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 L 2,3 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.

RIXS investigations of liquids, solutions, and liquid/solid interfaces

“…Soft X-ray techniques have proven to be very powerful for the investigation of applied systems such as thin-film solar cells [66][67][68][69][70][71], since they give detailed information about the local chemical environment of specific elements and the band gap in the surface-near bulk region [72]. In such applied systems, it would be especially interesting to perform in situ investigations, e.g., during wet-chemical treatment or deposition steps, of catalysts in operation, of batteries during charge/discharge cycles, and many more.…”

Spectroscopic investigation of the deeply buried Cu(In,Ga)(S,Se)2∕Mo interface in thin-film solar cells

“…XES spectra provide detailed information about the local chemical bonding of the S atoms. 22,23 The spectrum of the CIGSSe front side shown in Fig. 4͑c͒ consists of the dominating S 3s → S 2p transitions labeled ͑1͒ and of the emission from the upper valence band filling the 2p holes, where transitions from In 5s-and Ga 4s-derived states labeled ͑2͒ and Cu 3d-derived states labeled ͑3͒ are visible.…”

Synchrotron-based spectroscopy for the characterization of surfaces and interfaces in chalcopyrite thin-film solar cells

“…Indium 5s derived valence states decaying into sulfur 2p core holes (indicative of S-In bonds [78]) and sulfur 3s-like electrons in hybrid valence orbitals of sulfur-oxygen bonds [79] can be responsible for signals around 155 eV. The feature at 160 eV is attributed to valence states with a strong copper 3d character (S-Cu bonds) [80]. One should keep in mind that S L 2,3 XES has an information depth of about 60 nm.…”