【作者】 王德生；

【导师】 贺德衍；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2012， 博士

【摘要】 伴随着经济社会的迅速发展,能源作为最基本的驱动力显得更为重要。传统化石燃料的大量开发和使用带来了严峻的环境污染和生态破坏。能源危机和环境恶化已然成为人类社会必须面对的巨大挑战。开发新型可再生能源以及实现能源的高效储存成为世界各国关注的焦点。由于太阳能储量丰富,清洁无污染,太阳能的利用成为能源发展战略的重中之重,太阳光伏发电成为太阳能利用最重要的途径。另一方面,二次电池特别是锂离子电池在电能的存储上发挥着巨大作用。开发新型高容量、大功率、长寿命的锂离子电池成为当今储能电池研究的热点。薄膜Si和纳米结构Si材料是新一代薄膜光伏电池的主导材料,Si基纳米材料也是高能量密度、高功率密度锂离子电池阳极材料的研究热点。本论文中,我们采用等离子体增强化学气相沉积(PECVD)方法在低温下制备了晶化硅薄膜和Si、Si1-xGex纳米棒阵列结构。研究了生长条件对样品结构、形貌和光学、电学性质的影响；采用Si基纳米棒结构作为锂离子电池阳极材料,研究了其电化学性能,通过调控Si1-xGex中的Ge含量进一步改善了SiGe电极的循环稳定性能。论文的主要内容及所获得的主要研究结果总结如下：1.晶化硅薄膜的制备及其结构、性质的研究分别采用射频(13.56MHz)和甚高频(VHF,81.36MHz)等离子体增强化学气相沉积(PECVD)技术制备了晶化硅薄膜。研究了传统射频PECVD过程中Ar稀释气的加入对薄膜结构和性质的影响。结果表明,Ar的加入可以有效提高薄膜的生长速率和晶化率,这主要归功于Ar*可以促进SiHH4的分解以及提供非晶硅晶化需要的激活能。另一方面,Ar稀释气的加入,会导致薄膜中SiHH2聚合集团成分的增多,产生较多的缺陷态,使光学带隙加宽和光吸收系数减小。相对传统的射频PECVD技术,采用VHF-PECVD技术可以在H2稀释率较低的情况下快速制备出高致密度、低缺陷态密度和较高光学吸收系数的晶化硅薄膜。2.Si纳米棒阵列的制备及其光学减反性能研究结合电化学沉积和VHF-PECVD技术,我们在电化学沉积的Ni纳米锥阵列衬底上制备了Si纳米棒阵列结构。通过对Ni纳米锥衬底形貌的调控,可以得到不同结构的Si纳米棒阵列。采用紫外-可见-近红外分光光度计对Si纳米棒阵列的光学反射性能测试表明,得到的直径约为200nm,高度700nm的Si纳米棒阵列在400-1100nm的波段反射率低于5%,这主要归因于纳米棒阵列结构所具有的增强的光学散射和有效调制的光折射率。这种纳米棒阵列结构有望在光电器件中得到应用。3. Si、Ge和Si1-xGex纳米结构的电化学性能研究采用VHF-PECVD技术对Ni纳米锥结构实施包覆,我们制备了Si和Si1-xGex纳米棒结构,并将其作为锂离子电池阳极材料,组装了标准的CR-2032纽扣式锂离子二次电池,测试了其电化学性能。研究表明,Si纳米棒电极具有较高的充放电容量,在C/5电流密度下的可逆循环容量达到3400mAh/g,经过30次循环之后仍高达3249mAh/g,保持率高于95%,相对于Si薄膜电极有明显的提高。然而,随着循环次数的进一步增加,Si纳米棒的容量迅速衰减。相比Si纳米棒,Ge纳米棒呈现出较好的循环性能。在C/2的电流密度下经过50次循环之后容量仍保持在约460mAh/g,保持率达100%。综合Si和Ge的优势,我们制备了Si1-xGex纳米结构,研究了不同Ge含量的Si1-xGex纳米棒的电化学性能。实验发现,随着Ge含量的增加,Si1-xGex的首次质量比容量明显减小,这主要是因为Ge的理论质量比容量低于Si的比容量。但是,Si1-xGex的循环稳定性随着Ge含量的增大逐步提高。对于Ge含量为29%的Si0.71Ge0.29电极,在0.5A/g的电流密度下,经过65次循环之后容量仍超过1000mAh/g。在4A/g的大电流充放电条件下,可逆容量达到1500mAh/g,经过50次循环之后,容量仍高达1100mAh/g。结果表明,Si1-xGex合金可以作为高容量的锂离子电池负极材料。通过调控Ge的含量,可以进一步改善其循环稳定性能。更多还原

