【作者】 李科慧；

【导师】 沈文忠；

【作者基本信息】 上海交通大学 ， 凝聚态物理， 2010， 硕士

【摘要】 能源是人类生活的基础,而传统的能源面临着枯竭和环境污染两大问题,难以满足人类社会可持续发展的需要。因此,开发和利用可再生的清洁能源成为全社会急需解决的大问题。太阳能作为取之不尽、用之不竭且无污染的绿色能源,成为首选目标之一。太阳电池是将太阳光能直接转换为方便使用的电能的一种器件。相比于第一代晶体硅太阳电池,第二代薄膜太阳电池——尤其是以硅为原料的太阳电池——具有易于大面积生长、生产成本低等优势,因而受到了广泛的关注,并且占据了一定全球光伏产业的市场份额。然而,商业上的非晶硅薄膜仍存在某些难以克服的缺点,如:能量转换效率低、电导率低,且易发生光致衰退现象(即所谓的Staebler-Wronski效应)。纳米硅薄膜在解决这些问题方面表现出色。纳米硅薄膜是一种由纳米量级单晶硅颗粒镶嵌于多晶硅网络中而组成的混相材料,目前已对其光学、电学性质进行了大量的研究。由于纳米晶粒中光学吸收截面增加和载流子传导率高,在nc-Si薄膜中发现了强的光学吸收和高的光电流。目前人们正在不断进行尝试,希望能制备出同时具有高转换效率和高稳定性的单结、多结第三代纳米硅薄膜太阳电池。在工业上应用纳米硅薄膜的难点之一是实现大面积均匀沉积,这一点对于提高太阳电池的性能极为重要。另一个影响纳米硅薄膜太阳电池性能的重要因素是对其光学带隙的调控,必须使其带隙与太阳光谱相匹配,以求更好地利用太阳能量。本文利用空间分辨率为微米量级的显微拉曼(Raman)面扫描技术和室温发光光谱(PL),研究了掺杂浓度对用等离子体增强化学气相沉积法生长的n型掺磷氢化纳米硅(nc-Si:H)薄膜的均匀性及光学带隙的影响。我们首先通过Raman光谱分析来提取磷掺杂nc-Si:H样品的均匀性以及晶粒尺寸分布、晶态比等结构特性随掺杂浓度变化的信息,并从生长的机制方面对其进行了分析。随着掺杂浓度CP从0%增加到5%,平均晶粒尺寸d先是逐渐降低,接着随着掺杂浓度CP超过5%进一步增加,平均晶粒尺寸d又逐渐上升;而从CP=0%变化到CP=20%,平均晶态比XC从54.9%连续下降到35.4%。同时,我们能很清楚地看到随着掺杂浓度的改变,平均晶粒尺寸和体现了标准差的分布线宽都在改变。本征样品D1的线宽最窄,均匀性最好;而在掺杂样品中,随着掺杂浓度的增加,相对偏差(即薄膜不均匀性)先减小后增大。而d和XC的平均值对CP的依赖关系同应力和Γ3的正好分别相反,说明提高掺杂浓度将会降低nc-Si:H薄膜的结构有序度。为了深入了解更多nc-Si:H薄膜的光学带隙等光学性质,我们还测量了nc-Si:H薄膜的PL光谱,这些PL谱能用晶粒尺寸呈对数正态分布情况下、同时考虑了量子限制效应、局域表面态和晶粒尺寸分布的I-K模型很好地拟合。发光光谱的结果进一步证实了Raman面扫描所得的样品均匀性。PL和拉曼分析的高度一致证实I-K模型是揭示纳米材料晶粒尺寸分布令人满意的手段。此外,拉曼面扫描能有效地提取薄膜均匀性方面的信息,而无论从基本物理还是未来应用的角度来考虑,均匀性在太阳电池的评估中都是不可或缺的。此外,我们还从PL峰位随掺杂浓度改变的角度,讨论了对nc-Si:H薄膜的带隙调控。这种峰移是由d、σPL/d和C这三个参数决定的。因此,合理选择掺杂浓度,能制造出同时具有高度均匀性和合适光学带隙的样品。希望本文的研究能对提高nc-Si:H薄膜太阳电池的性能有所帮助。而如何通过改变气体压力、氢稀释比和衬底温度等相互依赖的生长条件,达到最优化nc-Si:H生长的目的,值得我们进一步深入研究。以上研究得到了科技部国家重大科学研究计划(纳米研究计划)(2010CB933702)、国家自然科学基金重点项目(10734020)和上海市优秀学科带头人计划(08XD14022)的资助。更多还原

