Optimization of CdTe thin-film solar cell efficiency using a sputtered, oxygenated CdS window layer

• Published in: Progress in photovoltaics Vol. 23; no. 11; pp. 1484 - 1492
• Main Authors: Kephart, Jason M., Geisthardt, Russell M., Sampath, W. S.
• Format: Journal Article
• Language: English
• Published: Bognor Regis Blackwell Publishing Ltd 01-11-2015; Wiley Subscription Services, Inc; Wiley Blackwell (John Wiley & Sons)
• Subjects: blue response; Cadmium; Cadmium sulfides; cadmium telluride; Cadmium tellurides; Conduction band; Devices; Oxygenation; Photovoltaic cells; Solar cells; Thin films; thin-film photovoltaics; window layer composition
• ISSN: 1062-7995; 1099-159X
• DOI: 10.1002/pip.2578

A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high‐efficiency devices on tin oxide‐coated soda‐lime‐glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short‐circuit current. Meanwhile, fill factor is maintained, and open‐circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%‐efficient cell on 3.2‐mm soda‐lime glass. This window material was shown to be compatible with SnO2‐based transparent conducting oxide and high resistance transparent coated substrates using in‐line compatible processes. Copyright © 2015 John Wiley & Sons, Ltd
Sputtered, oxygenated CdS has been used with commercial tin oxide‐based transparent conducting oxide/high‐resistance transparent coated substrates to produce cells with efficiency over 15%. Process sensitivities to performance and stability, which include thickness, oxygen content, and copper back‐contacting procedure, are described. As oxygenation increases, the window layer becomes fully transparent, while a conduction band offset leads to a current‐voltage kink and increased VOC.