【Abstract】 Accompanied by rapid economic and social development, energy as the basic driving force for modern society shows of great importance. The massive consume of the traditional fossil fuels has posed serious problems of both environmental pollution and ecological destruction. Energy and environmental issues have been the two important problems that human society must pay attention to and have to solve. Development of new energy sources and energy storage become the focus for the world. Because of its abundant, clean and non-polluting, the develop of solar cells is a top priority of the strategy. On the other hand, electricity can not be directly stored. In the development of storage batteries, new high-capacity, high power and long-life lithium-ion batteries become today’s focus. Si thin films and Si nano-structured materials have become the dominant materials of the next-generation thin-film photovoltaic cells. At the same time, Si nano-materials are research focus for the high energy density, high-power lithium-ion battery anodes.In this thesis, the crystallized silicon films and Si, Si1-xGex nanorod arrays were prepared at low temperature by plasma enhanced chemical vapor deposition (PECVD) method. The influences of growth conditions on microstructure, morphology and the opto-electronics properties of the samples were investigated carefully. The electrochemical properties of the Si based nanorod arrays as anode were evaluated by galvanostatic battery tastings for Li ion batteries. It was showed that the electrochemical performance of Si1-xGex anodes can be improved by regulating the Ge content in the Si1-xGex. The main contents of the paper and the corresponding results are summarized as follows:1. The microstructures and properties of the prepared crystallized silicon filmsThe crystallized silicon films were deposited on glass substrates by the conventional radio frequency (13.56MHz) and very high frequency (81.36MHz) PECVD method. Silane diluted in H2or H2+Ar was used as the source gas for preparation of crystallized Si films by conventional PECVD. The results showed that the addition of Ar in dilution gases efficiently improves the deposition rate and crystallinity due to an enhanced dissociation of the source gas as well as the energy of deexcitation of Ar*released within the growth zone. Meanwhile, excess Ar dilution will lead to the polymerization and also a bad passivation of the hydrogen on the dangling bonds which causes the increase of the optical gap and defect states in the μc-Si films. The crystallized Si films can be deposited under a lower H2dilution by the VHF-PECVD method. It is found that the crystallized Si films showed better optical properties due to the better compact and the lower defect density by using very high excitation frequency of81.36MHz.2. The preparation and antireflection of the Si nanorod arraysSi nanorod arrays were prepared by the VHF-PECVD on the electrodeposited Ni nanocone arrays. By adjusting the preparation conditions of the Ni nanocone arrays, the morphology and the density of the Si nanorods can be controlled. It is found that the well-separated nc-Si:H nanorods provide excellent low reflectance over a wide wavelength range of300-1100nm due to enhanced light scattering and appropriate modulation of the effective refractive index between air and nc-Si:H nanorod structures. The novel nc-Si:H nanorod arrays may be suitable for low-cost solar cells devices and other applications thanks to the excellent antireflection.3. The electrochemical performance of Si, Ge and Si1-xGex nanorod structuresSi, Ge and Si1-xGex nanorod structures were prepared by the VHF-PECVD on the electrodeposited Ni nanocone arrays. The obtained Si, Ge and Si1-xGex samples were directly assembled against Li metal into half cells of CR-2032with Si, Ge and Si1-xGex acted as active materials, respectively. The electrochemical performance was evaluated by the galvanostatic battery testing. It is found that the Si nanorod anode shows a high reversible capacity of3400mAh/g at C/5rate. A capacity of3249mAh/g at C/5rate is attained with retention of95%after30cycles. The result is a great improvement compared to the Si film anode. But the capacity of Si nanorod shows a drastical decrease after30cycles. Compared to Si anode, Ge anode shows a stable capacity even at C/2rate for50cycles with a capacity retention of100%, but the capacity is only460mAh/g. Herein, Si1-xGex nanorod electrodes were applied for Li-ion batteries. It is found that the capacity retention is greatly improved with increasing Ge content of the Si1-xGex electrode. For Sio.71Geo.29nanorod electrodes, The charge capacity is above1000mAh/g even after65cycles at0.5A/g. The reversible charge capacity is about1500mAh/g even at high rate of4A/g. The capacity of Sio.71Geo.29is still as high as1100mAh/g after50cycles. The binder-free Si1-xGex nanorod electrodes could be further improved for a promising candidate for high-capacity and high-power Li-ion batteries.更多还原