【作者】 罗培青；

【导师】 窦晓鸣； 周之斌； H.Stiebig；

【作者基本信息】 上海交通大学 ， 光学工程， 2009， 博士

【摘要】 硅薄膜太阳电池主要包括氢化非晶硅(a-Si:H)太阳电池、氢化纳米硅(nc-Si:H)太阳电池以及由它们构成的双结或多结叠层太阳电池。硅薄膜太阳电池具有原材料丰富、耗材少、耗能小、无毒、低成本和易于大面积沉积等特点,是一种非常具有潜力实现大规模产业化的太阳电池。本论文对热丝化学气相沉积(HWCVD)制备硼掺杂nc-Si:H及银纳米粒子增强硅薄膜太阳电池光谱响应这两个方面的内容进行了研究。在硅薄膜太阳电池中,高电导率和高晶化率的p型nc-Si:H对提高太阳电池的光电转换效率起着至关重要的作用。与通常的等离子体增强化学气相沉积(PECVD)相比较,无等离子体辅助的HWCVD具有先天无尘和无等离子轰击等优点,近年来在硅薄膜太阳电池的本征层研究中取得了很多优异的成果,引起了国内外同行的广泛关注。而采用HWCVD制备硼掺杂nc-Si:H的报导却比较少,且大多缺乏系统深入的研究。本文通过广泛调节HWCVD的沉积参数,成功实现了硼掺杂nc-Si:H从接近a-Si:H到高晶化率nc-Si:H的相转变,并综合采用拉曼(Raman)光谱、红外光谱,特别是精密的霍尔(Hall)效应测试以及二次离子质谱(SIMS)等表征手段对样品的微结构、电学性质、硼掺杂浓度以及它们之间的相互关系进行了系统深入的研究。结果表明,最高电导率的硼掺杂nc-Si:H并非是通常认为的具有最高晶化率的样品,而是具有中等晶化率的薄膜。其次,HWCVD同时实现了硼掺杂nc-Si:H的高晶化率、高硼掺杂浓度、高掺杂效率和高载流子浓度,解决了常用PECVD一直存在的困难。最后,HWCVD制备的p型nc-Si:H可以实现比PECVD样品更高的电导率。这些成果展示了HWCVD比常用PECVD在制备高晶化率和高电导率硼掺杂nc-Si:H上的优势,对进一步提高硅薄膜太阳电池的转换效率具有重要的指导意义和实用价值。另一方面,由于a-Si:H和nc-Si:H在长波段的吸收系数比较小,而且硅薄膜太阳电池的厚度十分有限,因此,采取合适的陷光措施增加太阳电池对光的吸收对提高太阳电池的光电转换效率有着十分重要的作用。常用的陷光方法是制作透明导电膜绒面,使太阳光在透明导电膜与硅薄膜的界面发生光散射,增加入射光在太阳电池内部的光程,从而增加太阳电池对光的吸收。最近,有人报导了一种新型的采用小颗粒(直径小于100 nm)金属纳米粒子增强光吸收的方法,它来源于光照引起金属纳米粒子局域表面等离子体共振(LSPR)。该方法已经在有机太阳电池和染料敏化太阳池的研究中得到了应用,并相应的增加了太阳电池的光谱响应。然而,在硅薄膜太阳电池的研究中,虽然发现了纳米粒子具有很强的LSPR增强光吸收以及表面增强拉曼散射,但并未发现量子效率的相应增加。本文采用易于大面积沉积的真空热蒸发方法制备小颗粒银纳米粒子,通过创新性的将其集成在特殊结构的a-Si:H太阳电池中,观察到了纳米粒子对太阳电池在红光和近红外光波段光谱响应的增强。同时,本文还对小颗粒银纳米粒子增强硅薄膜太阳电池光谱响应的增强机理以及影响因素进行了深入讨论。该研究为采用新型的、非常具有吸引力的小颗粒金属纳米粒子增强标准硅薄膜太阳电池的光谱响应奠定了坚实的基础,具有重要的理论和实践指导意义。除了小颗粒金属纳米粒子LSPR增强光吸收以外,大颗粒金属纳米粒子(直径大于100 nm)也有一个引人注目的效应,那就是LSPR增强光散射。该散射与几何散射不同,是一种与入射光的波长有关的散射。有人在晶体硅和非晶硅太阳电池表面制备了大颗粒金属纳米粒子,并通过纳米粒子LSPR增强光散射使太阳电池光吸收和光谱响应得到增加。本文将真空热蒸发制备的大颗粒银纳米粒子和银纳米结构(大颗粒银纳米粒子相互连接)集成在n-i-p结构的nc-Si:H和a-Si:H太阳电池内部,使太阳电池在长波段的光吸收和光谱响应得到了明显增强。研究发现,当纳米粒子和纳米结构直接与太阳电池n层接触时,纳米粒子和纳米结构还存在着表面等离子体共振光吸收损耗,该损耗可以通过覆盖一层比硅薄膜折射率低的介质层在纳米粒子上使共振光吸收损耗蓝移而得到有效抑制。这为我们提高硅薄膜太阳电池的光电流和减少光吸收损耗提供了一种新的思路和方法,具有重要的应用价值。更多还原

