Investigating a CIGS Solar Cell with XRD and EDXRF

Copper indium gallium selenide (CIGS) is one of the three most commonly used materials for thin-film solar cells, along with cadmium telluride and amorphous silicon. These types of solar cells offer greater cost efficiency in comparison to bulk solar cells comprised of crystalline silicon because the amount of material needed to produce the solar cells is low. Despite this, these solar cells still exhibit lower cell efficiencies because of less mature upscaling compared to crystalline silicon-based solar cells.

CIGS materials are still intensively investigated to enhance their properties. This investigation is usually carried out through the use of chemical substitution and by designing the interfaces of the solar cell layers.

The typical structure of the device is made up of a glass substrate, or flexible substrates, such as polyimide. A layer of Mo serves as the back electrode, followed by the absorber (CIGS). Then, a buffer layer of CdS and ZnO is used as window material (intrinsic ZnO) and transparent electrode (ZnO:Al).

ARL EQUINOX 100 X-Ray Diffractometer

Figure 1. ARL EQUINOX 100 X-Ray Diffractometer.

In order to guarantee the product’s optimal quality and performance, it is vital that the chemical and structural composition of the product is controlled, as well as the thickness of the layers. The simplest and most convenient solution for routine QC/QA processes, along with more sophisticated analyses in research labs, is seen in both energy dispersive X-ray fluorescence spectroscopy (EDXRF) and X-ray diffraction (XRD) working in tandem.

Using XRD and EDXRF to Investigate a CIGS Solar Cell

About the Thermo Scientific ARL EQUINOX 100 X-Ray Diffractometer

The Thermo Scientific™ ARL™ EQUINOX Series represents a portfolio of XRD instruments, ranging from simple and user-friendly bench-top systems for routine analysis, to advanced, floor-standing, high-performance research-grade systems. The Thermo Scientific™ ARL™ EQUINOX 100 X-ray Diffractometer (c.f. Figure 1) uses a custom-designed Cu (50 W) or Co (15 W) high-brilliance micro-focus tube with mirror optics.

A low-wattage system such as this does not necessitate the addition of external water chillers or other peripheral infrastructure. This allows the instrument to be easily transported between labs, and from laboratories to the field. The Thermo Scientific™ ARL™ EQUINOX 100 X-ray Diffractometer can be equipped with a thin film attachment, providing a computer-controlled ω and ᴢ movement for sample alignment and measurement (c.f. Figure 2).

In comparison to conventional diffractometers, the Thermo Scientific™ ARL™ EQUINOX 100 X-ray Diffractometer provides rapid data collection times due to its unique curved position sensitive detector (CPS), that measures all diffraction peaks in real-time simultaneously. As a result, it is well suited for fast screening of thin-film samples using GIXRD, with screenings typically complete within minutes.

The Thermo Scientific™ ARL™ QUANT’X EDXRF Spectrometer uses a highly sensitive silicon drift detector (SDD) to discriminate between the energy of the incoming radiation. Because of this, the instrument facilitates the measurement of all elements between F (Z = 9) and U (Z = 92). It is also equipped with a 50 W Rh or Ag tube, which is operational at voltages up to 50 kV. The conversion of spectra into elemental or oxide concentrations and layer thickness is made possible with the Fundamental Parameters (FP) analysis program included in the Thermo Scientific™ WinTrace software.

Its rugged and compact design, as well as low demand on additional, peripheral infrastructure, make the ARL QUANT’X a perfect industrial environment solution.

Case Study

A CIGS solar cell pre-assembly sample, consisting of a Mo back electrode and CIGS absorber layers, was measured for 2 minutes using an ARL EQUINOX 100 with Cu-Kα radiation (1.541874 Å) after the careful alignment of ᴢ and ω on the thin film stage.

All data processing and evaluation was carried out with SYMPHONIX for data acquisition and MDI JADE 2010 equipped with the pdf4+ database for the qualitative phase analysis.

Thin film attachment

Figure 2. Thin-film attachment.

Results

Using changing grazing angles in successive GIXRD measurements can allow individual layers to be exhibited exclusively and in sequence from the top to the bottom layers. Only a CIGS phase was shown in a measurement performed with a 1° grazing angle (c.f. Figure 3, green), but increasing a grazing angle of 5° (Figure 3, black) shows additional peaks related to Mo. As a result, the individual investigation of the arrangement and the crystallography of each of the layers is possible.

Table 1 illustrates the results from the EDXRF scans investigating the layers’ elemental concentrations and thickness. The chemical analysis reveals pristine CIGS and thicknesses that are typical for these kinds of modules.

GIXRD pattern (3 - 110° 2?; grazing angle 1° green; 5° black) with 2 min measurement time

Figure 3. GIXRD pattern (3 - 110° 2θ; grazing angle 1° green; 5° black) with 2 min measurement time.

Table 1. Results of the EDXRF measurement. 

Layer Thickness (µm) Elements Concentration (wt%)
CIGS 1.43 Cu 19.50%
Se 46.80%
In 23.50%
Ga 10.20%
Mo 0.34 Mo 100.00%

 

Summary

The ARL EQUINOX 100 X-ray Diffractometer facilitates the acquisition of a full GIXRD scan of CIGS solar cells, all within a measurement time of just two minutes. This allows users to distinguish between different structure types or the amorphous and crystalline character of the sample. It even enables users to distinguish between the different layers in the same module.

Aligning the sample is a straightforward process, and does not require users to already possess advanced knowledge of the operation. EDXRF analysis offers insight into the chemical composition as well as the thickness of the individual layers, which enables users to completely control the quality of the product, as well as track the concentrations of dopants.

Using XRD investigations and EDXRF in conjunction is an easy-to-use solution for basic thin film and coating investigations, not only in industrial applications but in academic research, too. It allows fast and reliable QC/QA procedures, even for untrained operators.

Acknowledgments

Produced from materials originally authored by Dr. Simon Welzmiller and Ju Weicai from Thermo Fisher Scientific.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

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