CdS and Cd(OH)2 formation during Cd treatments of Cu(In,Ga)(S,Se)2 thin-film solar cell absorbers

Abstract

The surface modifications induced by treating Cu(In,Ga)(S,Se)2 films in an aqueous ammonia hydroxide-based solution of Cd2+ ions—as used in record Cu(In,Ga)(S,Se)2 solar cells without a CdS buffer layer—have been investigated for different Cd2+ concentrations. Employing a combination of x-ray photoelectron spectroscopy, Auger electron spectroscopy, and x-ray emission spectroscopy, it is possible to distinguish two different surface modifications. For Cd2+ concentrations below 4.5 mM in the solution we observe the formation of a CdS monolayer, while higher Cd2+ concentrations lead to the additional deposition of a cadmium hydroxide film on the CdS/Cu(In,Ga)(S,Se)2 surface.