Coupling of X-AES Transitions and XPS Photopeaks to Assess the Oxide Formation of Ga and in CuIn_0.7Ga_0.3Se₂ Material During Air Aging

The solar absorber Cu (In_0.7Ga_0.3)Se₂ (CIGS) undergoes a process of evolution upon exposure to the atmosphere, resulting in the growth of oxide phases. This phenomenon can potentially affect the interfacial properties of CIGS, which in consequence may impact the efficiency of the solar cell. X-ray photoelectron spectroscopy (XPS) is an appropriate method to analyze the degradation of CIGS upon air aging. However, many photopeaks and Auger lines of the constitutive elements are distributed along the energy scale, and the exact determination of the degradation within the CIGS absorber requires specific care to select peaks to consider to ensure that information arise from similar escape depths. In this study, we propose to investigate the kinetics of degradation of Ga and In at similar depths probed by coupling not only photopeaks but also X-Auger electron spectroscopy (X-AES) transitions in the absence of photopeaks in the same energy range. If photopeaks modeling is well established for In and Ga, a decomposition procedure of the X-AES transitions must be developed. Both linear and nonlinear least square fitting were used and compared, starting to model CIGS, In₂O₃, and Ga₂O₃ references to deploy it after on Auger transitions measured on aged samples. Thanks to the determination of the degradation ratios (oxide phase over CIGS phase) at 3, 7, and 9 nm depth, we show that both In and Ga exhibit similar kinetic of oxide formation, which proceeds gradually by O penetration through the subsurface of the material, this penetration being more and more attenuated deeper.