|Title:||Development of novel front contact silver pastes for crystalline silicon solar cells based on nanomaterials|
Photovoltaic power generation.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Building Services Engineering|
|Pages:||xxvi, 141 leaves : ill. ; 30 cm.|
|Abstract:||Crystalline silicon solar cells have attracted significant attention in the past years due to their rich raw materials, low cost and relatively high photo-electric conversion efficiency. Typically, the structure of a crystalline silicon solar cell fabricated on silicon wafer is mainly a p-n junction. Metal contacts to both the n- and p-type sides of the junction are used to collect electricity. For commercial crystalline silicon solar cells, metal contacts are usually formed by screen-printing a silver paste which is then densified by a firing treatment process at high temperature. There are some problems and shortcomings preventing the screen-printed solar cells towards high-efficiency: (i) Due to the significant shrinkage of silver paste during firing treatment, it is difficult to achieve silver electrodes with high aspect ratio. This means that there would be high shading losses caused by a large percentage of front-side metal coverage. (ii) The inferior conductivity of the fired silver paste, which is several times lower than that of pure silver, largely limits the optimization of the metal contacts. In fact, these problems are mainly due to the formulation of the silver paste consisting of a suspension of fine particles of silver and glass frit in organics, which tends to result in non-homogeneous metallization contacts and low conductivity. Therefore, it is necessary to research and develop novel silver pastes based on new components and technologies to overcome the shortages and strive for a breakthrough to get better achievements in the development of high efficiency crystalline silicon solar cells. The objective of this thesis is to improve the photovoltaic performances of crystalline silicon solar cells by introducing novel front-side silver pastes based on nanomaterials. For better understanding and development of high performance front-side metallization silver pastes for solar cells, all the functional components (i.e. silver particles and glass frit) were prepared in this project. A series of silver pastes based on the as-developed materials were prepared and systematically studied. Lead-free nano-glass frit powders (NGFPs) were developed for the first time in this project to replace the conventional micron glass frit which tends to result in non-continuous contact formation in Si-Ag alloys. The NGFPs with glass transition temperature (Tg) of 376°C were prepared by sol-gel method from a multicomponent gel in the Bi₂O₃-SiO₂-B₂O₃-Al2O₃-ZnO system. The X-ray diffraction (XRD) pattern demonstrated that the xerogel was completely converted into amorphous glass after heat treatment at 500°C. To determine the wetting behavior and etching effect of the NGFPs, the test glass paste was prepared and deposited on silicon wafers with SiNx antireflective coating, followed by firing process. Finally, it was found that the NGFPs showed excellent wetting behavior and etching effect on silicon wafers and SiNx antireflective coating layer. The silver paste containing the as-developed NGFPs was prepared by mixing the NGFPs, silver particles and organic vehicle with the ratio of 4/80/16 (wt%) using a rotary evaporator and a three-roll mixer. In the I-V measurement, the fabricated solar cell based on the NGFPs showed higher conversion efficiency (15.7%) than that (14.6%) resulting from the reference sample (solar cell containing conventional micron glass frit). Besides, spherical silver nanoparticles (SSNPs)-aided silver pastes were developed using the SSNPs as a sintering aid. High-dispersive SSNPs were prepared by the solvothemal method. The silver pastes composed of micron silver particles (70 wt%), the SSNPs with an average size of 50nm (10 wt%), lead free glass frit (4 wt%) and organic vehicle (16 wt%) were screen-printed on single crystalline silicon wafers with SiNx antireflective coating, followed by firing treatment and tests. The experimental results demonstrated that the SSNPs with high surface energy are favor to the sintering of micron silver particles. Moreover, the size and numbers of pores in the silver thick films were reduced due to the SSNPs’s unique sintering behavior, contributing to better conductive network. In the I-V measurement, the fabricated solar cell based on the SSNPs-aided silver paste exhibited higher conversion efficiency (16.1%) than that (15.4%) resulting from the reference sample (solar cell containing no SSNPs).|
Moreover, silver nanowires (SNWs)-aided silver pastes were also developed by adding the SNWs into micron silver particles. The SNWs were prepared by reducing AgNO₃ with ethylene glycol in the presence of Pt seeds and polyvinylpyrrolidone (PVP). Pt nanoparticles as crystal seeds play a crucial effect on the anisotropic growth of silver nanowires. The silver pastes composed of micron silver particles (70 wt%), SNWs (100nm to 200nm wide and 0.5μm to 4μm long, 10 wt%), lead free glass frit (4 wt%) and organic vehicle (16 wt%) were screen-printed on single crystalline silicon wafers with SiNx antireflective coating, followed by firing treatment and tests. The fabricated solar cells based on the SNWs-aided paste generated an open-circuit voltage (Voc) of 621mV, a short-circuit current density (Jsc) of 34.3mA/cm², a fill factor (FF) of 74.1% and a conversion efficiency (Eff) of 15.8%. The solar cell performance improvement could be attributed to unique electronic property and “bridge-effect of the one-dimensional (1-D) metal nanostructures. In addition, to further improve the photovoltaic performances of crystalline silicon solar cells, the agglomeration of silver particles in the front-side paste should be resolved. In this thesis, high-dispersive spherical micron silver particles were developed in hot ethylene glycol. Experimental results demonstrated that the polyol process tends to obtain silver particles with better dispersibility compared with the aqueous reduction system. The silver pastes composed of high-dispersive silver particles with an average size of 0.5μm (80 wt%), lead free glass frit (4 wt%) and organic vehicle (16 wt%) were screen-printed on single crystalline silicon wafers with SiNx antireflective coating, followed by firing and tests. The fabricated solar cell based on the as-developed high-dispersive silver particles generated a conversion efficiency of 16.4%. In summary, the novel developments and their processes of the developed silver pastes reported in this thesis demonstrate that the optimum formulation for better conductivity and continuous contact formation of the silver front-side paste for crystalline silicon solar cells could be made by addition of silver nanoparticles and adoption of nano-glass frit and high-dispersive silver particles. Moreover, the results provide a basic understanding and development of synthesis of silver particles with various shapes in different sizes and nano-glass frit powders, which have also great potential for applications in conductive inks, adhesives and pastes for electronic devices.
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