Influence of Ga and Ag on the KF Treatment Chemistry for CIGS Solar Cells

The KF postdeposition treatment (KF-PDT) is compared on Cu(In,Ga)Se-2 (CIGS) and CuInSe2 (CIS), as well as Ag alloyed CIGS and CIS (ACIGS and ACIS), with the focus on surface chemistry and resulting device performance. Ga does not affect the surface K content; however, X-ray photoelectron spectroscopy shows a preferential reaction of F with Ga, in which CIGS+KF forms GaF3 and CIS+KF forms InF3. KF-treated devices have lower Cu and Ag concentration on the surface, but Cu is reduced more strongly than Ag. Films with Ga show a greater Cu and Ag reduction on the surface than CIS or ACIS. The KF-PDT improves CIS and CIGS device performance and allows for both CIS and CIGS to tolerate reduced CdS thicknesses. In ACIS and ACIGS, device performance decreases with KF, and the open-circuit voltage and fill factor loss is reduced with thinner CdS layers.