The effect of stoichiometric variations on performance in CIGS solar cells measured using X-ray microscopy

Scanning X-ray microscopy measurements are an indespensible tool of solar cell research to-day, and the compatibility with complementary measurement modes is a particular strength. This Bachelor’s thesis investigates a series of thin-film solar cells with a Cu(In,Ga)Se2(CIGS) absorber layer that differ in their overall ratio of gallium to indium, resulting in a varyingbandgap. Additionally, CIGS solar cells suffer from local fluctuations in the elemental con-centration which are of special interest in CIGS research, as they can be both beneficial anddetrimental to the solar cell performance. X-ray beam induced current (XBIC) and X-ray fluorescence (XRF) were assessed subsequentlyas a measure for the charge collection efficiency and elemental distribution, respectively, the first measurement being optimized for X-ray beam induced current and the second for X-rayfluorescence. Image registration served as a tool to align the two-dimensional datasets from XBIC and XRF measurements at nanoscale. The Enhanced Correlation Coefficient(ECC) maximization method was utilized and the results were evaluated with respect to the effect of Gaussian filters, resultingin an optimal alignment of two-dimensional maps for each solar cell of interest. This allowedfor the examination of the impact of stoichiometric variations on the solar cellperformance, employingk-means clustering and point-by-point correlation. As a result, a higher gallium-to-indium ratio and an increased concentration ofrubidium werefound to be locally present in low-performance regions of the solar cells, regardless of their over-all gallium-to-indium ratio. Regions with similar gallium-to-indium ratio can be high- or low-performing depending on the overall gallium-to-indium ratio the solar cell was produced with. Rubidium accumulates alongside grain boundaries and mitigates recombination losses

Image Registration in Multi-Modal Scanning Microscopy: A Solar Cell Case Study

Scanning probe measurements are an indispensable tool of solar cell research today, and the compatibility with simultaneous acquisition of complementary measurement modes is a particular strength. However, multi-modal data acquisition is often limited by different scan-parameter requirements. As a consequence, the modalities may be assessed subsequently rather than simultaneously. In this instance, image registration serves as a tool to align two-dimensional datasets at nanoscale. Here, we showcase an example of two subsequent scanning Xray microscopy measurements of solar cells with a Cu(In,Ga)Se2 absorber, the first measurement being optimized for X-ray beam induced current and the second for X-ray fluorescence. We discuss different approaches and pitfalls of image registration and its potential combination with Gaussian filtering. This finally allows us to proceed with the investigation of point-by-point correlations

Quantifying the Elemental Distribution in Solar Cells from X-Ray Fluorescence Measurements with Multiple Detector Modules

Within the analysis of solar cells with multi-modalX-ray microscopy, X-ray fluorescence (XRF) measurements havebecome a reliable source for evaluating elemental distributions.While XRF measurements can unveil the elemental distributionat unparalleled sensitivity and spatial resolution, the quantitativeanalysis is challenged by effects such as self-absorption and furthercomplicated by the inclusion of multiple detector modules.Here, we showcase the exemplary analysis of XRF spectraobtained from a Cu(In,Ga)Se2 solar cell utilizing four detectormodules. After cataloging typical features found in XRF spectra,we demonstrate the inclusion of detector modules with individualabsorption correction. This results in quantitative stoichiometricratios of the critical absorber elements Cu, In, and Ga that arein good agreement with the nominal ratios.These results are particularly relevant in view of futuremeasurements at diffraction-limited synchrotron beamlines: inorder to profit from the boost of nano-focused photon flux, XRFmeasurements will require multiple detector modules, for whichwe demonstrate an approach of quantitative analysis

Elucidating Materials Paradigm of CIGS by Structure-Composition-Performance Correlations

Recent developments in focusing hard X-rays to nanoscale beams have enabled scanning X-ray microscopy modalities and their simultaneous exploitation in multi-modal measurement campaigns. Specifically, X-ray beam induced current and X-ray fluorescence measurements have been established for the correlation of the electrical performance with the distribution of absorber and trace elements for thin-film solar cells with absorbers from CIGS to CdTe and perovskites. For CIGS, the composition is in an especially complex interplay with the synthesis conditions and the crystallographic structure due to the tetragonal lattice distortions, steep vertical In/Ga gradients, and lateral inhomogeneities that introduce lattice strain and structural defects. For this contribution, we have added scanning X-ray nano-diffraction to the multi-modal envelope of scanning X-ray microscopy to assess crystallographic properties of a solar-cell series with a varying In/Ga ratio. For the first time, this combination has been used to characterize a statistically significant number of CIGS grains embedded in as-deposited solar cells: mapping out the real and reciprocal space, we have isolated nearly 500 individual grains. This enabled us to elucidate Materials Paradigm of CIGS, by (1) correlating the lateral Cd and In/Ga distribution with the local performance and lattice spacing with unprecedented sensitivity, (2) differentiating voids in the absorber layer that appear (not) to be filled with CdS, and (3) evaluating the crystallographic properties including the grain orientation and grain-boundary classification with sub-grain resolution and powerful statistics in fully assembled devices. In the full presentation, we will elaborate on our methodological advances and unveil performance-relevant findings from the CdS coverage to the strain distribution at small- and large-angle grain boundaries. Beyond applications to CIGS, our work highlights the latest developments in the field of X-ray imaging and paves the way for advanced correlative nanoscopy at diffraction-limited storage rings that will become operational within the next few years

Comparison of XBIC and LBIC measurements of a fully encapsulated c-Si solar cell

A fully encapsulated c-Si solar cell was evaluatedusing focused X-ray and laser beams to probe the microscopicelectrical performance and composition. Particular emphasiswas placed on the influence of the silver fingers on the laser(LBIC) and X-ray beam induced current (XBIC). Therefore,an uncommonly high X-ray energy of 28 keV was utilized formaximum sensitivity to the Ag distribution measured by X-rayfluorescence through the back sheet. The direct comparison ofLBIC and XBIC measurements yields a comprehensive pictureof these techniques, illustrating the advantages and challenges ofboth approaches. Specifically, the effect of heavy elements actingas a secondary photon source that increase the XBIC signal isdiscussed and supported by Monte-Carlo simulations

Four-Fold Multi-Modal X-Ray MicroscopyMeasurements of a Cu(In,Ga)Se2_2 Solar Cell

Inhomogeneities and defects often limit the overall performance of thin-film solar cells. Therefore, sophisticated microscopy approaches are sought to characterize performance and defects at the nanoscale. Here, we demonstrate, for the first time, the simultaneous assessment of composition, structure, and performance in four-fold multi-modality. Using scanning X-ray microscopy of a Cu(In,Ga)Se$_2$ (CIGS) solar cell, we measured the elemental distribution of the key absorber elements, the electrical and optical response, and the phase shift of the coherent X-rays with nanoscale resolution. We found structural features in the absorber layer—interpreted as voids—that correlate with poor electrical performance and point towards defects that limit the overall solar cell efficiency

Erratum: Ossig, C., et al. Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In, Ga)Se₂ Solar Cell. <i>Materials</i> 2021, <i>14</i>, 228

In the original version of our article [...