籽晶辅助多晶硅定向凝固生长初期晶体缺陷形成与控制研究

Multicrystalline silicon (mc-Si) solar cell has occupied the mainstream of the photovoltaic market, but its photoelectric conversion efficiency is lower than that of monocrystalline silicon solar cell due to the crystal defects. Seed-assisted directional solidification is the major technique to control the defects of mc-Si at present. Experimentally, it has been demonstrated that the key point of this technique is to control the nucleation and crystal defect formation in the earlier stage of silicon solidification. In this proposed project, molecular dynamics simulations will be performed to investigate the nucleation and crystal defect formation in the earlier stage of seed-assisted directional solidification of mc-Si. The atomic mechanism of the nucleation and the crystal defects, such as grain boundaries, twin boundaries and dislocations, will be explored with different growth conditions, such as seeds, temperatures, and stress/strain conditions; the effects of these growth conditions on the nucleation and crystal defect formation in mc-Si will be discussed. On basis of these works, the laws and conditions for the nucleation and defect formation in the earlier stage of seed-assisted directional solidification will be proposed, and an effective method for controlling crystal defects will be suggested, followed by experimental verification. The present project is innovative in both the objectives and the methodology. The expected results will supply information for the theories of solidification and crystal defect at atomic scale, and the results will be of great importance to improve the quality of quasi-crystalline silicon and high performance multicrystalline silicon, which are both cast with the seed-assisted directional solidification method, and hence their photovoltaic performance.

占据市场主流的多晶硅太阳能电池因晶体缺陷的影响，其光电转换效率比单晶硅太阳能电池的效率要低。籽晶辅助定向凝固生长是目前控制多晶硅晶体缺陷的主要技术手段，实验上已证实该技术的关键是控制凝固初期晶核和晶体缺陷的形成。本项目运用分子动力学方法，就籽晶辅助多晶硅定向凝固生长过程中的形核和生长初期晶体缺陷的形成进行模拟研究。探索不同籽晶、温度、应力/应变等生长工艺条件下多晶硅晶核形成的原子机制，凝固生长初期晶界、孪晶界和位错等缺陷结构的形成机制；探讨籽晶、温度、应力/应变等生长工艺条件对多晶硅凝固形核和晶体缺陷形成的影响；在此基础上归纳籽晶辅助多晶硅定向凝固生长时形核和缺陷形成的规律与条件，提出对籽晶辅助多晶硅凝固生长初期晶体缺陷的有效控制方法并加以实验验证。本项目在方法和内容上均有创新，研究成果为凝固和缺陷理论提供信息补充，对提高籽晶辅助生长的准单晶硅和高效多晶硅的质量及其光伏性能有重要意义。

光伏市场上的两个主流产品—铸造单晶硅和高效多晶硅都是采用籽晶辅助定向凝固生长技术。该技术的关键是控制凝固初期晶核和晶体缺陷的形成，而实验上无法对其进行在线分析检测。本项目运用分子动力学方法，就籽晶辅助多晶硅定向凝固生长过程中的形核和生长初期晶体缺陷的形成进行模拟，并辅助物理实验进行研究。结果发现：.（1）不同温度下硅熔体均匀形核呈现出两种模式：高温自发形核模式和低温发散形核模式。存在形核率最大临界温度。生长阶段，不同温度下的晶粒数随时间变化呈现出三种不同的模式。均匀形核生长多晶硅的位错主要来自于晶界，孪晶原子含量超过40 %。.（2）硅作为籽晶时，分析了生长的过程中孪晶和位错的含量、形成、演变规律以及两者的伴生关系。归纳发现了两种典型的缺陷构型。碳化硅作为籽晶时，观察到均质形核和异质形核两种形核方式。不同生长面上诱发的硅平行孪晶与生长界面的夹角不同，晶体结构也略有不同。理论计算和模拟结果较好地预测了温度与形核之间的关系。.（3）横向温度梯度增加，形核数目随着温度梯度增加而减少；晶体生长速率呈先增后减的趋势，固液界面形状也在变化。纵向温度梯度增加，晶体生长速率呈先增后减的趋势。生长速率顺序和形成孪晶的难易程度、结构均与恒温下基本一致。.（4）应力限制了二维晶核的形核速率，抑制了硅晶体的初期生长，且易诱发形成堆垛层错区。铁和碳杂质都会降低硅晶的生长速率，并易形成应力集中，诱发孪晶和层错形核，降低硅晶体质量。.（5）分别采用多晶硅棒和颗粒作为籽晶，发现棒料籽晶的铸锭，位错更少且少数载流子寿命更高。根据模拟结果和实验摸索，我们设计并提出了两种新型的籽晶铺设方法，有效提升了铸造硅晶体的质量。另外，我们引入大角度的籽晶小条和在坩埚和石墨护板之间增设石墨隔热毡有效地阻挡边缘多晶的侵入，提升了全单晶硅片率和降低了硅块的位错比例。相关技术在企业得到推广应用。