【作者】 刘志刚；

【导师】 崔容强； 孙铁囤； 李国欣；

【作者基本信息】 上海交通大学 ， 光学工程， 2006， 博士

【摘要】 能源是人类社会向前发展的基础,环境是人类可持续发展的必要条件。传统能源日益紧张,人们不断探求新的洁净能源。各种新能源都有其优劣,光伏发电作为新能源的一个分支,越来越引起人们的关注。最近几年,太阳能电池的产量以前所未有的速度增长,晶体硅电池依然占据市场的主流,其中多晶硅占据市场的53%。在晶体硅电池制造工艺中,酸液腐蚀技术越来越多的应用于多晶硅太阳电池工业化生产。但酸液的自催化性使其在应用过程中出现可控性差、重复性差等问题,人们不得不用一些辅助设备以保证其大规模生产,增加了生产成本。本论文首次提出了用弱碱氨水（NH₃·H₂O）控制氢氟酸（HF）/硝酸（HNO₃）/水（H₂O）体系腐蚀硅片速度的新思路。用正交试验的方法研究了新型腐蚀液（HF/HNO₃/ NH₃·H₂O /H₂O体系）中各成份对腐蚀速度的影响,氨水的加入并不影响HF、HNO₃、H₂O在硅片腐蚀过程中所起的作用。重点讨论了氨水对腐蚀速度的影响。氨水含量增大的过程中,腐蚀速度先增大后减小。定量计算了新型腐蚀液的成份,并将Pitzer理论应用于离子活度系数的计算。结合化学反应速度理论可以看出,氨水增大过程中HNO₃浓度和活度系数的减小是腐蚀速度降低的主要原因。通过比较腐蚀过程中铵离子的消耗量、硅片的消耗量和硝酸根的消耗量可以看出,硅片腐蚀过程中,硝酸铵（NH₄NO₃）不断分解产生一氧化二氮（N₂O）气体,这是本文得出的最重要的结论。建立了三相体系模型,大量N₂O气体附着在硅片表面,起到气泡掩蔽作用,增加了硅腐蚀的动力学阻。而且,大量的N₂O气体增加了HF的传质阻,降低了腐蚀速度。另外,用气体搅拌模型讨论了N₂O在升起的过程中起到的搅拌作用。气体搅拌降低了HF的扩散阻力、HF的浓度梯度及扩散层的厚度,增加了腐蚀速度。氨水体积增加的过程中,HNO₃的浓度及活度系数、N₂O气体的阻止作用及HF的扩散性质共同影响了硅片的腐蚀速度。研究了硅片腐蚀速度随时间的变化规律。硅片在酸液中的腐蚀速度与反应放热、体系与外界的交换热、溶液中的氟离子浓度有关。硝酸铵在腐蚀过程中的分解是造成HF/HNO₃/H₂O体系和HF/HNO₃/ NH3·H₂O /H₂O体系对硅片腐蚀情况不同的根本原因。将新型腐蚀液应用于多晶硅太阳能电池表面织构工艺。NH₄NO₃不断分解产生的N₂O气体有利于腐蚀坑结构的形成。得到的多晶硅表面均一,晶向依赖性小。与其它方法制备的多晶硅表面相比,虽然300-1200nm波段范围内的反射率不是最低,但制备的电池性能最好。从电压、电流和填充因子等方面分析了电池性能不同的原因。新型腐蚀液在应用过程中稳定、可控性好、重复性好,不需要温控设备和循环设备,降低了生产成本。介绍了背腐蚀法分离p-n结的原理,将新型腐蚀液应用于晶体硅太阳能电池背腐蚀技术。其对腐蚀速度的抑制作用,可有效消除腐蚀过程中对正面n⁺层的破坏。设计了一种碱液腐蚀法,对电池背场进行检测。与等离子刻蚀方法比较,背腐蚀背场均一,长波的光谱响应较高,电池输出电压明显提高。背腐蚀法得到电池的背反射器平坦,长波反射后的光程减小,反射率降低,电流降低。从综合效果上来看,背腐蚀法得到的电池性能较等离子刻蚀法明显改善了。最后,本文提出了一些后续工作的建议,希望新型腐蚀液早日应用于晶体硅太阳能电池的工业化生产。更多还原

