  Cadmium Telluride (CdTe) solar cell technology is a promising candidate to
help boost green energy production. However, impurities and structural defects
are major barriers to improving the solar power conversion efficiency. Grain
boundaries often act as aggregation sites for impurities, resulting in strain
localization in areas of high diffusion. In this study, we demonstrate the use
of scanning 3D X-ray diffraction microscopy to non-destructively make 3D maps
of the grains, their phase, orientation, and local strain within a CdTe solar
cell absorber layer with a resolution of 100 nm. We quantify twin boundaries
and suggest how they affect grain size and orientation distribution. Local
strain analysis reveals that strain is primarily associated with high
misorientation grain boundaries, whereas twin boundaries do not show high
strain values. We also observe that high-strain grain boundaries form a
continuous pathway connected to the CdS layer. Hence, this high-strain region
is believed to be associated with the diffusion of sulfur from the CdS layer
along grain boundaries. This hypothesis is supported by SEM EDS and X-ray
fluorescence experiments. The method and analysis demonstrated in this work can
be applied to different polycrystalline materials where the characterization of
grain boundary properties is essential to understand the microstructural
phenomena.
