【作者】 杨文继；

【导师】 王岩国；

【作者基本信息】 湖南大学 ， 凝聚态物理， 2013， 博士

【摘要】 提高太阳能的利用率，尤其是薄膜硅太阳电池的光电转换效率是人们研究的一个重要课题。有关提高太阳电池的光电转换效率的方法很多，其中包括在太阳电池表面镀制减反射膜或是制作绒面结构，以及对整个电池引入陷光结构，以减少太阳电池表面对光的反射损失，同时增加太阳电池活性层对光的吸收。因为太阳光具有宽谱、广角、多变的复杂特性，如何优化太阳电池减反射膜（或绒面结构）及其陷光结构是一个难题，解决这一难题将有助于减反射膜和陷光结构的设计。本论文在通过数值模拟分析了薄膜硅太阳电池基本工作原理及其输出特性，以及影响其光电转换效率的各种因素的基础之上，对同时具有广角前透射特性和随机背反射特性的这一常见陷光结构的陷光能力进行了研究；作为具体例子，对基于玻璃的具有双减反射膜结构的一体式薄膜硅太阳电池的光吸收特性进行了研究；在广泛分析了各种绒面结构的减反射特点的基础之上，运用严格耦合波方法，对亚波长周期界面结构的光学行为进行了详细的研究。获得的主要结果如下：用试射法分析了薄膜硅内建电场和空间电荷的精确分布，并提出用反问题法由已知电场或电势的分布来确定半体中的掺杂浓度分布。为给出合理的解，掺杂浓度的值必须满足特定的条件。数值分析表明，只有处在一些特定狭窄范围内的电势积分因子才满足这些条件，而对于某些电场分布则完全不能给出满足这些条件的正确解。分析了光强、表面复合、以及各种掺杂浓度的搭配对于电池光电转换效率的影响，并由此给出了优化的参数配置。利用光在粗糙表面的反射具有随机性的特点，提出一种几率算法，对具有平的减反射面和粗糙背反射结构的硅薄膜的光吸收特点进行了理论和数值分析。采用给定波长下，光吸收率对入射角的积分均值即角均吸收率作为品质因子，发现角均吸收率作为给定波长下的角依赖透射率的泛函，不存在极大值，因而无法用这一方法确定这一结构的最大陷光能力。但分析发现角依赖吸收曲线和角依赖透射曲线之间存在着一种共同的渐近趋势，利用这一趋势的上限值确定了这一结构的最大陷光能力。结合转移矩阵和几率算法，对硅膜生长在玻璃一侧，硅与玻璃之间以及玻璃的另一侧均镀有减反射膜，硅的背侧为粗糙反射面的双减反射膜结构的陷光效果进行了分析和优化模拟，优化的结果接近于前述一般理论得出的最大陷光值。对于5μm厚的硅膜，当入射角从0°变化到75°时，在AM1.5G太阳光下的谱平均吸收率（在0.3–1.2μm波长范围内）只减小约２.５８％，显示很好的广角特性。而在较长波段，发现尽管透射曲线之间存在很大差异，但吸收曲线却趋于集中在其上限的邻域，这意味着对长波段的减反射要求可以显著降低。上述研究解决了光吸收率具有复杂的角依赖和谱依赖关系的薄膜硅太阳电池陷光结构的最大陷光能力的问题，所得出的结论可直接用于太阳电池结构的优化设计，所采用的理论方法也为其它不同的陷光结构的优化提供了参考。最后，运用严格耦合波理论，提出倒格空间中的双坐标系统，对亚波长周期界面结构的光学行为进行了详细分析。研究结果定量地给出了亚波长界面结构与多层减反射膜等效的条件，以及作为起减反射作用的太阳能玻璃和硅太阳能电池表面的亚波长界面结构相关参数的范围。对空气-玻璃以及空气-硅亚波长周期界面结构的减反射特性进行的分析和计算表明，在很宽的谱范围和角范围内，总体上亚波长界面结构可比平表面具有更低的反射率，显示良好的减反射效果，但在大入射角下其反射率总比优化后的折射率渐变多层减反射膜要高，尚需寻求其它结构进一步优化。更多还原

