From atomic structure to photovoltaic properties in CdTe solar cells

Aberration corrected scanning transmission electron microscopy (STEM) has been used to determine the structures of a variety of dislocation cores in CdTe, including 30° and 90° Shockley partial dislocations, positive and negative Frank sessile partial dislocations, and steps on twin boundaries. Structure models have been constructed from the images and electrical activity has been investigated with density functional calculations. An integrated electron energy loss spectroscopy, cathodoluminescence and electron beam induced current system has been designed and built to probe electrical and optical properties of individual defects. The first STEM-cathodoluminescence result shows strong impurity segregation between the CdTe and the glass. The correlation between the scanning electron microscopy-electron beam induced current and electron backscatter diffraction maps proves that the grain structures and boundaries dominate the electrical activity. After heat treatment in CdCl2, Cl is found to segregate to the grain boundaries, and they show higher efficiency than the bulk material.