Bradley West

Latest developments in the x-ray based characterization of thin-film solar cells

We present the latest developments in the characterization of thin-film solar cells based on the combination of elemental mapping from fluorescence measurements using synchrotron x-rays, with beam induced current from electron and x-ray beams. This is a powerful method to directly correlate compositional variations with charge collection efficiency. We compare different approaches for mapping solar cells both in cross-section and in plan view on CIGS and CdTe solar cells. Based on examples from our latest research, we discuss the experimental approaches and highlight the advantages and limitations of each technique. Finally, we present an outlook to experiments that will allow x-ray based characterization to enter new fields of research that were not accessible before.

X-ray fluorescence at nanoscale resolution for multicomponent layered structures: a solar cell case study

The study of a multilayered and multicomponent system by spatially resolved X-ray fluorescence microscopy poses unique challenges in achieving accurate quantification of elemental distributions. This is particularly true for the quantification of materials with high X-ray attenuation coefficients, depth-dependent composition variations and thickness variations. A widely applicable procedure for use after spectrum fitting and quantification is described. This procedure corrects the elemental distribution from the measured fluorescence signal, taking into account attenuation of the incident beam and generated fluorescence from multiple layers, and accounts for sample thickness variations. Deriving from Beer-Lamberts law, formulae are presented in a general integral form and numerically applicable framework. The procedure is applied using experimental data from a solar cell with a Cu(In,Ga)Se2 absorber layer, measured at two separate synchrotron beamlines with varied measurement geometries. This example shows the importance of these corrections in real material systems, which can change the interpretation of the measured distributions dramatically.

Charge Collection in Hybrid Perovskite Solar Cells: Relation to the Nanoscale Elemental Distribution

Unveiling the correlation between elemental composition, Fermi-level splitting, and charge collection in perovskite solar cells (PSCs) when exposed to different environments is crucial to understanding the origin of defects. This will enable defect engineering to achieve high-performance and long-lasting PSCs. In this paper, we measured, for the first time, the spatial distribution and charge-collection efficiency at the nanoscale by synchrotron-based X-ray fluorescence (XRF) and X-ray beam-induced current (XBIC) with subgrain resolution, and we observe a correlation between Pb/I ratio and charge-collection efficiency. In contrast with other thin-film solar cells, PSCs are highly sensitive to ambient conditions (atmosphere and illumination). As the XRF and XBIC measurements were conducted in vacuum under an X-ray source illumination, the impact of measurement conditions on the cells needs to be taken into account. Furthermore, necessary conditions for quantification of XRF/XBIC measurements, such as film homogeneity, are not fulfilled in the case of PSCs. Therefore, we will discuss fundamentals of XRF/XBIC measurements of PSCs that will enable reliable, quantitative, high-resolution measurements of elemental distribution and charge collection.

Development of an in situ temperature stage for synchrotron X-ray spectromicroscopy

In situ characterization of micro- and nanoscale defects in polycrystalline thin-film materials is required to elucidate the physics governing defect formation and evolution during photovoltaic device fabrication and operation. X-ray fluorescence spectromicroscopy is particularly well-suited to study defects in compound semiconductors, as it has a large information depth appropriate to study thick and complex materials, is sensitive to trace amounts of atomic species, and provides quantitative elemental information, non-destructively. Current in situ methods using this technique typically require extensive sample preparation. In this work, we design and build an in situ temperature stage to study defect kinetics in thin-film solar cells under actual processing conditions, requiring minimal sample preparation. Careful selection of construction materials also enables controlled non-oxidizing atmospheres inside the sample chamber such as H2Se and H2S. Temperature ramp rates of up to 300 °C/min are achieved, with a maximum sample temperature of 600 °C. As a case study, we use the stage for synchrotron X-ray fluorescence spectromicroscopy of CuIn(x)Ga(1-x)Se2 (CIGS) thin-films and demonstrate predictable sample thermal drift for temperatures 25-400 °C, allowing features on the order of the resolution of the measurement technique (125 nm) to be tracked while heating. The stage enables previously unattainable in situ studies of nanoscale defect kinetics under industrially relevant processing conditions, allowing a deeper understanding of the relationship between material processing parameters, materials properties, and device performance.

Correlation between grain composition and charge carrier collection in Cu(In,Ga)Se2 solar cells

Understanding the spatial composition inhomogeneity in CIGS solar cells can provide insight into a potentially large cause of decreased cell efficiency. We used synchrotron based nano x-ray fluorescence and nano x-ray beam induduced current to investigate the correlation between grain-to-grain compositional variations in Cu(In,Ga)Se2 absorber layers and variations in carrier collection efficiency. We found compositional variations as large as 5 at. % and current variations as large as 10%. Grains with higher average copper concentration and lower indium concentration showed increased carrier collection than neighboring grains with lower copper and higher indium concentration. We also investigated the overall composition dependence of carrier collection in CIGS films and show that 30% gallium content devices show a correlation between charge carrier collection, while 60% content devices do not.