【Abstract】 Energy is the basis of our daily life, while the problem of environment pollution and energy source shortage are becoming increasingly serious. Therefore, the exploitation of clean power and renewable energy resources becomes the urgent problem. As an inexhaustibility and green energy, solar energy provides a promising prospect.Due to their advantages over the first generation crystalline Si solar cells in ease of large area and low manufacturing cost, the second generation thin film solar cells, especially those based on Si, have attracted much attention and are ready to make a substantial contribution to the world’s photovoltaic market. For commercial amorphous Si thin films, however, there still remain some unavoidable disadvantages, i.e., low energy conversion efficiency, low conductance, and light induced degradation of cell performance (the so-called Staebler-Wronski effect). Nanocrystalline Si thin film, a mixed-phase material consisting of nanocrystals embedded in amorphous tissue, presents very promising features in solving these problems. Extensive optical and electrical investigations of nc-Si thin films have been carried out. Strong optical absorption and high photocurrent are found in nc-Si films and attributed to the enhancement of the optical absorption cross section and good carrier conductivity in the nanometer grains. There are numbers of attempts to realize high efficiency and good stability single-junction and tandem third generation nc-Si thin film solar cells.Uniform deposition over a large area, which is one of the principle challenges in applying nc-Si thin film on an industrial scale, plays a significant role in promoting the performance of solar cells. Another critical factor that influences the performance of nc-Si solar cells is the controllable optical bandgap of nc-Si to make full use of the sunlight.In this paper, micro-Raman mapping with a spatial resolution of micrometer and room-temperature visible photoluminescence (PL) has been carried out on phosphorous-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma enhanced chemical vapor deposition. Both the thin film uniformity and structural properties are revealed.We start with Raman analyses to get information about the structures, i.e., the distribution of grain sizes and crystalline volume fraction, of our P-doped nc-Si:H thin films with different doping concentrations and physically interpreted on the basis of growth mechanism. We notice the first gradual decrease of the average grain size d when CP increases up to 5%, and then gradual increase with the further increase of CP, whereas the average crystalline volume fraction XC decreases continuously from 54.9% to 35.4% as CP increases from 0% to 20%. At the same time, we can identify easily the changing of the average grain size and the linewidth of the distribution, which indicates the standard deviation, with the doping concentration. The intrinsic sample D1 exhibits the narrowest linewidth in the distribution profile, i.e., the most homogeneous with the least relative deviation of only 1.88%, while in doped samples the relative deviation (thin film nonuniformity) first decreases and then enhances with increasing doping concentration. Interestingly, the mean values of d and XC appear to have an opposite dependence on CP to those of strain andΓ3, respectively, which demonstrates that increasing doping concentration reduces the degree of structural order within these nc-Si:H thin films. In order to gain more insight into the optical properties such as the bandgap of these nc-Si:H thin films, we performed photoluminescence measurements. The PL profiles can be well reconstructed by using the model proposed by Islam and Kumar (I-K model), which takes into account the combined effects of quantum confinement, localized surface states, and grain size distribution, together with a lognormal rather than normal crystallite size distribution. The PL profiles yield microstructural information in good agreement with the Raman analysis, demonstrating that PL spectra is a convenient method to acquire the size dispersion, while the Raman mapping can further identify the thin film uniformity in nanomaterials, which is indispensable in solar cell evaluation from the viewpoints of both fundamental physics and future applications. Additionally, we have investigated the shift of the PL peak energy stemming from the variation of P doping concentration, which is determined by the values of d,σPL/d, and C.Therefore, selection of appropriate doping concentration allows the production of samples exhibiting high uniformity and suitable optical bandgap. We expect that this study may benefit to the improvement of the nc-Si:H thin film solar cell performance, although controlling the growth conditions such as gas pressure, hydrogen dilution ratio, and substrate temperature, which are not independent of each other for optimizing the growth of nc-Si:H, still needs further investigations.This work was supported by the National Major Basic Research Project of 2010CB933702, Natural Science Foundation of China (contract No. 10734020), and Shanghai Municipal Commission of Science and Technology Project of 08XD14022.更多还原