Chemical and Electronic Properties of the Mg_xZn_1-xO/CuGa₃Se₅ Interface with a Cd²⁺-Treatment

Ordered-vacancy wide band gap CuGa₃Se₅ is a promising candidate as absorber material for the top cell in tandem solar cells as well as in photoelectrochemical water-splitting devices. For wide band gap absorber materials, it is increasingly important to also use a wide band gap buffer layer to avoid absorption losses. Instead of the traditionally used CdS buffer layer, a conductive oxide Mg_xZn_1-xO (MZO) was thus chosen in this study, deposited by RF sputtering on the absorber. To further improve the interface properties, a Cd²⁺ partial electrolyte treatment (CdPE) can be applied to the CuGa₃Se₅ surface prior to MZO layer deposition. Further optimization of the interface requires a more detailed understanding of the chemical and electronic properties at the MZO/absorber interface, such as the band edge positions, band gaps at the interface and, in particular, the band alignment.<br/><br/>In this contribution, we investigate the CuGa₃Se₅ absorber after CdPE, as well as its interface with the MZO layer using a toolchest of electron spectroscopies, including laboratory-based x-ray and UV photoelectron spectroscopy (XPS and UPS), x-ray-excited Auger electron spectroscopy (XAES), and inverse photoemission spectroscopy (IPES). With this combination of techniques, a detailed electronic and chemical picture of the interface is painted and will be discussed in view of its impact on the performance of CuGa₃Se₅ solar devices.