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High-Pressure Behavior of Kesterite-Type Photovoltaic Materials

Subject Area Solid State and Surface Chemistry, Material Synthesis
Physical Chemistry of Solids and Surfaces, Material Characterisation
Theoretical Chemistry: Molecules, Materials, Surfaces
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 409759379
 
Final Report Year 2024

Final Report Abstract

The imminent increase of the human population in the coming years demands for exploring ways in serving basic needs out of the limited Earth’s capacities. One of the major problems currently being investigated worldwide is the generation of sufficient energy through the use of alternative sources, away from the almost depleted and environmentally-harmful fossil fuels. The utilization of solar energy spearheads these efforts. The current photovoltaic technology, however, based almost exclusively on silicon, has reached its threshold, asking for alternative materials with suitable properties. Among the various compounds, the kesterite-type Cu2ZnSnS4 ticks several boxes, owing to its optimal optical band gap, high absorption in the visible range, and its earth-abundant, non-toxic constituent elements. Despite ongoing efforts worldwide, however, this system remains difficult to ‘handle’ due to a plethora of inherent point defects, cationic disordering, and even secondary phases arising from the synthesis routes. The latter constitute detrimental factors, hindering the efforts of optimizing its photovoltaic efficiency. As a means of an in-depth understanding of this system, we have investigated several different compositions of this quaternary material, corresponding to both end-members with different cations, as well as selected solid solution series. Our investigations have shown that even in a fully ordered modification, the presence of even moderate stress/strain may lead to cationic disordering in this material, thus directly affecting its electronic/photovoltaic properties. Further compression will eventually lead to a denser disordered modification with metallic conductivity. These discoveries should be considered when using this material for applications, where the size mismatch between the photovoltaic thin film and its underlying substrate will always result in some local strain. A potential way of overcoming this potentially detrimental effect might be either with proper engineering of the thin film/substrate system so as to induce tensile stress/strain (unexplored here), or by appropriate cationic exchange (i.e. chemical pressure) in order to overcome the effect of compressive stress, and preserve at the same time the desired electronic properties. The analysis of the effect of pressure on the stannite-type and Ag-bearing compounds might shed more light into this direction

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