Materials for solar energy conversion

Compound semiconductors, such as Cu(In,Ga)X2 (CIGS/Se) and Cu2ZnSnX4, (X = Se and S; CZTSSe) are due to their chemical flexibility and high optical absorption used as absorber materials in thin film solar cells (TFSC). Due to the chemical complexity of these compounds the analysis of structure-property relations is a crucial task for the further development of chalcogenide thin film solar devices. Moreover binary and ternary compounds, which may occur as secondary phases in CZTSSe thin films, like Cu-Sn-sulfides/selenides, influence the electonic properties of the final device.

The investigation of these materials is mainly done on polycrystalline powder materials prepared by solid state reaction in sealed and evacuated silica tubes. The preparation of powder materials of these compounds gives the advantage of a resulting material with a well-defined chemical composition. The latter is analyzed by wavelength dispersive X-ray analysis using a JEOL JXA 8200 electron microprobe system, using calibrated elemental standards. For the structural analysis classical X-ray diffraction in Bragg-Brentano geometry on a Panalytical X’pert MPD system with Cu-Kα radiation and subsequent Rietveld analysis is applied. The analysis of the crystal structure and microstructure of thin films is done by gracing incidence X-ray diffraction (GIXRD), which leads to depth dependent structural information of the films.

For the differentiation of isoelectronic cations in the Rietveld refinement procedure advanced diffraction techniques such as neutron powder diffraction and anomalous X-ray diffraction are used.