【Abstract】 Silicon thin film solar cells mainly include hydrogenated amorphous silicon (a-Si:H) solar cells, hydrogenated nanocrystalline silicon (nc-Si:H) solar cells and tandem or triple junction solar cells made of a-Si:H and nc-Si:H. Because of abundant raw material, less material and energy consumed, non-toxic, low cost and easy to deposit in large area, there is a great potential to realize the mass production of silicon thin film solar cells. This thesis contains two aspects of contents, one is boron doped hydrogenated nanocrystalline silicon prepared by hot-wire chemical vapor deposition (HWCVD) and another is enhanced spectra response of silicon thin film solar cells by Ag nanoparticles.In silicon thin film solar cells, it is crucial to have highly conductive p-type nc-Si:H with high crystallinity to improve the cell efficiency. Compared with common plasma enhanced chemical vapor deposition (PECVD), HWCVD appears to be a promising deposition method because of absence of powder formation and ion bombardment. In recent years, there have been a great of fruits of intrinsic layer deposited by HWCVD in silicon thin film solar cells. While there are few and short of systematical researches on boron-doped nc-Si:H prepared by HWCVD. In this thesis, it is successful to obtain boron-doped silicon thin films transited from a-Si:H to highly crystallized nc-Si:H by widely adjusting the deposition parameters of HWCVD. The microstructural, electrical, doping properties of the samples and the relationship of them are systemically investigated by Raman spectra, infrared absorption spectra, and special precise Hall effects measurements and second ion mass spectra (SIMS). It is showing that the highest conductive boron-doped nc-Si:H don’t have commonly believed the highest crystallinity, but have moderate crystalline volume fraction. Second, HWCVD realized the boron-doped nc-Si:H with high crystallinity, high boron concentration, high doping efficiency and high carrier concentration which is a long time existed problem of the common PECVD method. Finally, the p-type nc-Si:H deposited by HWCVD other than PECVD can have higher conductivity. Theses results exhibit the merits of HWCVD compared with the common PECVD in preparing boron-doped nc-Si:H with high cyrstallinity and high conductivity. This work has instructive significance and application value to further improve the efficiency of silicon thin film solar cells.On the other hand, in order to improve the conversion efficiency of silicon thin film solar cells it is very important to adopt a suitable light trapping method to enhance the light absorption because of low absorption coefficiency of a-Si:H and nc-Si:H in long wavelengths range and limited thickness of solar cells. A common used light trapping method is texturing transparent conductive oxide (TCO) which makes the incident light scattered at the interface between TCO and silicon film. In this case the light path is prolonged inside solar cells, and thereby the light absorption is enhanced. Recently, it has been reported a novel enhancing light absorption method by small (the diameter smaller than 100 nm) metallic nanoparticles which originates from the light induced metallic nanoparticles localized surface plasmon resonance (LSPR). This novel method was applied in organic and dye-sensitized solar cells and the spectra response were corresponding enhanced. However, it was not observed enhanced spectra response in silicon thin film solar cells by metallic nanoparticels even though a huge improved light absorption and surface enhanced Raman scattering were reported. In this thesis, small Ag nanoparticles are prepared by thermal evaporation which is easy to deposit in large area. And it is observed enhanced red and near-infrared spectra responses when the Ag nanoparticles are innovatively integrated in special structural a-Si:H solar cells. Meanwhile, it is deeply discussed the mechanism and influence factors of enhancing spectra responses of silicon thin film solar cells by small Ag nanoparticles. This work establishes fundaments of enhancing light absorption and spectra response for standard silicon thin film solar cells. It has important significance in theory and practice.Besides enhanced light absorption by small metallic nanoparticles, large metallic nanoparticles (diameter larger than 100 nm) have an attractive LSPR enhancing light scattering effect. Different from geometry light scattering the light scattering of large metallic nanoparticles are wavelength dependent. It has been reported that light absorption and spectra response were enhanced by employing large metallic nanoparticles on top of crystalline silicon and a-Si:H solar cells. In this thesis, large Ag nanoparticles and Ag nanostructure (large Ag nanoparticles connected to each other) prepared by thermal evaporation are integrated inside n-i-p nc-Si:H and a-Si:H solar cells, and the light absorption and spectra response in long wavelengths are distinctly enhanced. Results show that there are surface plasmon resonance light absorption losses of nanoparticles and nanostructure when they directly connect with n-layer. Such light absorption losses can be restrained by covering a thin layer of medium, which has smaller refractive index in comparison to silicon film, on tope of nanoparticles and nanostructure to blue shift resonance absorption losses. This work provides a new way and method to increase photocurrent and reduce light absorption losses in silicon thin film solar cells. It has significant application value.更多还原