Chemical and Electronic Properties of an Exfoliated Cu(In,Ga)Se₂-Based Thin-Film Solar Cell

Cu(In,Ga)Se₂ (CIGSe) thin-film solar cells have shown high energy-conversion efficiency, scalability, low material consumption, and can be produced on flexible material, altogether making them a continued appealing option for photovoltaic devices. In the area of photoelectrochemical water splitting (PEC), the tunability of their band gap makes them an important component in a tandem device architecture.

In this investigation, a CIGSe-based thin-film solar cell was exfoliated between the CIGSe absorber and the Mo back contact, resulting in two separate sample surfaces for analysis. Mechanical exfoliation techniques for thin-film semiconductors involving physical cleavage between distinct layers can be used to separate sequentially deposited layers with the goal of stacking them in tandem devices. Furthermore, this separation technique provides a means by which regions of complete devices can be analyzed that are typically inaccessible, allowing for crucial insights into their chemical and electronic properties.

We have utilized x-ray photoelectron spectroscopy (XPS), x-ray-excited Auger electron spectroscopy (XAES), ultraviolet photoelectron spectroscopy (UPS), and inverse photoemission spectroscopy (IPES) to perform an in-depth chemical and electronic structure characterization of the back of the CIGSe absorber and the surface of the Mo back contact. XPS spectra reveal strong Se-related signals on the Mo back contact surface, indicative of the formation of a MoSe₂ interfacial layer, which facilitates mechanical separation between the CIGSe and MoSe₂ surfaces. In analyzing the chemical and electronic properties of these surfaces, secondary constituents, such as C and Na, need to be taken into account, especially when interpreting the UPS and IPES spectra of CIGSe for a determination of the valence band maximum (VBM) and conduction band minimum (CBM). The obtained results will be discussed in view of the possible band alignment at the interface before exfoliation and their integration into a stacked PEC device.