K-Alpha: Compositional XPS Analysis of a Cu(In,Ga)Se2 Solar Cell

Importance of the Topic

Thin-film solar cells based on CIGS alloys offer high photovoltaic efficiency combined with low material usage and lightweight design. Understanding the chemical composition and layer interfaces is critical to optimize performance and reproducibility.

Objectives and Study Overview

This study employs XPS depth profiling to map elemental distribution across a CIGS solar cell stack. The analysis encompasses the Mo back contact, CIGS absorber with graded composition, CdS buffer, and ZnO window layers.

Methodology

The sample was sputtered using argon ions in a calibrated routine guided by SEM cross-section measurements. Spectra were captured at successive depths with a rotating stage to maintain uniform etching and maximize depth resolution.

Instrumentation

• Thermo Scientific K-Alpha XPS system
• Argon ion sputter source with computer alignment and low energy capability
• 128-channel electron energy analyzer for rapid spectral acquisition
• Reflex Optics imaging module for etch crater visualization
• Rotating sample stage operating off-axis for consistent profiling

Main Results and Discussion

Depth profiles clearly resolve individual layers and reveal composition gradients of gallium and indium within the absorber. Stoichiometry shifts near interfaces indicate possible interlayer reactions that may affect device bandgap and charge transport.

Benefits and Practical Applications

This approach provides precise depth-resolved chemical information essential for quality control in thin-film PV manufacturing. It enables detection of interfacial chemistry changes that influence device efficiency.

Future Trends and Potential Applications

Advancements may include integration of in situ monitoring, lower-damage sputtering techniques, and coupling with complementary imaging modalities. These developments will further enhance understanding of multilayer thin-film systems.

Conclusion

XPS depth profiling with the K-Alpha system offers detailed compositional mapping of CIGS solar cells. The method supports optimization of thin-film architectures and interface engineering for improved photovoltaic performance.

Reference

1. D. Abou-Ras et al. Elemental distribution profiles across Cu(In,Ga)Se2 solar-cell absorbers acquired by various techniques. In: M. Luysberg, K. Tillmann, T. Weirich (Eds.), EMC 2008, Vol 1: Instrumentation and Methods, Proc. 14th European Microscopy Congress 2008, Aachen, Germany, Sept 1–5, 2008, Springer, 2008, p. 741.