Cadmium Selective Etching in CdTe Solar Cells Produces Detrimental Narrow-Gap Te in Grain Boundaries
Cadmium Selective Etching in CdTe Solar Cells Produces Detrimental Narrow-Gap Te in Grain Boundaries
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CdTe 太阳能电池中的镉选择性蚀刻会在晶界中产生有害的窄带隙 Te
DOI：
10.1021/acsaem.9b02224
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发表时间：
2020
影响因子：
6.4
通讯作者：
Scarpulla, Michael A.
中科院分区：
文献类型：
--
作者：
Misra, Sudhajit;Aguiar, Jeffery A.;Gardner, Sophia;Sang, Xiahan;Unocic, Raymond R.;Munshi, Amit;Sampath, Walajabad S.;Ferekides, Chris S.;Scarpulla, Michael A.
设计和加工技术的最新进展使多晶(px)-CdTe作为光伏吸收剂的商业化成为可能。晶界（GBs）是这些器件中最突出的结构缺陷，在器件制造过程中发生重大变化。然而，器件制造工艺对这些gb的影响尚不完全清楚。薄膜光伏中的主流GBs模型认为单个GBs在其区域内具有均匀性。在这里，使用基于像差校正扫描透射电子显微镜（STEM）的低损耗和核心损耗电子能量损耗谱（EELS），我们发现CdTe的背表面蚀刻导致单个晶界内的不均匀性。我们观察到，在背表面蚀刻后，GBs区域从CdTe转变为元素Te，其带隙仅为0.33 eV，距背表面深度为1 μm。在吸收层深处的gb中元素Te的存在将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。然而，用于反向接触形成的添加剂方法，如Te、ZnTe或其他材料的沉积，保留了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，加性背接触方法是优越的。
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.
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