Monitoring chemical reactions at a liquid–solid interface: Water on CuIn(S,Se)2 thin film solar cell absorbers

The chemical and electronic structure of the interface between liquid water and a CuIn(S,Se)2 thin film surface was studied with synchrotron-based, high energy-resolution soft x-ray emission spectroscopy (XES). By probing the local environment around the sulfur atoms, an x-ray-induced sulfate formation at the CuIn(S,Se)2 surface can be monitored, correlated with a substantial enhancement of sodium impurity atoms from the CuIn(S,Se)2 film and its glass substrate. The results demonstrate that, with XES, an experimental probe is available to in situ study chemical reactions at liquid–solid interfaces or at surfaces in a high-pressure gas environment in a chemically sensitive and atom-specific way.