Wide band-gap Cu-chalcopyrite/In-based transparent conductive oxide interfaces: What affects the gallium oxide formation and its properties?

Thin film (Ag,Cu)(In,Ga)(SSe)2 has promising potential applications as a top cell in tandem solar cell devices. The chalcopyrite film must be deposited on a transparent back contact (TBC) for such applications, and therefore the well-established back contact structure must be re-examined and reoptimized. In this contribution, we examine the formation of gallium oxide interface species and the impact of process variables on the resulting chemical and electronic structure. By cleaving the absorber/TBC layer stack, the backside of the absorber and the exposed TBC can be made accessible to be characterized by surface-sensitive energy dependent X-ray photoelectron spectroscopy (XPS). Significant changes in the thickness of the oxide layers were revealed as a function of absorber Ga content, Na content, and other variables related to synthesis. The link between oxide properties and device performance will be discussed.