Controllable Synthesis And Photovoltaic Property Of ZnO Nanostructures

Nowadays, the sustainable development of energy and environment are the two important strategies for the social development and human culture in the world. With the increase of the energy demands of human and depletion of fossil resource, the development of new energy is attracting more attentions. Solar energy is considered as one of the most promising new energies because of its inexhaustible, environmental friendly and wide distribution. Dye-sensitized solar cells (DSSC) are thought to be favorable for the use of solar energy due to their non-toxic, simple fabrication procedure and low cost, etc. The photoanode film with different nanostructures is a significant part of DSSC, and the conformation and component of the photoanode film have a great impact on the properties of DSSC. Recently, the design and optimization of photoanode film are one of the hottest issues in DSSC field. In this thesis, to solve the key issues of the design and optimization of photoanode film, we chose zinc oxide (ZnO) with micro/nano-structure as the research materials, synthesized ZnO nanograss and hierarchical ZnO nanostructures by hydrothermal method, and explored the underlying growth mechanism of ZnO nanostructures. Then, the effect of ZnO nanostructures on the photovoltaic property was systematically discussed. The main points can be summarized as follows.1. ZnO nanograss films were synthesized by means of hydrothermal method, and the effects of capping agent on the morphologies, sizes and aspect ratios of ZnO nanograss were investigated by FE-SEM, HR-TEM and XRD analyses. The FE-SEM results indicated that the diameter and length of ZnO nanorods increased with increasing growth time. When the concentration of PEI in the growth solution increased from 0 to 7 mM, the higher ratio of c-axial growth rate to lateral one led to an increase in aspect ratio from 34.34 to 93.83. Furthermore, it was found that the shape of ZnO nanorod was hexagonal rod under the adjusting of lower PEI concentration, while it trended to be cylindrical needle at higher PEI concentration. The results of Al3+ as an additive demonstrated that the mean diameters of ZnO nanorods were significantly decreased from 465 nm to 210 nm when adding a lower concentration of Al(NO3)3 (0.25 mM) in the growth solution. There was no obvious changes with further increasing Al(NO3)3 concentration. It was found that the absorbed additives of ZnO crystal plane were the main reason of the changes in the aspect ratio of ZnO nanograss.2. ZnO nanorod-nanosheet (ZnO NR-NS) hierarchical nano-array was for the first time prepared through a two-step trisodium citrate assisted hydrothermal process at a mild temperature. The results showed that the smooth surfaces of ZnO nanorods were surrounded by nanosheet, which was oriented assembled by nanoparticles. The growth temperature played an important role in the formation ZnO NR-NS hierarchical nano-array. Finally, ZnO NR-NS hierarchical nano-arrays on some flexible substrates (zinc foil and stainless steel mesh) were successfully synthesized via a two-step trisodium citrate assisted hydrothermal technique.3. The photovoltaic properties of ZnO nanograss with different aspect ratios, ZnO NR-NS hierarchical nano-array and flexible ZnO NR-NS hierarchical nano-arrays were systematically investigated. The short-circuit current density and photo-to-electric conversion efficiency of ZnO nanograss with high aspect ratio were both enhanced due to the large surface area leading to higher dye loading. The short-circuit current density and conversion efficiency increased from 1.93 mA·cm-2 and 0.47% to 2.90 mA·cm-2 and 0.73% Compared with ZnO nanograss, the photo-to-electric conversion efficiency of ZnO NR-NS hierarchical nano-array was increased by nearly 70%, increasing from 0.66% to 1.13%, owing to higher dye loading and better light scattering in the ZnO NR-NS hierarchical nano-array. The results of flexible ZnO NR-NS hierarchical nano-arrays also confirmed that the hierarchical structure was beneficial to improve photovoltaic property.4. It has been developed a simple approach of trisodium citrate assisted hydrothermal method for fabricating ZnO nanostructures including microrods, hierarchical microspheres and hollow microsphere. The growth mechanism and photovoltaic properties of these ZnO nanostructures were also compared. The results indicated that the growth direction of ZnO crystal would change from [001] to [100] when introducing citrate ion into the growth solution. As the concentration of citrate ion was 1 mM, ZnO hierarchical microsphere with diameter of 2-3 μm was obtained. The growth mechanism of ZnO hierarchical microspheres could be attributed to nanosheets interlaced each other and assembled into microspheres. As the concentration of citrate ion up to 4 mM, ZnO hollow microsphere with diameter of 2-4 μm was obtained. And the mechanism of ZnO hollow microspheres could be owing to oriented assembly followed by an Ostwald ripening process. The superior photovoltaic performance (1.42%) of ZnO hierarchical microspheres compared to those of ZnO microrods or hollow microspheres (0.41% and 0.79%, correspondingly) was attributed to the higher dye loading, better light scattering and relatively longer electron lifetime.5. ZnO/PANi hybrid photoanode was prepared using a dipping technique. The effect of PANi hybridization on the photovoltaic property of ZnO nanograss was investigated. According to the results of FT-IR and Raman spectra, an intense interaction and the chemical-adsorbed PANi structure caused an interface hybrid effect between PANi and ZnO. The photoelectrochemical measurement results showed that the photo-to-electronic conversion efficiency of PANi (100 mg·L-1) hybridized ZnO nanograss photoanode had been increased by 60%, i.e. from 0.40% to 0.64%. It was found that more effective charge separation and faster interfacial charge transferring occurred in the hybrid photoanode, leading to higher conversion efficiency of the hybrid photoanode.