Depth Profile Analysis of Thin Film Solar Cells by XPS

The depth profiling through the thin film stack was performed using the Thermo Scientific K-Alpha XPS, which analyzed the elemental and chemical structure of Cu(In,Ga)Se₂ (CIGS) solar cells which are the best alternative to thick silicon cells as they are efficient, cost-effective and light-weight.

CIGS solar cells, as seen in Figure 1 are made up of a thin-film stack on a substrate. In order to produce thin film solar cells, it is important to control film composition. Reproducing the needed layer design in commercial volumes is a major challenge. This is a critical factor since the electrical properties of the cell are based on the exact layer composition. Both the composition through the device and the interfacial chemistry can be determined by XPS depth profiling.

Figure 1. Scanning electron micrograph of a Cu(In,Ga)Se₂ solar cell (cross-section) and its mode of operation.

Experimental

The Thermo Scientific K-Alpha XPS was used to depth profile the CIGS sample. Argon ions and the K-Alpha rotating stage were used to profile the sample to get the best depth resolution in a thick multilayer sample. An SEM cross-section of the device was used for calibration. The off-axis sample rotation enabled several profiles to be carried out on the different or the same sample without removing them from the instrument. Figure 2 shows the images taken with K-Alpha’s unique Reflex Optics system following depth profiling. The two different light sources of K-Alpha enables the user to view a range of features and the high quality of the etch crater on the surface. Snapshot spectra acquired using the 128- channel detector in the depth profile at each level enables the collection of spectral regions in seconds while the optimal chemical state information is maintained. Excellent ion flux even at low energies is possible with the computer aligned and integrated argon ion source.

Figure 2. CCD images of the etch crater at the end of the profile, taken using the K-Alpha Reflex Optics and the co-axial lighting with the side lighting (left image) and just the side lighting (right).

Analysis of insulating samples is made simple and also maintains stable analysis conditions throughout the profile by a simple turn-key charge compensation system in K-Alpha.

Results

K-Alpha’s ability to maintain the excellent depth resolution throughout the profile and sputter complex multi-component films has been demonstrated. Figure 3 shows the results of the depth profile of a thin film CIGS solar cell. A change in stoichiometry of the layer near the interface is probably caused by interaction with the lower layers.

Figure 3. Depth profile of a CIGS solar cell. The depth scale has been calibrated by using Ta₂O₅ standard.

Summary

The Thermo Scientific K-Alpha XPS instrument was used to depth profile a CIGS solar cell. By quantifying and identifying elemental components as a function of depth, this technique can be easily used to determine the multilayer structure of the solar cell. Accurate characterization of the interfacial chemistry is possible by the excellent depth resolution of the acquired data. This information is applicable for all thin film photovoltaic devices.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).

For more information on this source, please visit Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).