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.

Automated summary

A carbon-rich layer at the back-contact interface of a solution-processed CZTSSe absorber was investigated, revealing a detailed chemical structure consisting of carbon, selenium, and sulfur. Selenium was found as elemental inclusions, possibly due to incomplete reaction during absorber production, while sulfur was traced back to residuals from the aqueous ATGL solution. Additionally, S-Mo bonds were found underneath the carbon-rich layer at the Mo back contact.