## 4 Summary and Outlook  
In summary, we employed _special quasirandom structures_ in combination with suitably sized supercells to accurately model the Cu-Zn disorder in CZTSe, a promising material for the next generation of thin-film solar cell devices. The results presented in this work have been obtained by first-principles calculations based on DFT, utilising the SCAN and hybrid HSE06 exchange and correlation functionals. In order to circumvent some flaws of earlier investigations, we additionally employed a recently devised new method to speed up structural relaxations based on hybrid functional calculations. In that way we were able to investigate the necessary amount of symmetry inequivalent cation arrangements to accurately model the Cu-Zn disorder in CZTSe. Our results show that the Cu-Zn disorder is a temperature effect stemming from experimental growth conditions, and that the Cu-Zn disorder leads to a slight increase in the unit cell volume compared to the conventional kesterite structure showing full cation order. The band gap reduction of about 0.2 eV due to the introduced disorder could be verified from the DOSs and the onsets in the absorption coefficients, thereby confirming earlier experimental and theoretical results. Lastly, the applied method of _special quasirandom structures_ in combination with suitably chosen supercell sizes has been shown to offer atomistic insights into Cu-Zn disorder in CZTSe,  
Figure 5: (**a**) Density of states for the ordered (upper panel) and disordered (lower panel) kesterite crystal structures, showing the total DOS as a shaded grey background, and the Cu (4d), Sn (2b), and Se (8i) partial DOSs in red, black, and green, respectively. (**b**) Absorption coefficients for the ordered (black) and disordered (red) kesterite crystal structures. In case of the disordered kesterite, the contributions show the weighted averages of the 10 symmetry inequivalent cation arrangements.  
thereby supplementing the previously prevalent approaches based on a combination of DFT and Monte-Carlo calculations based on cluster expansion methods. The significant decrease in required computing times of the employed method would also allow for feasible investigations of the whole solid solution series CZTSSe. The CZTSSe mixing parameter could be adjusted in line with the Shockley Queisser limit and verified in future experiments, thereby furthering our knowledge about kesterite-based solar cell materials.  
This research received no external funding.  
Not applicable.  
The data that support the findings of this study are available from the corresponding author upon reasonable request.  
This work made use of computational resources provided by the North-German Supercomputing Alliance (HLRN, www.hlm.de, accessed on 12 January 2022), and the ZEDAT [45] and DIRAC high-performance computing facilities of the Freie Universitat Berlin and the Helmholtz-Zentrum Berlin, respectively.  
The author declares no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} CZTS & Cu\({}_{2}\)ZnSnS\({}_{4}\) \\ CZTSe & Cu\({}_{2}\)ZnSnSe\({}_{4}\) \\ CZTSse & Cu\({}_{2}\)ZnSn(S\({}_{x}\)Se\({}_{1-x}\))\({}_{4}\) \\ DFT & Density functional theory \\ DOS & Density of states \\ \end{tabular}