Electrodeposition Of ZnO Thin Films And Their Applications In Quantum Dot-sensitized Solar Cells

Energy has always been an important driving force for the development of human society. We have entered the era of high energy consumption since the middle of twentieth century, and the existing energy has been unable to meet the needs of economic and social development. The energy crisis has become the biggest challenge. Developing high efficiency, low cost, sustainable solar cells is not only an effective way to solve the energy crisis, but also a significant step to protect the ecological environment. Quantum dots sensitized solar cell?QDSC?, which was developed from Dye sensitized silar cells?DSSCs? has great potential and have attracted much attention due to its high theoretical efficiency, low cost and stable ability.Nano-semiconductor oxide film is an key component of QDSC when used for collecting electrons.Currently TiO₂ is the best material for photoanode of QDSC. With the development of nano materials science, more and more materials and nano structures have been used in QDSC.Among them, Zn O nano materials are favored because of their excellent electron mobility and easy control of nano morphology.In this paper, we first electrodeposited ZnO nanosheets, and then synthesized Zn O nanosheet-nanorod arrays and nanospheres based on it. The ZnO nanosheets were also used as template for preparing composite film which contains SnO₂ nanosheets and TiO₂ nanoparticles, and then the composite film and ZnO nanosheet-nanorod arrays were sensitized with CdS and CdSe respectively to explore the effects on QDSC performance when they were used as photoanodes.The main research contents and results are listed as follows:?1? ZnO nanorods were grown on the electrodeposited ZnO nanosheets and the effect of precursor concentration on the structure of ZnO nanorods was investigated. It was observed from SEM that he process of the protrusion on the ZnO nanosheets were gradually transformed into the nanocone, and finallychanged into the nanorods with the increase of precursor concentration.The length of ZnO nanorods were also optimized by changing the hydrothermal reaction time. The diameter of the nanorods was too large when the hydrothermal time was too long,and a large number of nanorods were crowded together so that the array of ordered nanorods was destroyed. The optimal hydrothermal reaction time was ultimately determined to be 10 hours.?2? By adjusting the concentration of precursor solution to 0.15 M, a kind of nanosphere composed of Zn O nanorods was also prepared.The growth process of nanospheres was investigated by controlling the hydrothermal reaction time.It wasfound that the excessive amount of hexamethylenetetramine?HMTA?decomposition occurs in the hydrothermal process, resulting in a strong alkaline environment, which led to the corrosion of the ZnO nanosheets, and the remodeling for morphology of the Zn O thin films.The influence of electrodeposition time on the formation of ZnO nanosphere was also investigated.The experimental results shown that the process of the growth of the nanorods could not be blocked, or nanorods can be free to grow in all directions to form the nanospheres composed of ZnO nanorods. It requires growing ZnO nanorods on the substrate surface with relatively large fluctuation.?3? In the end,the as-prepared ZnO nanosheet-nanorod arrays film was also treated with?NH₄?₂TiF₆,sensitized with CdS and CdSe, and then used as photoanodes of QDSC. The final results shown that the shortcircuit current?Jsc? of photoanode based on Zn O nanosheet-nanorod arrays had been greatly improved compared with the photoanode based on the simple ZnO nanosheet. The photoelectric conversion efficiency of 2.5% was finally obtained. It can be seen from absorption spectra of films and IPCE spectra of QDSC that the absorption and IPCE based on ZnO nanosheet-nanorod arrays was much higher than those of other photoanodes in the wavelength range of 400nm-500 nm, which indicated that the structure of ZnO nanosheet- nanorods was favorable for the deposition of CdS using successive ionic layer adsorption andreaction?SILAR? method.?4? The ZnO nanosheet films were prepared by electrodeposition and used as template for preparing SnO₂ nanosheets. It was observed that the SnO₂ nanosheets are verticaland contain plenty of nano-voids.After treated with TiCl₄ and deposited TiO₂,it can be seen from the SEM for cross section of SnO₂/TiO₂ composite film that TiO₂ nanoparticles were full filled in the intervals between SnO₂ nanosheets, and the SnO₂ nanosheets almost could not be found.The whole film exhibited a porous structure with a large specific surface area, which was conducive to the deposition of quantum dots. The element distribution diagrams also show thatTiO₂ was homogeneously filled in the SnO₂ frameworks. The composite film was used as phaotoanode of QDSC after co-snsitized with CdS and Cd Se, and the results indicated that the photoelectron lifetime of the composite thin films is comparable to that of pure TiO₂ films, but the former have a higher IPCE and the total photoelectric conversion efficiency is 3.49%.