【作者】 姜婷婷；

【导师】 杨德仁； 余学功；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2014， 博士

【摘要】 光伏发电是近年来发展最快的可再生能源技术之一,而晶体硅太阳电池占据了全球光伏市场90%以上的份额。硅材料尤其是多晶硅材料内的金属杂质和缺陷对于硅片质量及太阳电池的性能影响较大,是学术界和产业界关注的重点和热点问题之一。因此,研究硅中金属杂质与缺陷具有重要的理论意义和实际应用背景,也是适应硅太阳电池低成本、高效率的发展趋势的必然要求。本文在综述前人研究成果的基础上,利用少子寿命测试仪、电流-电压／电容-电压测试仪、光学显微镜等技术,结合铸造多晶硅生产实际中的问题,系统地研究了金属杂质与位错的相互作用、金属杂质与晶界的相互作用及金属杂质对于对硅片和太阳电池电学性能的影响,取得了如下主要的创新成果：(1)研究了铸造多晶硅边缘区域(红边区)低少数载流子寿命的起因及对太阳电池性能的影响。实验发现：来自坩埚与坩埚涂层中间隙铁杂质的扩散是导致边缘区域硅晶体少子寿命降低的主要原因；通过磷扩散吸杂,可以有效减少边缘区域的间隙铁浓度,从而提高该区域的有效载流子寿命；经历太阳电池制备工艺后,含边缘区域的太阳电池片的性能可以得到大幅度改善,仅仅比普通电池片效率略低。(2)研究了金属杂质对硅中晶界电学性能的影响。阐述了Fe、Cu、Ni、Cr、 Co、Mn几种金属沾污对于(110)/(100)大角晶界的电学性能参数的影响规律。发现：在几种金属沾污后,晶界的能级分布均向深能级范围移动,晶界态密度有不同程度的提高,载流子捕获截面也有所增大,其中Fe杂质对于晶界态密度的影响最大；随着金属沾污含量的增大,导致晶界的能级分布更深、晶界态密度增大；Cu/Ni在共沾污时晶界的缺陷能级分布比Cu与Ni单种金属沾污导致的晶界能级位置更深,晶界态密度略有增大。实验还指出：通过特定条件的热处理,可以降低金属沾污晶界的态密度及载流子捕获截面；通过低温退火调控晶界上金属沉淀的尺寸及密度分布,进而可以调控晶界的电学性能、改善金属沾污晶界的多晶硅器件性能；通过氢钝化,晶界引入的能级分布在更窄的范围,空穴捕获截面也降低了两个数量级左右,晶界态密度略有降低。(3)研究了位错对晶体硅电池性能的影响。结合普通铸造多晶硅、高效铸造多晶硅以及铸造单晶硅,得到了位错分布对于硅片质量及太阳电池的性能影响。实验发现：高效多晶硅由于具有更低的位错密度,更均匀的晶粒分布,表现出均匀的少子寿命分布,电池效率高于普通多晶硅电池；铸造准单晶硅材料硅锭中间的单晶区域不含有晶界,位错密度较低,硅片性能最好、电池效率最高；而铸造准单晶硅边缘含有多晶和大量的位错聚集体,缺陷密度极高,对少子寿命影响很大、电池效率也最低。实验还说明：位错密度与位错分布的均匀性对于硅材料的质量以及太阳电池的性能有着显著地影响,位错密度越高、位错的聚集体越多,电池的效率越低。(4)研究了硅中金属杂质的磷扩散吸杂效应。实验发现：对于n型硅晶界上的金杂质沾污,磷扩散吸杂可以有效吸杂出晶界处的杂质,但吸杂效率受沾污程度影响；在低浓度沾污下,一步磷吸杂可以有效吸杂出晶界上的金杂质；而在严重沾污下,两步变温磷吸杂更为有效。实验证明：在普通工艺磷扩散样品中,表面死层中会存在SiP沉淀,显著增强载流子的复合及电池性能；增加后续退火以及改变扩磷工艺参数两种方法可以通过消除SiP沉淀有效改善太阳电池的性能。更多还原

【Abstract】 Cast multicrystalline silicon (mc-Si) occupies more and more market shares of solar cells because of its higher cost-effectiveness. However, since me silicon inevitably contains large quantities of metallic impurities and structural defects, solar cells based on such materials usually have lower photoelectric conversion efficiency than those based on single crystalline Czochralski (CZ) silicon. It is necessary to engineer the impurities and defects for improving the wafer quality and hence the cell performances.In this dissertation, based on some important problems associated with the practical solar cell manufacturing, we have studied the interactions between metallic impurities and structural defects in crystalline silicon and their impact on the performances of solar cells by combining microwave photocurrent decay, current/capacitance-voltage, optical microscope, and spreading resistance profiling techniques. Some innovative results have been achieved, which are illustrated below:(1) The causes of low minority carrier lifetime regions corresponding to the edge of casting me silicon ingots and its influence on the performance of solar cells have been explored. It is found that the distribution of interstitial iron exactly coincides with the minority carrier lifetime, indicating that iron contamination is mainly responsible for the lifetime degradation. After phosphorus diffusion gettering process, the low carrier lifetime region became narrower, and the concentration of interstitial iron was reduced by nearly one order of magnitude. After the celling process, the internal quantum efficiency of the edge zone has a lower response to the long wavelength light, in accordance with the minority carrier lifetime distribution. As a result, the solar cells based on edge zone wafers exhibit slightly lower efficiency than those conventional ones.(2) The effect of various kinds, forms and concentrations of metallic impurities on the electrical properties of a silicon grain-boundary (GB) formed by direct bonding technology has been investigated. It is found that compared to that of the clean GB, the density of interface states and their majority carrier capture cross-section for the contaminated GB are increased at different levels, dependent on the contaminated metal type. Meanwhile, the density of GB states in metal contaminated samples increases with the increase of metal content. The detrimental effect of a metal-contaminated GB on the electrical properties of me silicon solar cells can be minimized by engineering the metal precipitate density and size distribution. Compared to the clean GB, the energy distribution of interface states at the GB subjected to hydrogenation becomes shallower, and the carrier capture cross-section can be reduced by about two orders of magnitude, while the density of GB states varies slightly.(3) Influence of dislocations in silicon on solar cell performance has been studied. It is found that in high performance me silicon wafer, the wafer quality and the solar cell efficiency are both higher than normal me silicon solar cell, due to the lower dislocation density and the more uniform grain size distribution. Generally, the normal quasi-mono solar cell presents the best performance, as a result of low dislocation density and absence of grain boundary. However, the conversion efficiency of the defective quasi-mono solar cell is even lower than normal me cell, due to the existence of high density dislocations.(4) Effect of phosphorus diffusion gettering on metallic impurites has been studied. It is found that at a low level contamination, a single-step phosphorus gettering can be effective for most of metal impurities at the GB by extending the diffusion time. However, at a high level contamination, a two-step phosphorus gettering is more effective to getter out the gold impurities at the GB. After the effective phosphorus gettering for metallic impurities, the GB electrical characteristics could be recovered to the original level. However, during the phosphorus diffusion process, the present of SiP precipitates has strong influence on the performance of junction. By modifying phosphorus diffusion technology, the SiP precipitates can be eliminated, and therefore the conversion efficiency of solar cells gets improved.更多还原