can exhibit a maximum PCE as high as 4.12%, which is 2.6 times higher than that of pure ZnO NAs.(3)A ZnO/TiO2 composite nanorods photo-anode with improved performance for DSSCsWe designed a double layers composite photoelectrode, which is consisting of two parts:one is the bottom layer based on TiO2 NRs, and the other is the top layer made from disorder ZnO NRs. We obtained TiO2 NRs on FTO via a chemical bath deposition (CBD) method. Then we treated the sample with thermal decomposition process. In the thermal decomposition process, we found that ZnO NPs not only occupied the surface of the TiO2 NRs, but also the space between NRs. DSSCs based on ZnO/TiO2 composite nanorods (NRs) photoanodes were assembled, and the power conversion efficiency (PCE) of these ZnO/TiO2 composite NRs film solar cells is 4.36%, which is obviously higher than that of DSSCs based on pure TiO2 NRs (0.6%) and ZnO NRs (3.10%). The enhanced performance of ZnO/TiO2 composite NRs film DSSCs can be attributed to the combined effects of ZnO and TiO2 NRs. In this architecture, the thick top-layer ZnO NRs offers a large surface area for enough dye absorption. The thin bottom-layer TiO2 NRs not only offers a direct and quick pathway for photo-injected electrons transfer along the photoanode, but also acts as a blocking layer which effectively blocks the direct contact between the substrates and the electrolytes, which results in lower carrier recombination.(4)A novel hierarchical ZnO disordered/ordered bilayer nanostructured film for DSSCsBased on the ZnO/TiO2 composite nanorods photoanodes, we made improved on the material. A novel hierarchical ZnO nanostructured film is synthesized on a FTO substrate. This hierarchical film is composed of disordered ZnO NPs (top layer) and ordered ZnO nanowires (bottom layer). The products possess the following features such as high specific surface area, fast electron transport, and pronounced light-scattering effect, which are quite suitable for applications on DSSCs. A light-to-electricity conversion efficiency of 5.16% is achieved when the hierarchical ZnO nanostructured film is used as the photoanode. This efficiency is found to be much higher than that of the DSSCs with pure ordered ZnO NWs (1.45%) and disordered ZnO NRs (3.31%) photoanodes. (5) Building smart TiO2 nanorod networks in/on the film of P25 nanoparticles for high-efficiency DSSCsWe herein present a useful and novel strategy to redesign the photoanodes by building smart TiO2 nanorod networks in and on the film of P25 nanoparticles (NPs) to optimize the comprehensive performance of DSSCs. By using a doctor-blade method followed by a facile hydrothermal treatment, an interesting hierarchical double-layered film consisting of P25 NPs and TiO2 NRs was on FTO substrates. In our strategy, the P25 NPs (bottom-layer) with large surface area can potentially enable large amount of the dye absorption, while TiO2 NRs (top-layer) as the scatting part would effectively strengthen the light harvesting ability. Moreover, TiO2 NRs inserted into P25 NPs film also provide conducting networks for fast photogenerated electrons transport. The research demonstrated that this hierarchical double-layered photoanode indeed exhibits superior DSSCs performances to that of pure P25 NPs film; the photovoltaic conversion efficiency increases up to 8.62% under illumination of one sun (AM 1.5 G,100 mW cm-2), which is quite higher than 6.12% for pure P25 NPs photoanode. This work highlights the significance of the rational design of photoanode electrodes for enhanced energy conversion applications.