Soft X-ray Spectroscopy of a Complex Heterojunction in High-Efficiency Thin-Film Photovoltaics: Intermixing and Zn Speciation at the Zn(O,S)/Cu(In,Ga)Se2 Interface

The chemical structure of the Zn(O,S)/Cu(In,Ga)Se₂ interface in high-efficiency photovoltaic devices is investigated using X-ray photoelectron and Auger electron spectroscopy, as well as soft X-ray emission spectroscopy. We find that the Ga/(Ga+In) ratio at the absorber surface does not change with the formation of the Zn(O,S)/Cu(In,Ga)Se₂ interface. Furthermore, we find evidence for Zn in multiple bonding environments, including ZnS, ZnO, Zn(OH)₂, and ZnSe. We also observe dehydrogenation of the Zn(O,S) buffer layer after Ar⁺ ion treatment. Similar to high-efficiency CdS/Cu(In,Ga)Se₂ devices, intermixing occurs at the interface, with diffusion of Se into the buffer, and the formation of S-In and/or S-Ga bonds at or close to the interface.

Characterization of Sulfur Bonding in CdS:O Buffer Layers for CdTe-based Thin-Film Solar Cells

On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices.