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Fabrication of Si-based Thin Films and Their Application in Lithium Ion Battery

Author: QinYanLi
Tutor: HeDeYan
School: Lanzhou University
Course: Condensed Matter Physics
Keywords: Microcrystalline silicon films Inductively coupled plasma Solar cell Lithium ion battery Silicon nanowire Optical property
CLC: TM912
Type: PhD thesis
Year: 2011
Downloads: 148
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Abstract


Due to materials abundant and non-toxic, silicon becomes a hot material in the field of energy exploitation and storage. The tandem solar cells and nanowire-based solar cell are representative in the third generation (high efficiency and low cost) solar cell. Microcrystalline silicon thin film is widely applied in the tandem solar cell as middle or bottom material due to its improved optical stability, higher electron mobility, narrower and modulatable band gap. And silicon nanowires can function as both emitter and antireflection coating owing to the strong broadband optical absorption and a direct path for charge transport by the geometry of such nanostructures. In order to improve the efficiency and reduce the costs of solar cell, the study on the preparation and optoelectronic properties of microcrystalline silicon films and silicon nanowires are important. Meanwhile, as a lithium storage material, the theoretical capacity of silicon is ten times higher than carbon. If it is used as anode material for lithium ion battery, the word time of mobile telephone battery is prolonged to ten times longer than the present one. Therefore, silicon is believed to be one of the most promising anode materials for lithium ion battery.In this thesis, microcrystalline silicon thin films and crystalline-amorphous core-shell silicon nanowires were prepared by ICP-CVD at relatively low temperature. The growth mechanism, microstructure and optoelectronic properties are analyzed. Meanwhile, electron irradiated silicon film and the as-deposited silicon film by VHF-PECVD was as anode to assemble lithium ion battery, respectively, and the performances were investigated. The outline of the thesis is shown as follows:The microcrystalline silicon thin films were deposited by ICP-CVD on low-cost substrates, such as glass and plastic. The low-temperature crystallization of silicon films in ICP-CVD and the effects of deposited condition on the structure and optoelectronic properties were investigated. The results reveal that abundant atomic H is produced in ICP-CVD, and it has important impacts on the microstructure and optoelectronic properties of silicon thin films. The optical bandgap of thin films can be adjusted between 1.55 eV and 2.1 eV, and conductivity is relatively high. These films could be used in the tandem solar cells as middle or bottom materials.Silicon nanowires were prepared by thermal CVD and ICP-CVD, respectively. The participation of plasma successfully lowers the growth temperature and enhances the yield. And nanowires prepared by ICP-CVD consist of crystalline core and amorphous shell, and silicon tips are tapering. In view of the above, a growth meachanism was suggested. An enhanced optical absorption of silicon nanowires was observed comparing with the amorphous silicon films, due to the strong light trapping and the graded refractive index effect.Electron irradiation techniques were used to modify the morphology of amorphous silicon films deposited by ICP-CVD, and the structure abates the volume expansion of silicon films during the insertion and extraction of lithium ion in a certain extent. The specific discharge capacity of the electron beam irradiated silicon film anode still remains 1819 mAh g-1 after 30 cycles, which is about 2.5 times of that of the as-deposited silicon film anode. At the same time, a loose structure with abundant voids was found in the films deposited by VHF-PECVD, which could improve the cycle performances in high current for lithium ion battery.

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