【作者】 张磊；

【导师】 沈鸿烈；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2012， 博士

【摘要】 传统化石能源的日益减少以及由其造成的环境的污染迫使我们必须开发清洁的可再生能源。太阳能光伏发电是一种直接将太阳能转化为电能的能源利用方式，具有广阔的应用前景。目前市场上占主导地位的太阳电池为体硅电池，由于硅片本身成本较高，导致太阳电池的发电成本仍然无法和传统发电成本相抗衡。利用晶硅薄膜代替体硅材料可以有效的降低太阳电池成本，从而降低太阳能发电成本。本文以太阳电池使用的晶硅薄膜作为研究对象，针对当前晶硅薄膜制备方法的不足，提出了一系列新颖的制备方法，并详细研究了不同参数对制备的薄膜性能的影响及影响机理。通过优化材料性能，制备了晶硅薄膜太阳电池器件并对器件制备工艺参数进行优化，制备了光电转换效率较高的晶硅薄膜太阳电池。系统研究了热丝温度、H2稀释比、衬底温度及衬底类型等对热丝CVD沉积硅薄膜性能的影响。研究发现适当的提高热丝温度可以有效的提高硅薄膜的结晶性能，而过高的热丝温度反而会使硅薄膜的结晶性能降低。H2稀释比对薄膜性能的影响表现出类似的趋势，即：适当的H2稀释比有利于硅薄膜结晶性能的提高，同时随H2稀释比的升高薄膜的沉积速率急剧下降。随衬底温度的升高，薄膜的结晶性能有一定的提高。通过优化硅薄膜的制备参数，制备了硅薄膜p-n结，发现其最高工作温度可达280oC。此外，不同类型的衬底对多晶硅薄膜的结晶性能也有一定的影响。研究发现，Si(111)衬底对热丝CVD制备多晶硅薄膜具有很强的诱导作用，Si(100)衬底次之，AZO衬底对多晶硅薄膜也具有一定的诱导作用，而普通玻璃则没有表现出对硅薄膜生长的诱导作用。对非晶硅薄膜进行固相晶化处理可以制备结晶性能优异的多晶硅薄膜，如固相晶化法(SPC)、金属诱导晶化法(MIC)等。然而长达数小时甚至数十小时的晶化时间使这种方法制备的多晶硅薄膜成本很高。本文对热丝CVD沉积的非晶硅薄膜进行快速热退火晶化处理，研究了不同处理参数对所制备薄膜性能的影响。研究发现，利用快速热退火可以在极短时间(<20s)内得到晶化率超过95%的多晶硅薄膜。在上述研究基础上，制备了硅薄膜p-n结，最高工作可达300oC。此外，通过优化工艺，有效抑制了厚度为微米级的非晶硅薄膜在快速热退火晶化过程中可能出现的微裂纹及脱落现象。通过双层多孔硅层转移技术，利用自行设计的电化学腐蚀装置实现对待腐蚀单晶硅片全面积腐蚀制备双层多孔硅，并以此为模板采用LPCVD技术外延高质量准单晶硅薄膜。同时，采用热丝CVD低温沉积非晶硅薄膜作为电池的发射极，形成薄膜型HIT (Heterojunction withIntrinsic Thin layer)结构太阳电池。这种技术一方面使用外延高质量晶硅薄膜作为吸收层可以有效降低硅材料用量，并且可以将制备的太阳电池转移至廉价衬底上，同时用于制备双层多孔硅的单晶硅片可以重复利用，从而降低电池的制备成本。另一方面，利用热丝CVD沉积发射极与传统的高温制备发射极工艺相比，具有能耗低、速度快的优点，在200oC的温度下，仅需要50s即可以完成发射极的制备，因此可以进一步降低太阳电池的制备成本。通过优化工艺，利用层转移技术制备了光电转换效率为9.6%的薄膜型HIT太阳电池。在掌握热丝CVD制备不同形态的Si薄膜参数的基础上，通过在沉积腔室内引入衬底偏压，研究衬底偏压对沉积硅薄膜性能的影响。研究发现，在较低H2稀释比下通过施加衬底偏压可以有效提高所沉积薄膜的结晶性能与致密度，因此可以实现在较低H2稀释比下高速沉积高质量微晶硅薄膜。另外，在较低H2稀释比下衬底偏压对不同衬底上沉积的微晶硅薄膜结晶性能提高的程度也有所区别，具体表现为：在不锈钢衬底上沉积的硅薄膜与玻璃衬底上的具有更优异的结晶性能。与之相反，在高H2稀释比条件下，不锈钢衬底上沉积的硅薄膜结晶性能随衬底偏压的升高逐渐降低，而玻璃衬底上的微晶硅薄膜结晶性能则先变好后降低。分析衬底偏压对所沉积硅薄膜影响的机理主要与热丝在高温下发射的电子在电场中加速运动有关，这些高速运动的电子与反应基元相互碰撞，而引起的沉积过程的变化。而两种衬底上沉积薄膜的结晶性能差异主要与衬底表面在电场中的电位有关。不锈钢衬底电位与石墨衬底盘相同，因此受衬底偏压影响更明显。在优化衬底偏压对热丝CVD沉积微晶硅薄膜性能的基础上，制备了n+nipp+结构的微晶硅薄膜太阳电池。研究发现适当的衬底偏压有利于微晶硅薄膜太阳电池转换效率的提高，通过优化参数，在不锈钢柔性衬底上采用单腔室热丝CVD技术制备了转换效率为6.07%的微晶硅薄膜太阳电池，而不加衬底偏压时同类太阳电池的转换效率仅为3.86%，可见衬底加偏压使转换效率相对提高了57.25%。更多还原

