## 3 Results
### Optimized CFS Thin Film  
To assess the reproducibility of the synthesis, Sample 18 was synthesized with the identical conditions as Sample 16, i.e., evaporating FeCu-03 ball milled powders with a lower vacuum (10\({}^{-6}\) mbar), using a two-step current of 105 A and 140 A, followed by sulfurization with a dynamic N\({}_{2}\) flow. Further characterization of surface and cross-section morphology, resistivity, carrier concentration, and mobility was carried out on this sample. Figure 7 shows the Rietveld refinement of the XRD data of Sample 18. The QPA shows that the sample consists mainly of the desired CFS chalcopyrite phase (PDF # 000250288), \(\sim\)93% by weight. Nevertheless, the CuFe-bornite (PDF #40080243) and pyrite FeS\({}_{2}\) (PDF #000060710) phases also formed along with CFS, with a weight percentage of \(\sim\)5 and 2%, respectively. The chalcopyrite phase also shows a preferred orientation along the [112] direction. Good agreement is observed for the profile fitting and experimental data, as shown by the featureless residual curve. Since FeS\({}_{2}\) requires the lowest formation energy among the different Cu-Fe-S phases (see Figure 3), its presence as a secondary phase is also expected. As is common for thin films, the contribution of crystallite size is better represented by a Gaussian profile function, which yields a volumetric crystallite size average (L\({}_{\rm vol}\)) of 61(1) nm. A relatively small microstrain of 0.4(9)% is observed for the chalcopyrite phase, with a predominant Lorentzian contribution to the line profile, see Figure 7.  
Figure 7: XRD pattern of the CFS-18 sample. Black dots stand for the experimentally obtained data in square root scale, while the red line represents the profile fitting. The grey line shows the residual (difference between the experimental and fitted model). The tick markers represent the Bragg reflections shown in the caption. (1 1 2) is the preferred orientation of CFS chalcopyrite. The SEM and EDXS analyses of the sulfurized Sample 18 are summarized in Figure 8. On the surface of the film, micrometer-sized grains form bigger polycrystalline agglomerates with an irregular plated shape microstructure (Figure 8a). Figure S31 shows that this morphology is extended throughout a large area of the sample. The chemical maps collected on the same surface suggest that Cu is homogeneously distributed throughout the sample, whereas Fe and S are more distinguished on the top surface of the grains. This reveals that even when the precursor powders are well mixed by the milling procedure, Cu tends to form the base layer of the film when evaporated, leaving the film surface Fe-rich. This also corroborates the slightly Fe-rich stoichiometry observed in the crystalline phases obtained using XRD. SEM and EDXS measurements of the CuFe precursor (FeCu-03, pre-sulfurized Sample 18) can be visualized in Supplementary Note S7. This film shows a smooth and clean surface morphology, with a slightly copper-rich stoichiometry on the surface. For the CFS film, the average thickness is \(\sim\)1.7 \(\upmu\)m, as seen in the cross-section micrograph of Figure 8b, left. The cross-section of the film is also Cu-rich, adding to the fact that it forms the base layer of the film. The electrical characterization of the sulfurized Sample 18 is shown in Figure 9. The conductivity of the CFS film increases with the temperature, and carrier concentration shows a positive value, confirming a non-degenerate \(p\)-type semiconductor nature. To explain the \(p\)-type nature, we see that Cu is forming as the base layer of the film, and CFS is believed to be Fe-rich. Since the oxidation state of CFS is often understood as Cu\({}^{+}\)Fe\({}^{3+}\)S\({}_{2}\)\({}^{-}\), the \(p\)-type nature of CFS can be stabilized by substituting Cu\({}^{1+}\) with Fe\({}^{3+}\) or, in general, by  
Figure 8: SEM and EDXS analysis of Sample 18. (**a**) Surface scan (**left**), EDXS and chemical mapping (**right**). The table contains information on EDXS analysis of the atomic proportion of each element.