Fabrication Of Ordered Micro/Nanostructures By Nanosphere Lithography And Their Photoelectrical Applications

The worldwide demand for energy is constantly increasing with the population growth, economicdevelopment and social progress. The development of clean and renewable energy sources are highlydesired in order to resolve the crisis of energy shortage and environmental pollution caused by theextensive use of traditional fossil energy. Among the various technologies nowadays, the solar cell isbelieved to be one of the most promising ways in achieving such goals. However, the energy proportion ofphotovoltaic cells is still relatively low in the component of global energy production. The lowerconversion efficiency and the higher fabrication costs are still the main bottlenecks for its large-scaleapplication. Fortunately, thin film solar cells provide us the possibilities to develop low-cost photovoltaicdevices. Since there is a tradeoff between the absorption and the collection of photo-generated carriers inthe thin film solar cells due to their film structure, the conversion efficiency of thin film solar cells is stillfar behind that of the crystalline silicon solar cells and cannot meet the standards for the real commercialapplication. Integration of ordered micro/nanostructure materials in thin film solar cells has beenconsidered as one of the most promising ways to solve the contradiction between light absorption and thecollection of carriers. Generally, the fabrication of ordered micro/nanostructures needs to rely on theexpensive micro-fabrication equipments, which is not appreciated for the large-scale application in thinfilm solar cells. In contrast, nanosphere lithography is widely used to construct ordered micro/nanostructures with various shape, size, and material due to its low cost, versativle, high-throughput, andlarge-scale feathers.In this paper, patterned nanostructures for several different materials were fabricated by usingnanosphere lithography. Among them, the ordered ZnO cavity-like nanostructure was used to fabricate theZnO/Cu₂O heterojunctional thin film solar cells. Compared with its plannar counterparts, the improvedshort-circuit current density （Jsc） and photoelectric conversion efficiency can be observed for the highlyordered nanoheterojunction structured devices. The better performance can be attributed to the increasedheterojunction area. In addition, ordered silicon nanowire arrays with controllable diameter and densitywere also fabricated by nanosphere lithography combined with the metal-assisted chemical etching technology. The influencing factors such as the doping type of the silicon wafer, metal catalyst, and theetching time on the formation of silicon nanowire arrays were also investigated. Mainly, three parts ofresearch works are carried out in this paper.（1） The polystyrene （PS） microspheres synthesis, the template fabrication, and the building ofordered micro/nanostructures using colloidal crystal template: The612nm diameter polystyrenenanospheres with good monodisperse were prepared using the emulsion polymerization method. Thelarge area close-packed monolayer template of PS nanospheres was transferred to the substrates usingthe self-assembled characteristic of PS nanospheres. Next, the non-close-packed PS sphere templatecan be fabricated by the reactive ion beam etching （RIE） processes. Based on the different PS spherecolloidal crystal templates, ordered gold, silver, SnO₂, and Ag₂S nanostructures were obtained. Inaddition, the ordered gold nanodot structure was also fabricated using the Ag₂S nanomesh as thesecondary template.（2） The fabrication of ordered ZnO cavity-like nanostructures and its application in Cu₂O/ZnOheterojunction thin film solar cell: The large-scale ordered ZnO cavity-like nanostructures werefabricated through the nanosphere lithography （NSL） technique combined with the hydrothermalgrowth. Nanostructured Cu₂O/ZnO heterojunction solar cells were fabricated by the electrodepositionof Cu₂O into the ordered ZnO cavity-like nanopatterns. Compared to its plannar counterpart, theimproved short-circuit current density （Jsc）, and photoelectric conversion efficiency （PCE） can beobtained for the ordered nanoheterojunction structure. The better performance can be attributed to theincreased heterojunction area.（3） The preparation of ordered silicon nanowire arrays with controllable diameter, density, andlength by NSL combined with metal-assisted chemical etching: Ordered arrays of silicon nanowirewith different diameter，density and length were prepared by nanosphere lithography combined withreactive ion beam etching （RIE） technology and metal-assisted chemical etching. The influencingfactors such as the doping type of the silicon wafer, metal catalyst, PS sphere templates and the etching time on the formation of silicon nanowire arrays were also investigated, which provided the basis forthe applications of silicon nanowires based solar cells and other optoelectronic devices.