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Development of plasma cleaning and plasma enhanced close space sublimation hardware for improving CdS/CdTe solar cells




Swanson, Drew, author
Williams, John, advisor
Sampath, W. S., advisor
Sites, James, committee member

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A scalable photovoltaic manufacturing process that employs a heated pocket deposition technique has been developed at Colorado State University. It allows for the economical manufacturing of single-junction thin-film CdTe solar cells with efficiencies over 13%. New techniques that further increase cell efficiency and reduce production expenses are required to make solar energy more affordable. To address this need a hollow-cathode plasma source was added to the load-lock region of the CSU single-vacuum in-line CdTe-cell fabrication system. This plasma source is used to clean the transparent-conductive-oxide layer of the cell prior to the deposition of the CdS and CdTe layers. Plasma cleaning enables a reduction in CdS thickness by approximately 20 nm, while maintaining an improved cell voltage. Cell current was improved and cell efficiency was increased by 1.5%. Maps generated by scanning white-light interferometry, electroluminescence, and light-beam-induced current all show uniformity improvement with plasma cleaning treatment. To further increase cell efficiency a hollow-cathode plasma-enhanced close space sublimation (PECSS) source was utilized to modify the CdS window layer material as it was being deposited. This was done by integrating PECSS into the CSU inline CdS/CdTe-cell fabricating system and by sublimating the CdS semiconductor material through a plasma discharge. To date oxygenated CdS (CdS:O) cells have been grown by sublimating CdS through a PECSS source operated on oxygen. Data are presented showing that PECSS CdS:O films have increased the band gap of the window layer therefore reducing absorption loss, increasing cell current, and improving efficiency by 1.2%.


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plasma cleaning
thin film


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