Cu(In,Ga)Se2 (CIGS) thin-film solar cells are an economical, lightweight alternative solution with superior efficiency to bulk solar cells based on silicon, which have 200 times higher thickness. Figure 1 shows a CIGS solar cell, which comprises a thin-film stack on a substrate, generally glass. The zinc oxide layer and the molybdenum layer make the electrical contacts.
Figure 1. Scanning electron micrograph of a Cu(In,Ga)Se2 solar cell (cross- section) and its mode of operation.
A thin n-type CdS layer forms a p-n junction, with a p-type CIGS film acting as a sunlight absorber layer. Manufacturing methods commonly used include simultaneous or sequential evaporation or sputtering of gallium, indium and copper. The final film composition is established following the reaction of vaporized selenium with the metals.
Controlling the film composition is a key challenge in the fabrication of CIGS solar cells. Reproducing the required layer design in commercial volumes is another critical issue because of the dependability of the cell’s electrical properties on the exact composition of the layers. Both these compositions and the interfacial chemistry of the CIGS solar cells can be determined using XPS depth profiling.
In this experiment, a Thermo Scientific K-Alpha XPS (Figure 2) was employed to depth profile a CIGS solar cell sample using argon ions. A SEM cross-section of the device was used to calibrate the depth scale. The use of the K-Alpha rotating stage to profile the sample provided the optimum depth resolution through a very thick multilayer sample.
Figure 2. The Thermo Scientific K-Alpha XPS.
The sample rotation was carried out off-axis (compucentric), which allows for performing several profiles on the same sample or different samples without removing them from the instrument. Figure 2 presents the images of the etch crater captured with the novel Reflex Optics system of the K-Alpha, subsequent to the depth profile.
Figure 3. 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).
Using the K-Alpha’s two different light sources enables observing the superior quality of the etch crater and various features on the surface. The 128-channel detector was used to acquire the snapshot spectra at each level in the depth profile, enabling acquisition of each spectral region within seconds without compromising the quality of the chemical state information.
The integrated argon ion source is fully computer aligned and controlled, delivering superior ion flux even at low energies. The ready-to-use charge compensation system featured in the K-Alpha simplifies the analysis of insulating samples, while maintaining the analysis conditions stable throughout the profile.
Figure 4 shows the results of the depth profile of a thin film CIGS solar cell, clearly depicting the structure of the solar cell consisting of the upper ZnO layer, the thin CdS layer, the CIGS layer, and the Mo substrate. Composition gradients of gallium (in red) and indium (in blue) can be seen in the CIGS layer, affecting the bandgap of the material.
Figure 4. Depth profile of a CIGS solar cell. The depth scale has been calibrated by using Ta2O5 standard.
From the results, it is evident that the K-Alpha is capable of sputtering complex multi-component films without compromising depth resolution throughout the profile. The results have also shown the change in stoichiometry of the layer close to the interface. This may be due to the interaction with the lower layers and could be crucial for device performance.
This article covered the depth profiling of a CIGS solar cell with the Thermo Scientific K-Alpha XPS instrument. The results clearly demonstrate the ability of this technique to easily determine the multilayer structure of the solar cell, determining and measuring elemental components as a function of depth.
This type of data is useful for all thin film photovoltaic devices. The interfacial chemistry of the thin-film solar cell can be accurately characterized, thanks to the unprecedented depth resolution of the acquired data.
This information has been sourced, reviewed and adapted from materials provided by Thermo Scientific – X-Ray Photoelectron Spectroscopy.
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