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Cadmium Selective Etching in CdTe Solar Cells Produces Detrimental Narrow-Gap Te in Grain Boundaries
ACS Applied Energy Materials ( IF 5.4 ) Pub Date : 2020-01-28 00:00:00 , DOI: 10.1021/acsaem.9b02224
Sudhajit Misra 1 , Jeffery A. Aguiar 2 , Sophia Gardner 3 , Xiahan Sang 4 , Raymond R. Unocic 4 , Amit Munshi 5 , Walajabad S. Sampath 5 , Chris S. Ferekides 6 , Michael A. Scarpulla 1, 3
Affiliation  

Recent advances in design and processing technology have made possible commercialization of polycrystalline (px)-CdTe as a photovoltaic absorber. Grain boundaries (GBs) are the most prominent structural defects in these devices and undergo significant changes during device fabrication. However, the effects of device fabrication processes on these GBs are not entirely understood. Prevailing models of GBs in thin-film photovoltaics consider individual GBs to have homogeneous properties in their area. Here, using an aberration-corrected scanning transmission electron microscope (STEM)-based low-loss and core-loss electron energy-loss spectroscopy (EELS), we show that back-surface etching of CdTe leads to inhomogeneity within individual grain boundaries. We observe that etching the back surface leads to the conversion of a region of GBs from CdTe to an elemental Te, which has an only 0.33 eV band gap, as deep as 1 μm from the back surface. The presence of elemental Te in GBs this deep into the absorber layer will increase recombination in the absorber layer and limit the extractable open-circuit voltage, thus reducing device efficiency. However, additive methods for back contact formation such as deposition of Te, ZnTe, or other materials preserve the CdTe stoichiometry of the GBs. Thus, especially for the next generations of CdTe-based cells having longer minority carrier diffusion length and/or thinner absorber layers, additive back contacting methods are superior.

中文翻译:

CdTe太阳能电池中的镉选择性蚀刻在晶界产生有害的窄间隙Te。

设计和加工技术的最新进展已使多晶(px)-CdTe作为光伏吸收剂的商业化成为可能。晶界(GB)是这些器件中最突出的结构缺陷,在器件制造过程中会发生重大变化。但是,尚未完全了解器件制造过程对这些GB的影响。薄膜光伏中GB的流行模型认为单个GB在其区域内具有均一的特性。在这里,使用基于像差校正的扫描透射电子显微镜(STEM)的低损耗和芯损耗电子能量损耗光谱(EELS),我们显示了CdTe的背面蚀刻会导致各个晶界内的不均匀性。我们观察到蚀刻背面会导致GBs区域从CdTe转换为元素Te,该元素Te的带隙只有0.33 eV,距离背面深1μm。深入吸收层的GBs中的Te元素会增加吸收层中的复合并限制可提取的开路电压,从而降低器件效率。但是,用于背面接触形成的附加方法(例如Te,ZnTe或其他材料的沉积)可以保留GBs的CdTe化学计量。因此,特别是对于具有更长的少数载流子扩散长度和/或更薄的吸收层的下一代CdTe基电池而言,加性背接触法是更好的选择。深入吸收层的GBs中的Te元素会增加吸收层中的复合并限制可提取的开路电压,从而降低器件效率。但是,用于背面接触形成的附加方法(例如Te,ZnTe或其他材料的沉积)可以保留GBs的CdTe化学计量。因此,特别是对于具有更长的少数载流子扩散长度和/或更薄的吸收层的下一代CdTe基电池而言,加性背接触法是更好的选择。深入吸收层的GBs中的Te元素会增加吸收层中的复合并限制可提取的开路电压,从而降低器件效率。但是,用于背面接触形成的附加方法(例如Te,ZnTe或其他材料的沉积)可以保留GBs的CdTe化学计量。因此,特别是对于具有更长的少数载流子扩散长度和/或更薄的吸收层的下一代CdTe基电池而言,加性背接触法是更好的选择。
更新日期:2020-01-28
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