| In spite of the relatively lower conversion efficiency for polycrystalline silicon (polysilicon) solar cells than monocrystalline silicon (monosilicon) solar cells, the fomer dominates the commercial market because of their lower fabrication cost. One of the most intensive reasearch tasks in the photovoltaic community is to further enhance the conversion efficiency of silicon-based solar cells. In general, there are two technical approaches to enhance the conversion efficiency of silicon-based solar cells:the solidification process optimization for polycrystalline silicon ingots, and the process improvement for fabricating solar cells.Three kinds of method have been used for preparing polysilicon ingots:casting method, the direct melt directional solidification method (Bridgman method) and electromagnetic casting (EMC). Direct melting directional solidification method is the mainstream of growth process, and this method can grow silicon ingots with preferable columnar orientation. It is still necessary to advance the solidification process in order to grow larger polysilicon ingots with better quality. Numerical simulations by computer for polycrystalline solidification is an economic and effective method, and many research results have shown that computer simulation work can be reliableto a large extent. This thesis adopts the CGSim software for polysilicon ingot furnace temperature field, temperature gradient and a numerical simulation of the flow field in the silicon melt in different crystal got their highly (heat insulation of the opening of the cage) and long crystal speed distribution, and intends to explain the different location of the polycrystalline silicon ingot microcrystalline provides experiment basis for the mechanism of further optimization for the future provided a reference for polycrystalline silicon ingot production process.High density of crystal defects existing in the directional solidification of polysilicon (dislocations, grain boundaries, microcrystalline), the existence of these defects influence the polycrystalline silicon solar cell conversion efficiency. This experiment we used an optical microscope to observe dislocations in the directional solidification was studied by the optical topography of dislocation characteristics of polycrystalline silicon ingot, before we found some dislocation characteristics are not reported in the literature, also observe the directional solidification of polysilicon in the presence of large amounts of slip dislocation, and this has not been reported before in the literature. We using scanning electronic microscope (SEM) and electron back scattering diffraction test results with electron backscattered diffraction (EBSD) technique to observe the surface morphology of directional solidification polycrystalline samples, and draw some crystallography data (type and number of grain boundary, orientation of intergranular etc); All of these will help us understand why the efficiency of polycrystalline silicon solar cell is lower than monocrystalline silicon solar cells. We also reported the growth of the polycrystalline silicon ingot of directional solidification process of the formation of the crystallite, combined with silicon ingot furnace temperature field, flow field and temperature gradient, and crystal nucleation and growth of related theory to explain the mechanism of formation of the crystallite. And studied the microcrystalline area percentage of effect on the performance of photovoltaic solar cells.Cell preparation technology improvements include:the surface of the wafer velvet, minus reflection film plating and grow various nanostructures on reducing reflector oxide; These can realize minus reflection of light, improve the efficiency of silicon-based solar cells. This experiment using SiH4and NH3as the reaction gas, chemical vapor deposition is used to the preparation of single and double hydrogenated silicon nitride (SiN:H) minus reflection film. By changing the reaction rate ratios of airflow control SiN:H film refractive index n value between1.8to3.3change, change the deposition time to control the double Nx:H film thickness of the top and bottom; We for polycrystalline silicon solar cell double SiN:H minus reflection film is optimized, the optimization results were obtained:the top refractive index n1=1.9, at the bottom of the n2=2.3(lambda=615nm); The thickness of the top and the bottom of the best=60nm d1and d2respectively=23nm. The optimized double SiN:H solar cells than single-layer SiN:H has better photovoltaic performance-higher conversion efficiency eta, the bigger the open circuit voltage Voc and stronger of the short circuit current density Jsc.In order to further decrease reflection of light, various nanostructures grown on SiNx:H is needed to designed and simulated. These have minus reflection effect of nanometer structure which has the advantage that can easily to design of the structure, minimize the reflection of light. ZnO nanostructures as reducing reflector has attracted great attention, ZnO on the substrate can easily grow into nanometer array, the morphology of the nanocrystalline easy control; In this experiment we’re SiN:H subtraction on the reflector with water bath method to grow ZnO nanostructure arrays film to form a gradient minus reflection film to further reduce the light reflection; Using rigorous coupled wave analysis (rigorous coupled wave analysis, referred to as RCWA) for design of this gradient membrane; Water bath temperature, time and solution solubility was studied on the properties of nanometer column growth and its influence; Got ZnO nanometer column diameter, length and the distance between the array and the relationship between the optical reflectivity. The mechanism to improve the performance of the cell is analyzed using the finite difference time domain (finite-difference time-domain, referred to as FDTD) mothod.In order to get the shape of ZnO nanorods and the relationship between the optical reflectivity, we use the water bath method in SiN:H layer was prepared successfully erbium doped ZnO nanoparticle array; To study the growth time of erbium doped ZnO nanoparticle must array, the influence of the microstructure found erbium doped ZnO nanoparticle must array length and diameter increases with growth time. When growth time increased to90and120minutes, erbium-doped ZnO must arrays of nano shall be at the bottom are connected together, the top by needle into a flat. Reducing reflection performance becomes poor.60minutes growth of erbium doped ZnO nanoparticle array must be presented the best minus reflection performance and photovoltaic performance, conversion efficiency increases from15.64%to17.41%. Parabolic morphology of Er-doped ZnO nanostructured arrays coating can be obtained by adding ammonia into reaction solution via hydrothermal method. The reflectance of the solar cells integrated with parabolic nanostructures is only0.1%with broad-wavelength and wide angles, and the photovoltaic conversion efficiency of the cells is relatively increased by20%. The results is explained using the optical model. |
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