TEM Studies of High-Efficiency CdTe Solar Cells on Commercial SnO 2 /Soda-Lime Glass
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TEM Studies of High-Efficiency CdTe Solar Cells on Commercial SnO2/Soda-Lime Glass Yanfa Yan, X. Wu, J. Zhou, and M.M. Al-Jassim National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA ABSTRACT The microstructure of high-efficiency CdTe solar cells on commercial SnO2/soda-lime glass was investigated by scanning transmission electron microscopy. The CdTe solar cells have a structure of soda-lime glass/SnO2/ZTO/CdS:O/CdTe. We found no interdiffusion between the SnO2 layer and ZTO layer. Weak diffusion of Zn from the ZTO layer into the CdS:O layer was observed; however, the diffusion was not uniform. Interdiffusion also occurred at the CdTe/CdS:O interface. In the back-side of the CdTe, a thin layer of Te was found, which formed during the nitric-phosphoric etching. In addition, a very thin layer of CdHgTe was observed at the CdTe/Te interface.
INTRODUCTION Cadmium telluride has been recognized as a promising photovoltaic material for thin-film solar cells because of its near-optimum bandgap of ~1.48 eV and its high direct-absorption coefficient. Commercial-scale CdTe modules with efficiencies of 6%–9% have been produced by several CdTe deposition techniques. Recently, NREL researchers developed a manufacturing-friendly process for fabricating high-efficiency CdTe devices on low-cost commercial SnO2/soda-lime (SL) glass substrates [1]. In this process, a nanostructured CdS window layer replaced the poly-CdS film in the conventional device structure, and a zinc-tinoxide film was integrated into a CdTe cell as a buffer layer located between the SnO2 and the nano-CdS:O film. The new process significantly improves short-circuit current density (Jsc) through the reduction of the CdS thickness. In addition, this process excludes conventional timeconsuming and expensive heat-up and cool-down segments that limit throughput and yield, and some “wet” processes that generate a large amount of liquid waste solution. The devices fabricated by this process achieved NREL-confirmed total-area efficiencies of more than 14%. In this paper, we report on our detailed microstructure analysis of high-efficiency CdTe solar cells on commercial SnO2/SL glass as investigated by scanning transmission electron microscopy (STEM).
EXPERIMENTS Cross-sectional TEM specimens were prepared by sandwiching together the device in two pieces of Si wafers. The sandwiches were first mechanically polished to ~100 µm thickness and then dimpled down to ~5 µm. The samples were subsequently thinned to electron transparency using a 4-kV Ar ion-beam at 13° inclination, then cleaned at a lower voltage (1.5 kV). STEM images and X-ray energy dispersive spectra (EDS) data were taken on a FEI Tecnai F20-UT microscope. The probe size of the electron beam was about 2 nm for the EDS.
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RESULTS The novel CdTe device manufacturing process has several important features. First, the modified SL glass/SnO2/ZTO/nano-CdS:O/CdTe device structure developed at NREL was used in this process. In the NREL device structure, a nanostructured CdS (nano-CdS:O)
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