Investigating nanoscale features in energy materials using scanning 3D X-ray diffraction microscopy

In the face of the climate crisis, the need for advanced high-performance energy conversion and storage materials is essential. Most of these materials are polycrystalline, where the combined properties of individual crystals directly influence the overall device performance.

Over the last two decades, considerable efforts have been made to develop nondestructive methods to characterize 3D polycrystalline microstructures, both in terms of experimental hardware and reconstruction techniques. These methods, commonly known as Three-Dimensional X-ray Diffraction (3DXRD) techniques, have gained significant popularity, especially within the metallurgy community. Scanning 3DXRD, a variation of 3DXRD, enables the measurement of not only grain-averaged information but also the visualization of internal inhomogeneities within grains and at grain boundaries.

The primary goal of this PhD project was to push the limits of the 3DXRD microscope at the ID11 beamline of ESRF to measure very small grains. This is much needed for energy materials, which often have nanoscale grain sizes. In this thesis, we present 3D imaging of the polycrystalline microstructure of two industrially relevant energy materials with an unprecedented spatial resolution of 100 nm. This enabled the visualization of small grains in CdTe solar cells (smallest grain size 300 nm) and Ni/YSZ solid oxide cells (average grain size 400 nm). A modified indexing procedure allowed precise characterization of small grains by utilizing neighboring grain orientation information. Additionally, twin domains were mapped by addressing challenges with shared diffraction peaks, and an improved 3D reconstruction method was developed to account for voids using X-ray fluorescence data.

In CdTe solar cells, 3D visualization revealed that strain tends to localize along high-misorientation grain boundaries, likely arising from sulfur diffusion. SEM and X-ray fluorescence experiments supported this hypothesis. A similar experiment on Se diffusion in a CdSeTe solar cell confirmed these findings.

The ultimate goal of imaging polycrystalline microstructures is to perform measurements while the device is operational. As a step toward this, we demonstrate that scanning 3DXRD can track microstructural changes at the 100 nm scale. An ex-situ heating experiment on solid oxide cells revealed how Ni grain growth contributes to Ni network coarsening, a key degradation process in these devices.