Formation of Cd x Hg 1-x Te Layers on CdTe after NP-Etching and HgTe-Graphite Pasting
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Formation of CdxHg1-xTe Layers on CdTe after NP-Etching and HgTe-Graphite Pasting Yanfa Yan, K.M. Jones, X. Wu, and M.M. Al-Jassim National Renewable Energy Laboratory (NREL), Golden, CO 80401 ABSTRACT We report on our investigation of the microstructure and chemical composition at the CdTe/Te-rich interfaces generated by NP-etching polycrystalline and single-crystalline CdTe films and followed with HgTe-graphite pasting and thermal annealing. We find that after this process, a thin layer of CdxHg1-xTe forms between CdTe and Te-rich layers, giving a structure like CdTe/Cd xHg1-xTe/Te. High-resolution electron microscopy reveals that the CdxHg1-xTe layer has an epitaxial relationship with the CdTe. No Cd xHg1-xTe layer has been observed in bromine/methanol-etched samples or samples with intentionally deposited Te layers. INTRODUCTION CdTe is a promising candidate for thin-film large-scale solar cells due to its nearly ideal bandgap, high absorption coefficient, and ease of film fabrication. Small-area CdTe/CdS cells with efficiencies of 16.0% have been demonstrated [1, 2]. Among many issues, fabricating a highquality back contact is important for achieving a high-efficiency device, but it has long been challenging [3,4]. A low-quality back-contact often results in the high series resistance of a device. Usually, etching of the CdTe surface using mixed nitric and phosphoric (NP) acids prior to applying the back contact can improve the series resistance associated with the back-contact interface. Thus, NP etching is commonly applied in most high-efficiency CdTe solar cell devices. NP etching is known to deplete a crystalline CdTe surface of Cd and creates a Te-rich layer [5,6]. So far, two scenarios have been proposed for the effects of NP etching on backcontact properties. The first scenario is that the formation of the Te-rich layer improve hole flow and reduce series resistance of the device [7]. The second is that the Te-rich CdTe layers improve conductivity and CdTe device stability by gathering Cu [8]. At NREL, researchers have found that to achieve high efficiency, both NP etching and HgTe graphite pasting are usually required. Though the NP etching is an important process in the CdTe device fabrication, the wet processing is highly unfavorable in industrial large-scale manufacturing. Understanding the mechanism of NP etching and HgTe graphite pasting is important for developing suitable industrial techniques for fabricating high-quality CdTe backcontacts. In this paper, we report on our transmission electron microscopy (TEM) study of the microstructure of the surface of CdTe thin films after NP etching and HgTe graphite pasting. EXPERIMENTAL The polycrystalline CdTe films studied in this paper are deposited on CdS using close-spaced sublimation. The CdS thin layer was grown on SnO2 -coated glass using chemical-bath deposition. Polycrystalline CdTe films and single-crystalline CdTe wafers are etched in an NP solution (HNO3:H3PO4) for 30 s. For comparison, some CdTe thin films were etched in a bromine/met
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