3-Dimensional Characterization of CdTe Solar Cells

Cadmium telluride (CdTe) is efficiently used for solar energy conversion in the form of thin film solar cells. Its particularly well-fitting optoelectronic properties (almost ideal bandgap, high absorption coefficient and high thermal stability) are still driving the interests of both scientific communities and industrial companies in the highly competitive world of photovoltaic materials. An extended knowledge of the layer microstructure formation is expected to lead to process-optimization to continue to push back the current world record of 21 % efficiency.

Left: CdTe grain boundary network reconstructed from 3D EBSD. Right: Schematics of the 3D EBSD milling on the thin film material.

CdTe solar cells in substrate and superstrate configuration produced at the Laboratory for Thin Films and Photovoltaics at the EMPA Duebendorf are investigated using conventionnal electron back-scatter diffraction (EBSD) and focused ion beam (FIB)-EBSD tomography, also referred as 3D-EBSD. The main advantage of this technique lies in the possibility to probe the evolution of a relevant set of parameters (grain size, texture, grain boundary character distribution...) through the whole thickness (around 4 microns) of a strongly microstructurally-anisotropic layer. It is, therefore, possible to propose a coherent model for the growth of the thin film and to identify the critical steps of the grain boundary network formation. These steps will be the cornerstones of a future grain boundary engineering approach applied to the design of such cells.

In addition, electron channeling constrast imaging (ECCI) is also extensively used to probe the arrangement and density of dislocations and stacking faults at the nanometer scale. With this, a direct analyses of extended defects is possible on bulk materials at a resolution not much worse than dark-field TEM.

Finally, electronic properties are also probed using cathodoluminescence and scanning kelvin probe microscopy in order to understand the way they are affected by the layer microstructure, especially by the character of grain boundaries.

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