Selenium chemistry modifications of Cu(In,Ga)Se₂ surfaces after bromine etch. High resolution XPS studies of buried interface with chemically deposited CdS

The formation of the interface Cu(In,Ga)Se₂ (CIGS)/CdS in solar cells is not yet well fully understood but found crucial in fixing the final device quality. This interface depends on various parameters such as the initial chemical state of the CIGS surface or the chemical bath deposition (CBD) conditions used to grow the CdS layer. In this paper, XPS studies of buried interfaces were carried out by using gradual sputter etch. We have investigated interfaces on CIGS submitted or not to an aqueous bromine solution treatment. This chemical treatment leads to the formation of a Se° film on the CIGS surface, leaving a specular surface, which can be removed by a KCN treatment. The Se3d core level, which is probably the most interesting probe to provide a fine XPS characterization of the CIGS surface, was used to follow a chemical reactivity of interface during CdS growth. In this work, we present a detailed study of the evolution of the Se signal at CIGS surface and across the CIGS/CdS interfaces.We discuss problems of nucleation and growth of CdS layer on well defined CIGS surfaces.