Residual stress analysis and bow simulation of crystalline silicon solar cells

Abstract

The pressure to reduce solar energy costs encourages efforts to reduce the thickness of silicon wafers. Thus, the cell bowing problem associated with the use of thin wafers has become increasingly important, as it can lead to the cracking of cells and thus to high yield losses. In this paper, a systematic approach for simulating the cell bowing induced by the firing process is presented. This approach consists of three processes: (1) the material properties are determined using a nanoidentation test; (2) the thicknesses of aluminum (Al) paste and silver (Ag) busbars and fingers are measured using scanning electron microscopy; (3) non-linear finite element analysis (FEA) is used for simulating the cell bowing induced by the firing process. As a result, the bowing obtained using FEA simulation agrees better with the experimental data than that using the bowing calculations suggested in literature. In addition, the total in-plane residual stress state in the wafer/cell due to the firing process can be determined using the FEA simulation. A detailed analysis of the firing-induced stress state in single crystalline silicon (sc-Si), cast, and edge-defined film-fed growth (EFG) multi-crystalline silicon wafers of different thicknesses is presented. Based on this analysis, a simple residual stress calculation is developed to estimate the maximum in-plane principal stress in the wafers. It is also proposed that the metallization pattern, Ag busbars and fingers screen printed on the front of a solar cell, can be designed using this approach. A practical case of a 3-busbar Si solar cell is presented.

抽象

目 的

建立一套系统的方法来模拟矽晶太阳能电池的翘曲行为, 进而分析因翘曲而产生的残留应力。

创新点

1. 利用纳米压痕实验及电子显微镜测量材料性质及结构尺寸, 帮助有限元分析更准确地模拟太阳能电池的翘曲行为; 2. 提出了2 个针对不同矽晶太阳能电池因翘曲产生的简易残留应力的计算公式。

方 法

1. 利用纳米压痕实验测量铝胶及银胶的材料性质, 使用电子显微镜测量铝胶及银胶的结构尺寸; 2. 建立非线性有限元分析模型并与实验结果进行比较(图6), 得出不同矽晶太阳能电池残留应力分布结果(图7); 3. 将简易残留应力公式(公式(6)和(7))和有限元分析得到的结果进行比较(图9)。

结 论

1. 建立了一套有效模拟矽晶太阳能电池翘曲行为的分析方法, 该方法包含3 个部分: (1)纳米压痕实验测量铝胶和银胶的材料性质; (2)电子显微镜测量细部结构尺寸; (3)非线性有限元分析。利用此方法模拟的翘曲行为较其它计算方法更贴近实验结果。2. 该方法不仅能分析不同矽晶太阳能电池的翘曲行为, 而且能提供除了翘曲以外的其它信息, 比如残留应力。本文提出了2个较为简易的残留应力计算公式, 计算不同矽晶太阳能电池因翘曲而产生的残留应力。3. 方法考虑了银胶对矽晶太阳能电池翘曲的影响, 在实际应用中可以帮助分析不同银胶的网印方式对矽晶太阳能电池翘曲的影响。本文以帮助某公司设计太阳能电池为例, 证明了利用该分析方法在实际应用中帮助公司分析银胶网印的可行性。