Atomic-scale structure of kesterites

• Applicant: Professorin Dr. Claudia Sarah Schnohr
• Subject Area: Experimental Condensed Matter Physics ; Mineralogy, Petrology and Geochemistry ; Theoretical Condensed Matter Physics
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 278882382

Photovoltaic energy conversion is one of the major routes towards a sustainable energy supply which is one of the big global challenges of the twenty-first century. CdTe and Cu(In,Ga)Se2 thin film solar cells have recently reached record efficiencies of more than 20% closing the gap to silicon based technologies. Apart from reduced material costs, such thin film solar cells can be produced on flexible substrates offering tantalizing new applications in architecture and product design. However, the use of CdTe is hindered by the toxicity of its constituents while the large-scale implementation of Cu(In,Ga)Se2 may be limited by the availability of In. Consequently, strong efforts are being made to find suitable material substitutes. Kesterites, such as Cu2ZnSn(S,Se)4, consist of nontoxic elements that are widely available in the earth crust. The efficiency of kesterite thin film solar cells has continuously increased over recent years up to the current record of 12.6% and it is believed that conversion efficiencies similar to those of CdTe or Cu(In,Ga)Se2 may be achieved in the future. Currently, the performance of kesterite thin film solar cells is limited by the occurrence of secondary phases and structural modifications that are difficult to detect with diffraction techniques and that adversely affect the electronic properties of the material. Furthermore, the local atomic arrangements in mixed semiconductors often show a striking deviation from the long-range crystallographic order. This structural inhomogeneity on the sub-nanometer scale also has a strong impact on the material bandgap and thus on the device performance. Therefore, it is the aim of this project to study compositional, structural and electronic aspects of a comprehensive set of kesterite materials in order to improve the understanding of structure-property-relations in these complex semiconductors and to point out optimized preparation conditions for high-efficiency thin film solar cells. The materials studied include four different compounds and their mixtures, stoichiometric and non-stoichiometric, as powders, thin films or nanoparticles. Using X-ray absorption spectroscopy (XAS), the presence of secondary phases can be determined quantitatively as a function of material composition and growth conditions providing valuable information about optimized preparation routes. XAS also yields atomic-scale structural parameters, such as element-specific bond lengths, which provide original insight into the correlation between long-range and short-range order when compared to results obtained from diffraction techniques. Combining local structural parameters with valence force field simulations and ab initio calculations further yields unique information about the relation between structural and electronic properties. A detailed understanding of these structure-property-relations is indispensable in order to exploit the full potential of this promising group of materials.

• DFG Programme: Research Grants
• Co-Investigators: Professorin Dr. Silvana Botti; Professorin Dr. Susan Schorr