X-Ray Emission Spectroscopy of Cu(In,Ga)(S,Se)2-Based Thin Film Solar Cells: Electronic Structure, Surface Oxidation, and Buried Interfaces

Abstract

The electronic and chemical structure of Cu(In,Ga)(S,Se)2 (CIGSSe) thin film surfaces and of relevant interfaces in CIGSSe-based thin film solar cells is investigated with a combination of X-ray emission spectroscopy (XES) and photoelectron spectroscopy. Examples of sulfur L2,3 XES spectra of CdS and CIGSSe are discussed in view of resonant excitation, surface oxidation, and chemical bonding. The combination of the two techniques proves to be a powerful tool to identify spectral features correlated to certain chemical states or bonds. By monitoring these features in interface formation sequences, chemical and electronic information about buried interfaces can be obtained, which will be discussed in detail for the ZnO/CIGSSe interface. The experimental results provide valuable information on the CIGSSe surface and the ZnO/CIGSSe interface and, in general, demonstrate some of the spectroscopic advantages of X-ray emission spectroscopy.