Unveiling the reactivity of Cu(In0.7Ga0.3)Se2 Absorbers Induced by High Relative Humidity and Illumination: Insights From Surface to Bulk.

Since Cu(InxGa1 - x)Se2 (CIGS) absorbers are among the most efficient materials for photovoltaic energy conversion, it is crucial to understand their degradation mechanisms and the key factors driving them. To do so, we present aging studies up to 2000 h at 80% relative humidity and 20°C, first under illumination and then in the dark. Three complementary techniques are used to unveil the evolution of the absorber at different probing depths. Surface and subsurface chemical reactivities are investigated, respectively, thanks to XPS (x-ray Photoelectron Spectroscopy) and HaXPES (Hard x-ray Photoemission Spectroscopy) characterizations (probing from 3 up to 41 nm). In addition, EDS (Energy Dispersive Spectroscopy) measurements unveil the bulk chemical reactivity at 0.7 and 1.3 µm. The main result is a greater sensitivity of the absorber's surface when RH is combined with light rather than dark exposure, which triggers the CIGS network faster, leading to oxide phase growth, Se0 side phases appearance, and Na diffusion. This shows that the degradation is boosted in the presence of light. On the other hand, the CIGS bulk results intact around 1 µm under all the ambient conditions investigated.