【Abstract】 It is an important subject of scientific research for people to maximize utilization of solarenergy wherein the enhancement of photoelectric conversion efficiency of thin silicon solarcells is of particular significance. Many ways exist to improve photoelectric conversionefficiency of solar cells and these include depositing anti-reflection coatings on their surfaces,structuring the surfaces with different patterning techniques, and introducing a light-trappingstructure into the cells, etc., by which one may reduce the fraction of reflected power andenhance the absorption of light in the active silicon layer. Due to the broadband, diffusive,and fickle nature of sunshine, optimization of the antireflection coatings (or a textured surfacestructure) and the light-trapping structures becomes a problem and solution to the problemmay be of help to the design of multilayer anti-reflection coatings and light-trappingstructures. The basic principles, the output properties as well as those factors that have impacton the photoelectric conversion efficiency for thin silicon solar cells are analyzed bynumerical simulation. On this basis, the light trapping ability for the common light trappingstructure with omnidirectional front transmission characteristics and random back-reflectionis investigated. As a concrete example, the optical absorption properties of a glass-basedall-in-one thin silicon solar cell with dual anti-reflection coatings are studied. Based on broadanalysis of the anti-reflection properties of a variety of textured structures, the opticalbehaviors for periodic subwavelength textured dielectric surfaces are invetigated using therigorous coupled-wave analysis method. The main results are given in the following.The built-in fields and the space-charge distributions in doped thin silicon are accuratelycalculated using the numerical shooting method, and the inverse-problem method is proposedto construct the doping profile with given field or potential distribution within thesemiconductor. To produce the right results, certain conditions must be satisfied for the valuesof the doping concentrations. Numerical analysis shows that this may limit the integratingfactor for the electric potential to a very small scope, and some other field profiles may notgive the right results.Influences of the intensity of light, the surface recombination and the dopingconfigurations on the photoelectric conversion efficiency are studied and the optimizedparameter in configuration is given.A probability method is presented to calculate the light absorption properties of a siliconlayer with a flat front anti-reflection coating and a random-reflective back reflector. Theangle-averaged overall absorptance (AAOA) is chosen as the figure of merit for light at agiven wavelength. We find that, although the AAOA is a functional of the angle-dependenttransmittance at a given wavelength, the functional has no extreme value. So the maximumlight-trapping ability for this design can not be determined by this method. But our furtheranalyses reveal that both the angle-dependent aborption curves and the angle-dependenttransmission curves show the same asymptotic behavior consistently, thus we are able todetermine the maximum light-trapping ability by the upper limit of the light absorptioncurves.We propose a light-trapping structure with dual anti-reflection design—a piece of glasswith both sides coated with anti-reflection coatings. On the anti-reflection coating for one side of the glass a silicon layer is deposited with its back surface roughened and covered witha reflective medium. Combining the transfer matrix method and the probability method, wecalculate and optimize the light-trapping properties for this structure and find that theoptimized absorptance is close to the maximum value of its kind derived from theaforementioned general light-trapping theory. With a5-μm thick silicon, thespectrum-averaged absorptance in the0.3–1.2μm wavelength range (under the AM1.5G sun)decreases only by2.58%when the incident angle varies from0°to75°, clearly showing theomnidirectional characteristics of the dual anti-reflection design. Particularly, it is found thatthe angle-dependent absorption curves in the longer wavelength region tend to converge to asmall neighborhood of the upper limit irrespective of significant differences between thetransmission profiles. This indicates the anti-reflection requirement in the longer wavelengthregion can be significantly relaxed.The current work resolves the problem of maximum light-trapping ability for thin siliconsolar cells with a light-trapping structure which exhibits complex angle-dependent andspectrum-dependent absorption characteristics. The results can be directly employed to helpdesign and optimize a solar cell structure. The theoretical approach adopted here alsoprovides a reference for the optimization of other light-trapping structure.Using the rigorous coupled-wave analysis method, we come up with the doublecoordinate systems on the reciprocal space of the periodic surface texture and analyze theoptical behaviors for these structures. The conditions are given quantitatively for thesubwavelength surface texture to be equivalent to a multilayer dielectric coating. Also therange of the parameters for the subwavelength surface texture as an antireflection structureare presented for solar glass and the silicon cell. The anti-reflection characteristics for theair-glass and air-silicon periodic interface indicate that on the whole an optimizedsubwavelength surface texture can generally give quite small reflectance in a wide spectraland angle range, but for the current texture design its performance at higher incident angles isusually poorer than an optimized multilayer AR coating with graded index along thethickness direction. So further work is needed to improve the performance at higher incidentangles by searching for other surface design.更多还原