Nano-XRF Analysis of Metal Impurities Distribution at PL Active Grain Boundaries During mc-Silicon Solar Cell Processing

Metal impurities are known to hinder the performance of commercial Si-based solar cells by inducing bulk recombination, increasing leakage current, and causing direct shunting. Recently, a set of photoluminescence (PL) images of neighboring multicrystalline silicon wafers taken from a cell production line at different processing stages has been acquired. Both band-to-band PL and sub-bandgap PL (subPL) images showed various regions with different PL signal intensity. Interestingly, in several of these regions a reversal of the subPL intensity was observed right after the deposition of the antireflective coating. In this paper, we present the results of the synchrotron-based nano-X-ray fluorescence imaging performed in areas characterized by the subPL reversal to evaluate the possible role of metal decoration in this uncommon behavior. Furthermore, the acquisition of a statistically meaningful set of data for samples taken at different stages of the solar cell manufacturing allows us to shine a light on the precipitation and rediffusion mechanisms of metal impurities at these grain boundaries.

Elemental distribution and charge collection at the nanoscale on perovskite solar cells

Unveiling the correlation between elemental composition, fermi-level splitting, and charge collection in perovskite solar cells (PSCs) exposed to different environments is crucial to understanding the origin of defects. This will enable defect engineering to achieve high performing and long lasting perovskite solar cells. In this contribution we measured for the first time the spatial distribution and charge collection efficiency at the nano-scale by synchrotron-based x-ray fluorescence (XRF) and x-ray beam induced current (XBIC) with sub-grain resolution, and we observe a correlation between Pb/I ratio and charge collection efficiency. In contrast to other thin-film solar cells, perovskite solar cells are highly sensitive to ambient conditions (atmosphere and illumination). As the XRF and XBIC measurements were conducted in vacuum under an x-ray source illumination, the impact of measurement conditions on the measurements need to be taken into account. Furthermore, necessary conditions for quantification of XRF/XBIC measurements are not fulfilled for perovskite solar cells. Therefore, we will discuss fundamentals of XRF/XBIC measurements of perovskite solar cells that will enable reliable quantitative, high-resolution measurements of elemental distribution and charge collection.

Growth of Cu(In, Ga)(S, Se) 2 films: Unravelling the mysteries by in-situ X-ray imaging

In-situ investigations of Cu(In, Ga)(S, Se)2 (CIGS) absorber layers during growth have the potential to provide insight into the origin behind elemental segregation and defect formation at grain boundaries and in grain cores. Our study focuses on CIGS films grown by an industrially relevant precursor reaction process. An in-situ stage developed for these synchrotron studies is used to image the growth of CIGS layers with nanoscale resolution. Utilizing synchrotron based x-ray fluorescence, we are able to rapidly monitor changes in elemental distribution and particle diffusion, with better than 200 nm spatial resolution throughout the 25 minute process. In this work, we highlight some challenges associated with this type of measurements and discuss solutions identified to overcome them.

Quantifying X-Ray Fluorescence Data Using MAPS

The quantification of X-ray fluorescence (XRF) microscopy maps by fitting the raw spectra to a known standard is crucial for evaluating chemical composition and elemental distribution within a material. Synchrotron-based XRF has become an integral characterization technique for a variety of research topics, particularly due to its non-destructive nature and its high sensitivity. Today, synchrotrons can acquire fluorescence data at spatial resolutions well below a micron, allowing for the evaluation of compositional variations at the nanoscale. Through proper quantification, it is then possible to obtain an in-depth, high-resolution understanding of elemental segregation, stoichiometric relationships, and clustering behavior. This article explains how to use the MAPS fitting software developed by Argonne National Laboratory for the quantification of full 2-D XRF maps. We use as an example results from a Cu(In,Ga)Se2 solar cell, taken at the Advanced Photon Source beamline 2-ID-D at Argonne National Laboratory. We show the standard procedure for fitting raw data, demonstrate how to evaluate the quality of a fit and present the typical outputs generated by the program. In addition, we discuss in this manuscript certain software limitations and offer suggestions for how to further correct the data to be numerically accurate and representative of spatially resolved, elemental concentrations.

Characterization of encapsulated solar cells by x-ray topography

Solar panels reliability studies focus mainly on the properties of the encapsulating such as gel content and transmittance, while ignoring the impact of encapsulation process on the solar cells themselves. The harsh lamination conditions apply high temperature and pressure on the wafers, which can induce increased stress, deformation and defects. The investigation of solar cells sealed inside modules calls for a non-destructive method. In this paper, we demonstrate that transmission X-ray topography (XRT) can be used as an accurate method to evaluate bending feature of encapsulated wafers and present in detail the experimental methods from capturing diffraction data to the data analysis.