氢化非晶硅薄膜局域结构特征及其形成的分子动力学模拟研究

The hydrogenated amorphous silicon (a-Si:H) film is the core structure of HIT (Heterojunction with IntrinsicThinlayer) solar cell. Its quality determinates the photoelectric conversion efficiency of HIT solar cell directly. Experimental studies have shown that the relationship between the process parameters and the cell performance is intricate and lacks of repetition. One of the key reasons is that it is a poor understanding of the structure characteristics and its formation mechanism of hydrogenated amorphous silicon films. In the present project, molecular dynamics simulations on the growth of the a-Si:H films on crystalline Si surfaces will be carried out. The following major factors that affect the structures of a-Si:H films will be investigated: deposit radicals; incident velocities, frequencies and angle ranges; Surface orientation, surface smoothness and temperatures of crystalline Si substrate. The local microstructure of a-Si:H film will be characterized by the methods of bond types, atomic clusters and polyhedron analysis: mainly including the silicon dangling bond, Si-H to Si-Hx, the Honeycutt-Anderson (HA) bond pair analysis, the Cluster-Type Index Method (CTIM), the Atomic Cluster Alignment Method (ACAM) and the Voronoi polyhedron analysis (VP). We expect to understand the atomic mechanism of the characterization local structures formation and evolution of the a-Si:H films on crystalline Si surfaces and the intrinsic relationships between the process conditions and the characterization local structures of a-Si:H films. The expected results will present important references for the manufacture technology research, developing and industrialization of the HIT solar cell, and will contribute greatly to the understanding and characterization of amorphous structure, which is also an important scientific field.

氢化非晶硅(a-Si:H)薄膜是非晶硅/晶体硅异质结(HIT)太阳电池的核心薄层，其质量直接决定了该电池的光电转换效率。实验研究发现该电池的制备工艺与性能之间关系复杂，重复性差，原因就是对a-Si:H薄膜的微观结构及其形成机制还缺乏认识。本项目拟运用分子动力学方法模拟a-Si:H薄膜在晶体硅表面的生长，考察沉积条件对a-Si:H薄膜结构形成的影响，包括沉积基团及其入射、生长速率、硅衬底表面和温度等；并对a-Si:H薄膜局域结构进行特征化分析，具体包括悬挂键、HA键型指数、Voronoi多面体和原子团簇类型等分析。以期得到晶体硅表面a-Si:H膜局域结构形成和演变的原子机制，以及工艺条件与a-Si:H薄膜局域结构之间的本质关联，架起工艺与性能之间的微结构桥梁。本项目研究对于HIT型太阳电池制造技术的开发与产业化将具有重要指导意义，对于非晶材料结构的认识与表征这一重要科学领域也将作出一定贡献。

氢化非晶硅(a-Si:H)薄膜是非晶硅/晶体硅异质结太阳电池的核心薄层，其质量直接决定了该电池的光电转换效率。实验上因缺乏薄膜的微观结构及其形成机制的认识，而无法理解该电池的制备工艺与性能之间的本质关联。本项目运用分子动力学方法模拟a-Si:H薄膜的生长，考察沉积条件和退火条件对a-Si:H薄膜结构形成的影响，并对a-Si:H薄膜局域结构进行特征化分析。结果发现：.硅衬底温度对a-Si:H薄膜形成和结构影响显著。随衬底温度升高，a-Si:H薄膜表面越光滑，内部越致密，同时SiH含量、H含量和悬挂键密度均下降。薄膜中Voronoi多面体拓扑结构更接近球形结构，不易出现孔洞，且高配位的次近邻Voronoi多面体含量更高。硅衬底取向对a-Si:H薄膜的致密性和有序度有影响，但仅限于界面以上的几个原子层，致密性和有序度由高到低的顺序为：Si(111)>Si(110)>Si(001)。.沉积基团同氢原子结合形成气相越容易，则沉积基团的吸附率越低。以SiH3作为沉积基团得到的a-Si:H薄膜质量最好。基团入射角和入射动能影响基团的表面扩散能力，随基团入射角降低或入射动能增大，薄膜由岛状生长变成层状生长，表面更光滑，内部更致密，悬挂键含量降低，Voronoi多面体排列趋于紧密。入射频率对薄膜结构影响甚微。.对a-Si:H薄膜的SiyHx结构组态进行了研究，提出了除常规SiHx组态外，还有HSiy组态（包括：HSi2(s)、HSi2(l)和HSi3），阳光照射可引起HSiy组态中的Si-H物理键断裂。由此产生了HAC双面太阳电池的想法，并设计了一条可行的HAC双面太阳电池的制备技术路线，成功制备出光电转换效应更高的HAC双面太阳电池。.退火使a-Si:H薄膜结构更加紧凑和有序，退火温度低于500K时，短程氢扩散导致界面H含量增加；高退火温度下，长程氢扩散以及氢溢出导致界面H含量降低。这样很好地解释了物理实验中少数载流子寿命随退火温度先升后降的变化趋势。.本项目研究结果架起了非晶硅薄膜制备工艺与性能之间的微结构桥梁。对于非晶硅/晶体硅异质结太阳电池制备技术的开发与产业化将具有重要指导意义，部分研究成果已在产业化推广应用中使用。