【Abstract】 Energy is the base for human society development. And environment is the necessary condition of continuable development. People have to search new clean energy as common energies are being used up. All kinds of new energies have their advantages and disadvantages. Photovoltaic as one kind of new energies are being cared by more and more people. The yield of solar cells increased greatly in recent years. Crystalline silicon solar cells are market mainstream all the same and the multicrystalline silicon cell occupies fifty-three percent of all the markets. Acid etching technology is more and more used in multicrystalline silicon solar cell industry. However, the auto-catalyzing character led to uncontrolled and un-repeating problems when common acid solutions are used. Then some assistant apparatuses have to be used to maintain large-scale production, which enhances the product cost. New idea that ammonia （NH3?H₂O） being added into hydrofluoric acid （HF）/nitric acid （HNO₃）/water （H₂O） system to control silicon etched rate was brought forward in this article for the first time. Orthogonal test method was used to find the influences of all components in the new etching solution （HF/HNO₃/NH3·H₂O/H₂O system） on the etching rate. It was found that ammonia was irrespective to the influences of other reagents on etching rate. Etching rate influenced by ammonia volume was discussed emphatically. It was found that etching rate increased at first and then decreased with increasing NH3·H₂O volume. Components in the new solution were calculated, and Pitzer theory was used to calculated ions active coefficients. Besides, Chemical reactive rate theory was also used. It could be seen that reductions of HNO₃ concentration and active coefficients were the main reason for the reduction of etching rate with increasing ammonia volume. Consumptions of ammonium ions, silicon and nitrate ions during etching were tested. It could be concluded that ammonium nitrate （NH₄NO₃） decomposed continually to produce nitrogen monoxide （N₂O） when silicon was etched. It was the most important conclusion in this article. Three-phase model was established. A large mount of N₂O gas covered on silicon surface to increase reactive resistivity. Moreover, N₂O gas in diffusion layer increased HF transfer resistivity. So, etching rate decreased. Air bubble disturbing model was used to discuss the disturbing effect caused by rising of N₂O. Disturbing effect reduced HF diffusion resistivity, concentration grads and diffusion layer thickness which led to the increasing of etching rate. Concentration and active coefficient of HNO₃, preventing effect of N₂O, diffusion character of HF all influenced etching rate of silicon with increasing ammonia volume. Silicon etched with etching time was also studied. Variation of etching rate was interactions between reaction heat, the heat exchanging with outside environment and fluorine in the solutions. That NH₄NO₃ decomposing accompanied with silicon etching was the most important reason leading to the difference between HF/HNO₃/H₂O system and HF/HNO₃/NH3?H₂O/H₂O system.The new etching solution was used to texturing multicrystalline silicon surface. N₂O gas decomposed by NH₄NO₃ was benefit to forming etching pits. Surface of multicrystalline was homogeneous and the crystalline direction dependence was very little. When compared with other method, the reflectivity between 300nm and 1200nm of new solution etched surface was not lowest while the solar cell was the best. The difference was analyzed from several factors such as, voltage, current, fill factor, etc.. New etching solution was stable, controllable and repeatable. It needed not temperature controlled and circular apparatuses which lowered production cost.Principle of p-n junction separated by back etching method was introduced. The new etching solution was used to the back etching technology of crystalline silicon solar cell. The low etching rate of new solution prevent front surface being etched. A new back field tested method by use of alkaline solution was designed for the first time. Al-back surface field of the solar cells were studied. When compared with plasma etching method, the back field of back etching was much more homogeneous. The internal quantum efficiency （IQE） of long wave light was higher, and so voltage was higher. The back reflector of back etching solar cell was flat which led the low reflectivity of long wave light and so the current was not so high. As a whole, the solar cell prepared by back etching was better than edge etching.At last, some other experiments to be done were brought forward. We hoped that the new etching solution could be used in crystalline solar cell production line as early as possible.更多还原