Chemical structure of a carbon-rich layer at the wet-chemical processed Cu₂ZnSn(S,Se)₄/Mo interface

The carbon-rich layer at the back-contact interface of a solution-processed Cu2ZnSn(S,Se)4 (CZTSSe) absorber is investigated with a combination of surface-sensitive X-ray photoelectron and bulk-sensitive X-ray emission spectroscopy. For absorber deposition, an aqueous ammonium-thioglycolate (ATGL) solution was used, and the 'buried' back-contact interface was accessed by cleaving in a liquid nitrogen environment. In the pertinent literature, it is reported that such a carbon layer at the absorber/back-contact interface could have beneficial effects, e.g., to reduce series resistance or increase the short circuit current. Here, a detailed picture of the chemical structure of this carbon-rich layer at the back contact is derived, which consists of carbon (74 ± 7%), selenium (19 ± 4%), and sulfur (7 ± 3%). The selenium in this layer is found as elemental inclusions, possibly from not fully reacted selenium during the absorber production. The sulfur content in this carbon-rich layer is twice that of sulfur in the absorber. A detailed analysis of the chemical environment suggests that residuals from the aqueous ATGL solution are the origin of sulfur in this carbon-rich layer. Furthermore, underneath the carbon-rich layer, S-Mo bonds are found at the Mo back contact.