Analysis of Synchrotron-Based X-Ray Fluorescence Measurements from Multiple Detector Modules

X-ray fluorescence measurements are an ideal method for measuring the elemental distributions in samples but complications like self absorption make the evaluation of these measurements non-trivial – especially when using multiple detector modules. This is further complicated when studying low concentrations of elements, where accurate background subtraction and elemental sensitivity is essential. In the scope of this work, the treatment of data from multiple detector modules was studied. It was found that for relative distributions of elements, it may suffice to leave out self-absorption during the interpretation. But for quantitative results, each detector module’s angle has to be considered to correctly account for self-absorption effects and obtain accurate results. This lead to quantitative stoichiometric ratios of the critical absorber elements inside a Cu(In,Ga)Se2 solar cell that are in good agreement with the nominal ratios. In addition, a variety of fit methods were proposed and analyzed. The usage of the method with the highest sensitivity lead to a successful characterization of a nanowire scan that previously posed a problem due to its low counts. Although the problem of accurate background separation could not be fully eliminated, a possible source of it was found and discussed. Finally, due to usage of the data from the multiple detector modules used during the measurement of the nanowire, the sensitivity was increased, reducing the problem of background separation further. This lead to the detection of bismuth, which is essential for evaluations of further measurements

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