Want to improve on structural disorder in Cu-based quaternary chalcogenides? Let’s look at the divalent cation! David Matzdorff 1 , Galina Gurieva 1 , Denis Cheptiakov 3 , Susan Schorr 1,2 1 Helmholtz-Zentrum Berlin für Materialien und Energie, Germany, 2 Institut für Geologische Wissenschaften, Germany, 3 Paul Scherrer Institute, Switzerland With the increasing demand for sustainable energy sources, the group of Cu-based chalcogenides stands out in the field of photovoltaic applications, because they consist of non-toxic and earth abundant elements. On top of that the related solar cells show excellent stability under environmental conditions. It was even possible to achieve a PCE of 13.2% [1]with CZTSSe-based solar cells (CZTSSe-Cu 2 ZnSn(S,Se) 4 ), however, CZTSSe absorbers face a major problem. The material show a low open circuit voltage (V oc ) and it has been established that the low V oc is attributed to the widespread Cu-Zn disorder (Cu Zn /Zn Cu ) in kesterite absorbers [2,3]. A complete replacement of Zn with Mn in the semiconductor could potentially solve this problem. For that reason, we studied the two related Cu 2 MnSnS 4 and Cu 2 MnSnSe 4 compounds. They are able to cover a bandgap range of 1.1-1.5 eV [4,5] and thus ideal for both single junction and potentially tandem solar cells. The bandgap energy is significantly controlled by crystal structure. Unfortunately, the data available for these materials in terms of in-depth structural analysis is very limited and urgently needs to be further explored. The kesterite-type materials (Cu 2 ZnSnS 4 , Cu 2 ZnSnSe 4 and CZTSSe) has been already studied in detail establishing the off-stoichiometry type model [6] to assess experimentally point defects and structural disorder (Cu-Zn disorder). This model was also applied here for the Cu 2 MnSnX 4 (X= S, Se) semiconductors. The structural analysis of Cu 2 MnSnS 4 and Cu 2 MnSnSe 4 by conventional X-ray diffraction (XRD) encounters difficulties because the distinct cation positions in the crystal structure are occupied by different electronically similar elements (Cu, Mn and Ge). Due to their similar atomic scattering factors they cannot be distinguished in the process of analyzing the XRD data. That’s why we performed neutron diffraction experiments on those compounds to make use of the very different neutron scattering length of Cu, Mn and Ge allowing to distinguish between them. We applied the average neutron scattering length analysis method [7] to determine the cation distribution in the unit cell, the basis to conclude on the crystal structure and structural disorder. We show that Cu 2 MnSnS 4 and Cu 2 MnSnSe 4 adopt the stannite-type crystal structure type which hinder the structural disorder observed in the Zn-containing counterparts References 1. Zhou, J. (2021). Nano Energy , 89 , 106405 2. Rey, G. (2014). Applied Physics Letters , 105 , 112106
3. Valentini, M. (2016). Applied Physics Letters , 108 (21), 211909 4. Beraich, M. (2020). Journal of Alloys and Compounds , 845 , 156216 5. Ramasamy, K. (2018). Chemical Communications , 54 , 11757–11760 6. Schorr, S. (2019). Journal of Physics: Energy , 2 , 012002 7. Schorr, S. (2011). Solar Energy Materials and Solar Cells , 95 , 1482–1488
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