Cadmium Selective Etching in CdTe Solar Cells Produces Detrimental Narrow-Gap Te in Grain Boundaries

Journal Article · Tue Jan 28 00:00:00 UTC 2020 · ACS Applied Energy Materials

DOI: https://doi.org/10.1021/acsaem.9b02224 · OSTI ID:1631240

Misra, Sudhajit  ^[1]; Aguiar, Jeffery A.  ^[2]; Gardner, Sophia ^[1]; Sang, Xiahan  ^[3]; Unocic, Raymond R.  ^[3]; Munshi, Amit ^[4]; Sampath, Walajabad S. ^[4]; Ferekides, Chris S. ^[5]; Scarpulla, Michael A.  ^[1]

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1. Univ. of Utah, Salt Lake City, UT (United States)
2. Idaho National Lab. (INL), Idaho Falls, ID (United States)
3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
4. Colorado State Univ., Fort Collins, CO (United States)
5. Univ. of South Florida, Tampa, FL (United States)

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.

• Research Organization: Colorado State Univ., Fort Collins, CO (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE); USDOE Office of Enterprise Assessments (EA); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• Contributing Organization: Oak Ridge National Laboratory; University of Utah; University of South Florida
• Grant/Contract Number: AC05-00OR22725; EE0008557
• OSTI ID: 1631240
• Journal Information: ACS Applied Energy Materials, Journal Name: ACS Applied Energy Materials Journal Issue: 2 Vol. 3; ISSN 2574-0962
• Publisher: American Chemical Society (ACS)Copyright Statement
• Country of Publication: United States
• Language: English