Compositional XPS analysis of Cu(In, Ga)Se2 solar cells

Solar cells based on Cu(In, Ga)Se₂ (CIGS) thin films have demonstrated excellent efficiencies and offer a low-cost potential as compared with bulk silicon-based solar cells. The most common manufacturing methods are simultaneous or sequential evaporation or sputtering of copper, indium, and gallium. Vaporized selenium reacts with the metals in order to establish the final film composition. A major challenge in producing solar cells is to control the thin film composition. Reproducibility of the required layer design in commercial volumes has proven to be problematic. This is critical as the electrical properties of the cell depend on the exact composition of the individual layers within the thin film stack. XPS is a highly surface sensitive qualitative and quantitative chemical analysis technique that can provide unique information regarding the surface composition of complex conductive and/or insulating materials. The nominal sampling depth of XPS is ≤ 10 nm; however, in-depth composition can be provided by combining XPS with argon ion sputter depth profiling. Modern XPS instruments are capable of providing high sputter rates and rapid data acquisition along with excellent depth resolution. Therefore, XPS is an ideal analytical technique for investigating the composition of thin film materials by identifying and quantifying elemental and chemical components as a function of depth. In this study, XPS depth profiling was used to determine the composition and interface chemistry through a typical CIGS device. XPS depth profiling results indicated changes in the stoichiometry within the individual thin film layers and at the interfaces. This information could be important for optimizing product performance and for identifying causes for device failures. XPS depth profiling and work function measurements were also used to characterize thin film delamination defects in a damaged CIGS device.