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3D X-ray diffraction tracks strain patterns in solar cells
09-10-2024
A study using scanning 3D X-ray diffraction microscopy at the beamline ID11 has revealed that localized strain in cadmium telluride solar cells is primarily associated with impurities diffusing along grain boundaries with high misorientation angles. This strain, caused by sulfur from the CdS layer, is significantly reduced at twin boundaries, where minimal strain is observed. These findings offer valuable insights for improving solar cell efficiency by better managing strain distribution.
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Cadmium telluride (CdTe) solar cells are promising candidates for boosting green energy production. However, structural defects and impurities in the cells present significant challenges to achieving optimal efficiency. Grain boundaries in the CdTe absorber layer are known to act as centres for impurity aggregation, leading to localized strain. Previous studies using various 2D techniques have reported significant sulfur diffusion along grain boundaries from the cadmium sulfide (CdS) layer into the CdTe layer [1,2].
However, these studies did not analyze individual grain boundaries to identify which ones facilitate impurity diffusion. Since grain boundaries are unavoidable in polycrystalline materials, it is important to distinguish between those that assist diffusion and those that do not.
In this study, recent advancements in scanning 3D X-ray diffraction microscopy (S3DXRD) at the ID11 beamline [3] were used to generate a detailed 3D map of the microstructure and grain boundaries in a CdTe solar cell (Figure 1a-d), with a spatial resolution of 100 nm. After the S3DXRD experiment, the 3D volume provided statistical data on grain size, orientation, and lattice parameters (Figure 1f-h).
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Fig. 1: a) Schematic of the scanning 3DXRD setup, defining the laboratory coordinate system. The rotation angle ω, the Bragg diffraction angle θ, and azimuthal angle η characterize the diffracted beam for a reflection from a grain within the sample. b) 3D visualization of the grain map in a p-type CdTe solar cell. c-d) Orthogonal slices through the centre of the sample, with yellow lines representing twin grain boundaries. e) Inverse pole figure (IPF) colour scale showing crystal orientation along the Z-axis. f) Histogram depicting grain size distribution. g) Magnitude of the IPF density function. h) Plot showing the variation in average unit cell parameters a, b, and c with distance from the CdS n-type layer.
As shown in Figure 1e, twin boundaries dominate the microstructure, with 88.7% of the analyzed volume comprising grains adjacent to a twin boundary. The grain-averaged lattice parameters indicate that all three lattice parameters – a, b, and c – decrease continuously and synchronously with decreasing distance to the CdS layer. The volumetric component of the strain tensor was analyzed to investigate compressive strain more thoroughly. Regions of high volumetric strain were found to form a continuous pathway, originating from the CdS layer and extending into the CdTe layer along grain boundaries with high misorientation angles. In contrast, twin boundaries did not exhibit significant localized strain (Figure 2).
Fig. 2: a) 3D visualization of strain within a selected region of the CdTe p-type layer, highlighting areas of high strain localization. The colour scale is saturated near the CdS layer to emphasize weaker strain in the bulk. b) 3D visualization of grain orientation in the same region as (a), using the IPF colour code for cubic symmetry. c) Vertical cross-section of (a) along plane 1, showing strain distribution and annotated boundary types. d) Horizontal cross-section of (a) along plane 2, showing strain distribution. e-f) Cross-sections of the same planes as (c-d), now showing grain orientations and grain boundary character. Grain boundaries are marked with lowercase letters, with neighbouring grains included to form the strained grain boundary in both visualizations.
The isotropic nature of the compressive strain (Figure 1e) indicates that the strain along these connected regions is caused by sulfur diffusion from the CdS layer into the CdTe layer. Energy-dispersive X-ray spectroscopy results confirmed the presence of sulfur in the bulk of the CdTe layer, supporting this mechanism. The diffusion process involves the substitution of tellurium with sulfur in the CdTe lattice, leading to a contraction of the unit cell. This contraction is consistent with a decreased Te/Cd ratio in the bulk material towards the CdS interface, as observed using high-energy X-ray fluorescence data obtained simultaneously with the S3DXRD data. Twin boundaries show low strain values, likely due to a lower density of dangling bonds and lower free energy at twin boundaries compared to grain boundaries with high misorientation.
In conclusion, 3D visualizations of grain orientations and strain in a CdTe solar cell reveal that strain localizes near grain boundaries with high misorientation but not along twin boundaries. This strain is attributed to sulfur diffusion from the CdS layer into the CdTe lattice. These findings improve the understanding of sulfur diffusion in CdTe/CdS devices and offer insights for optimizing solar cell deposition processes.
Principal publication and authors
Grain boundary strain localization in a CdTe solar cell revealed by scanning 3D X-ray diffraction microscopy, A. Shukla (a), J. Wright (b), A. Henningsson (c), H. Stieglitz (d), E. Colegrove (e), L. Besley (a), C. Baur (a), S. De Angelis (a), M. Stuckelberger (f), H.F. Poulsen (a), J.W. Andreasen (a), J. Mater. Chem. A 12, 16793-16802 (2024); https://doi.org/10.1039/D4TA01799D
(a) Technical University of Denmark, Lyngby (Denmark)
(b) ESRF
(c) Lund University, Lund (Sweden)
(d) Helmholtz-Zentrum Geesthacht, Geesthacht (Germany)
(e) National Renewable Energy Laboratory, Golden (USA)
(f) Deutsches Elektronen-Synchrotron DESY, Hamburg (Germany)
References
[1] Y. Yan et al., Appl. Phys. Lett. 78 , 171-173 (2001).
[2] A.A. Taylor et al., Sol. Energy Mater. Sol. Cells 141, 341-349 (2015).
[3] J. Hektor et al., Materials 12, 446 (2019).
About the beamline: ID11 |
The Materials Science beamline, ID11, is dedicated to high-energy X-ray diffraction (XRD) and imaging studies. The beamline was upgraded with the addition of a nano-resolution station located 100 metres from the source, enabling diffraction measurements with very high spatial resolution (<100 nm). ID11 offers a comprehensive suite of X-ray diffraction techniques, including 3DXRD, diffraction contrast tomography, diffuse X-ray scattering, imaging, pair-distribution function analysis, and powder diffraction. The beamline supports the analysis of a wide range of samples, from amorphous and powder materials to poly-crystals and single crystals with sample size ranging from nano- to micro- to larger-scale specimens. Users also have access to a variety of sample environments. |