RF sputter etch as a surface cleaning process for CdTe solar cells

Thin film CdTe/CdS solar cells benefit from the use of a wet chemistry step prior to the application of the back contact electrode. A nitric-phosphoric acid mixture and a bromine solution are the most commonly used etchants. Following the CdCI₂ heat treatment the CdTe/CdS structures are briefly etched in one of the above solutions in order to remove native oxides and leave the CdTe surface Te-rich. A higher carrier concentration at the surface of CdTe is believed to be a key factor in facilitating the formation of an effective back contact. The primary objective of this work is to investigate the potential of an alternative dry, vacuum-based process, as a surface modification step for CdTe prior to the application of the back contact electrode. RF-sputter etch has been used as an alternative surface cleaning process. The process was optimized for the type of etchant gas, pressure, time, and sputter power. Solar cells exhibited V_OC´s greater than 830 mV and FF´s in the high 60´s. This performance demonstrates the potential of this process for the development of an all-dry vacuum based CdTe technology.