【Abstract】 The finite supply of the fossil fuels and the pollution caused by the consumption of these fuelsmake it urgent to develop the clean renewable energy. Photovoltaic (PV) technology is a promisingtechnology which can convert the solar radiation directly into electricity. The dominant products inthe PV industry are the c-Si wafer based solar cells. However, high cost of the c-Si wafers blocks thedevelopment of the PV industry. Replacing the c-Si wafer by crystalline silicon films is an alternativeway to reduce the cost of the solar cells.In this paper, we promoted some novel methods to fabricate high-quality crystalline silicon filmswith lower fabrication cost than the commonly used fabrication technologies. We have studied theinfluence of the fabrication parameters on the property of crystalline silicon films and discussed itsmechanism in detail. Based on the property optimization of the crystalline silicon films, we fabricatedcrystalline silicon thin film solar cells with relative high conversion efficiency.The influence of the temperature of hot-wires and substrates, H2dilution ratio, and type ofsubstrates on the properties of the Si films deposited by hot-wire chemical vapor deposition (HWCVD)was systematically studied. We found that properly high temperature of the hot-wires was beneficialto the enhancement of the crystallinity of the Si films. However, too high temperature of hot-wireswill deteriorate the crystallinity. The H2dilution ratio exhibits similar tendency, proper high H2dilution ratio is helpful for enhancing the crystallinity of the Si films, but the deposition rate isrelatively low. As the increasing of the temperature of the substrate, the crystallinity of the Si filmswas gradually improved. By optimizing the parameters, we fabricated the silicon film p-n junctions.We found that the highest working temperature of the junction was as high as280oC. Different type ofsubstrates also has an effective impact on the crystallinity of the poly-Si films. We found that thepoly-Si films deposited on Si (111) obtained the best crystallinity, while the following were on the Si(100), AZO and glass substrates. This may be caused by the substrate induction effect.An effective method to fabricate the poly-Si films is to crystallize the a-Si:H films, which alwaysrefers to solid phase crystallization (SPC) or metal induced crystallization (MIC). However, a quitelong duration which always took several or several tens hours is usually necessary for these processes.In this paper, the rapid thermal annealing was applied to crystallize the a-Si:H films. Influence ofdifferent parameters on the properties of the poly-Si films was studied. We have found that poly-Sifilms with high crystalline fraction (>95%) were obtained in a very short time (<20s). Based on the optimization above, p-n junctions have been fabricated by this technique. The working temperaturewas as high as300oC, which was greatly higher than that of silicon wafer based junctions. Byoptimizing the fabrication process, the micro-cracks and the peeling-off phenomenons of whichappeared in the fabrication of micrometer-order poly-Si films were effectively avoided.Through the double-layer porous silicon layer transfer technique, thin film based HIT(Heterojunction with Intrinsic Thin layer) solar cells was fabricated. The double-layer porous siliconwas formed on the whole area of the Si wafer by electrochemical etching process in the self-designedetch tank. This double-layer porous silicon can act as the template for the epitaxial growth ofhigh-quality crystalline silicon film by LPCVD. After the formation of this film, the emitter layer ofthe solar cell was deposited by HWCVD at low substrate temperature. By using the high-quanlitycrystalline film as the absorber layer, the consumption of the Si material can be effectively reduced.Besides, the fabricated solar cells can be attached to the cost-effective substrates and the Si wafer forfabricating double-layer porous silicon can be reused, which can also reduce the cost of the solar cell.Morever, compared to the trainditional emitter fabrication process, emitter layers fabricated byHWCVD have the advantages of low temperature and high growth rate (only50s in this paper), whichcan further reduce the cost of the solar cells. By optimizing the parameters during the fabricationprocess, solar cell with the conversion efficiency of9.6%was obtained.After mastering the parameters for depositing different type of Si films by HWCVD, a dc substratebias was introduced into the HWCVD system to investigate the influence of substrate bias on theproperties of the deposited Si films. The results show that the substrate bias can effectively improvethe crystallinity of the films deposited at low H2dilution ratio. And this improvement is moreeffective for the films deposited on stainless steel (SS) than that on glass, which make it possible todeposited high crystallinity microcrystalline Si (μc-Si:H) films at high growth rate. On the other hand,however, at high H2dilution ratio, the crystallinity of the Si films deposited on SS was graduallydeteriorated as the increasing of the negative substrate bias, while that on glass was firstly improvedand then deteriorated. The mechanism of the influence of substrate bias on the crystallinity of theμc-Si:H films is related to the electrons emitted from the hot wires. These electrons are accelerated inthe electrical field and collide with the reactant radicals make the deposition process different with thetraditional HWCVD. The differences of the crystallinity of the films deposited on SS and glass aremostly attributed to the different electrical potentials between these two kinds of substrates. Afteroptimizing the properties of the μc-Si:H films deposited with substrate bias by HWCVD, solar cellswith the n+nipp+structure were fabricated on SS. By optimizing the parameters during its fabrication,the solar cell with a conversion efficiency of6.07%was obtained. Compared to that fabricated without the applying substrate bias, which attained the conversion efficiency of3.86%, the solar celldeposited with applying substrate bias showed a57.25%higher efficiency.更多还原