Controllable Fabrication Of Silicon Nanowire Arrays For Application To Novel Heterojunction Solar Cells

Solar energy is a renewable cleaning power, which is expected to be one of the major alternative energy in the21st century. Photovoltaic cells are the key device to convent solar energy to electric one. In the conventional commercial application, the most comment photovoltaic devices are monocrystalline or polycrystalline silicon solar cells, whose development has been obstructed due to high generation cost, limited theoretical conversion efficiency, fragile and so on. Silicon nanowire(SiNW) arrays have been used to design and contrust silicon-based solar cells due to excellent optical, optoelectronic, mechanical flexibility, which provides a novel idea for fabricating silicon-based solar cells with various advantages of low cost, high efficiency and flexible function. In view of these points, this paper used wetting etching technique to fabricate SiNW arrays, explored their formation mechanism. Furthoremore, we adjusted the size, surface qualities and morphologies of SiNW arrays and optimized their antireflective and light-obsorption properties. Fanally, we fabricated the SiNW/PEDOT:PSS heterojunction solar cells and further explore their flexible function. The main research work is as follows:Silicon surface was mordified by UV-Ozone pretreatment, which improves deposition of uniform silver nanoparticles catalysts on silicon surface. The silver-assisted etching technique are used to fabricate uniform SiNW arrays on4inch wafer. Based on these, control of size, filling ratio and surface qualities of SiNW arrays are systematically investigated. Furthermore, tapered SiNW arrays were fabricated by using the gradual disolution characteristic of silver nanoparticles in the etching process. Based on the mask of spontaneously precipitated K2SiF6crystal in etching, a novel template-free method was proposed to fabricate silicon micropillar/nanowire composite arrays.Catalytic etching characteristic of noble metal with different morphologies, including nanoparticles, continous film and the film with nanoholes, was studied. The results indicated that the morphology of the film with nanoholes is effective to generate the SiNW arrays. Meanwhile, it was experimentally vertified that silver nanoparticles was oxidized and dissolved to silver ions during the process of catalytic etching, revealing the formation mechanism of tapered SiNW arrays. According to the study on the effect of oxidizing agents on the morphologies of SiNW arrays, the formation mechanisms of the porous SiNW, the polished silicon wafer and the silicon micropillar/nanowire composite arrays were clarified. Furthermore, from the study on the movement direction of silver nanoparticles into the silicon wafer, it is proposed that growth direction of the SiNW arrays can be tuned by doping alcohol solution into etchant or chosing the silico wafer with different crystal orientions.Effect of length, filling ratio, doping types and doping concentration of SiNW arrays on their antireflective properties was inverstigated. Porous nanowire and tapered nanowire exhibit the excellent antireflective properties in the wide range of spectrum and angle. The ability to reduce the reflectance in near-infrared range can be improved by the mordification of noble metal nanoparticles on SiNWs. Furthermore, the light-absorption properities of SiNW arrays without silicon substrate were characterized by transferring the film of SiNW arrays on glass. At the wavelength of550nm, light absorption rate of SiNW arrays is nearly up to95%. Light absorption of porous SiNWs is better than that of smooth SiNW arrays.The SiNW/PEDOT:PSS heterojunction solar cells were fabricated by infilling the PEDOT:PSS solution into SiNW arrays. Effect of upper electrodes, thickness of PEDOT:PSS films, structural parameters of SiNW arrays on the photovoltaic performance of these devices was investigated systematically. According to the optimization, the conversion efficiency of the optimal device was up to6.1%. The corresponding conversion efficiency was increased to9.7%through the comprehensive optimization of process parameters.The key fabrication process of single-crystalline Si micro-nanostructures film was studied. One was that chemical etching or electric field-assisted chemical etching were proposed to induce the lateral etching of silver nanoparticles on the interface between the root of SiNW arrays and the silicon substrate, which was beneficial to improve the transferred quality of the SiNW arrays film. Another is that ultra-thin silicon wafer having surface roughness of13nm, thickness below30μm was fabricated by a self-developed noble metal nanoparticles polised etching technique. Various morphologies of SiNW arrays and some patterns (i.e.,―NCEPU‖) were machined on these ultra-thin silicon wafers. Furthermore, the flexible device prototype of SiNW/PEDOT:PSS heterojunction solar cells was fabricated and it shows rectifying characteristics.