Synthesis Of ZnO And PbS Nanostructures Via Solution Approaches And Their PV Device Applications

In recent years, semiconductor nanowires and quantum dots are currently receiving widespread attention due to their excellent properties superior to their bulk materials counterparts, and gained rapid development in the field of electronic components and photoelectric conversion devices. At present, methods commonly used in semiconductors synthesis processes are gases-phase and solution phase approaches. But gases-phase approaches generally require highly sophisticated equipment, and often face limitations in terms of sample uniformity and low product yield. The solution phase approaches are relatively simple with lower temperature compared to gases-phase ones, but need some transition steps of poor reproducible. In addition, some chemicals that used in the synthesis process are unstable, toxic and expensive. In this dissertation, we have obtained the results as listed as following:Firstly, we prepared ZnO nanorod arrays using a hydrothermal reaction with zinc nitrate hexahydrate and hexamethylene tetramine as precursors at90℃. The ZnO nanorod samples have a single crystal structure of wurtzite and the arrays are of good verticality and uniform distribution.The diameter of ZnO nanowire grown in process with precursor concentration of0.025M and growth time of2hours is50nm, with an aspect ratio of5:1. Experimental results revealed that with the increase of the reactant concentration and growth time, the diameter of the ZnO nanorods have obviously increased as well as the average length and the aspect ratio. But, when reaction time exceeds a certain range (approximately5hours in our experiment), the growth rate of nanorods slowed down because of the continuous consumption of reactant and eventually stopped growing. Thickness of the seed layer also played an important role in the arrays growth process due to the corrosive effect of the growth solution. Only when the seed layer was thicker than a critical value of100nm, can nanoarrays of high quality be prepared. Besides,large ratio of Zn2+/HMTA can reduce the growth rate of the nanorods, but Zn2+/HMTA had little effect on the growth process when the value is close to1.Secondly, we synthesized PbS QDs using S powder as S source instead of the expensive and dangerous chemical of hexamethyldisilylthiane (TMS) in the temperature range of85℃to170℃. By adjusting the synthetic process, oleylamine and oleic acid capped PbS QDs with size distribution between3nm and20nm were successfully synthesized. Experimental results showed that, in higher reaction temperatures, the oleylamine capped quantum dots became larger ones because of an increasing reaction speed corresponding to a higher precursor reaction activity, due to the increasing temperature. Besides, as a result of the combined influence of the temperature and capping agents, the (111) plane had a higher growth rate relative to other crystal planes. Accordingly, as the temperature rose, quantum dots morphology transformed from spherical to polyhedron, and ultimately into regular cubic structure.Finally, we fabricated PVdevices using ZnO nanoarrays and PbS QDs synthesized in former sections. ZnO nano-array film about200nm thick was synthesized with process described in the previous section. PbS quantum dots with average diameter of5nm and15nm were deposited on the array using a spin coating method to form films of different thicknesses for photovoltaic devices fabricating. Device test results showed that, only when smaller (5nm) quantum dots films completely cover the ZnO arrays could efficient photovoltaic structures be formed, because thinner QDs films would cause a direct contact between the nanowire and Ag electrode. Short-circuit will occur in the direct contact structures. Moreover, quantum dots with larger particle size will destroy the energy band structure of the device, both of which cannot produce the photovoltaic effect.