With their lightweight and flexible structures, thin-film solar cells represent a promising alternative to traditional silicon solar cells. One common type of semiconductor used to manufacture these devices is copper indium gallium diselenide (CIGS), a material that has demonstrated excellent power conversion efficiency.

Seeger et al. describe a spectroscopic technique for the non-destructive analysis of the individual layers in thin-film solar cells. Called angle-resolved electroreflectance spectroscopy (ARER), this method allows for the determination of bandgap energies of very thin (∼25-50 nm) zinc sulfide oxide [Zn(O,S)] buffer layers in CIGS solar cells.

Traditional electroreflectance spectroscopy measures how the reflectance of a structure changes due to an applied electrical bias. However, multiple reflections within the layered device architecture lead to distortions in the experimental spectra, masking the signal of the tiny buffer.

The researchers measured a set of ARER spectra and numerically calculated an averaged modulus spectrum by applying a special transformation and averaging procedure. The resulting undistorted spectra could then be used to obtain buffer bandgap energies and provide information about the composition of the buffer layer, and could also detect unwanted secondary phases in the solar cells.

The researchers found that ARER can suppress distortions better than previous modifications of electroreflectance spectroscopy and help optimize the fabrication process of CIGS solar cells. The new method has important applications for research on cadmium-free buffer layers, which utilize environmentally friendly materials like Zn(O,S) with properties that can improve the efficiency of CIGS devices.

Source: “Averaged angle-resolved electroreflectance spectroscopy on Cu(In,Ga)Se2 solar cells: Determination of buffer bandgap energy and identification of secondary phase,” by Jasmin Seeger, Jonas Grutke, Nico Weber, Stefan Schützhoff, Xiaowei Jin, Reinhard Schneider, Dagmar Gerthsen, Wolfram Witte, Dimitrios Hariskos, Oliver Kiowski, Manuel Schweiger, Heinz Kalt, and Michael Hetterich, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5123380.