Abstract
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.
Original language | English |
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Pages (from-to) | 1749-1758 |
Number of pages | 10 |
Journal | ACS Applied Energy Materials |
Volume | 3 |
Issue number | 2 |
DOIs | |
State | Published - Feb 24 2020 |
Funding
This work at the University of Utah was supported in part by NSF, grant ECCS 1711885. The STEM-EELS characterization of the samples was performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This work is supported in part through the INL Laboratory Directed Research & Development (LDRD) Program under DOE Idaho Operations Office Contract DE-AC07-05ID145142. The authors at Colorodo State University acknowledge support from NSF AIR, NSF I/UCRC and DOE SIPS programmes. The work at Colorado State University was supported by NSF award 1540007, NSF PFI: AIR-RA program 1538733 and DOE awards DE-EE0008177 and DE-EE0008557.
Keywords
- aberration-corrected STEM
- electron energy-loss spectroscopy
- grain boundary
- polycrystalline CdTe
- thin-film solar cell
- wet etching