## 3 Results  
### Evaporation Optimization  
#### 3.1.1 Non-Milled Cu-Fe Powder Evaporation  
To optimize the CFS thin film deposition, it is necessary to have a prior understanding of the thermal evaporation of the elemental Fe and Cu powders. Therefore, the thermal evaporation was carried out on pristine Fe and Cu, as well as in a manual mix of Fe-Cu powders (atomic ratio 1:1). The XRD patterns of the evaporated films are shown in Figure 2a, and their respective evaporation parameters are summarized in Figure 2b (with full details in Table S2 of the Supplementary Material). Through iterative experimentation, the evaporation current for Cu and Fe was established as 105 A and 140 A for Samples (1) and (2), respectively. The same currents were employed during Cu-Fe blend powder evaporation  
Figure 1: (**a**) TEG dimensions and layout. (**b**) Homemade setup for the I-V-P measurements, circuit diagram, and real-size comparison of the TEG with a one-euro coin for scale. for Samples (3) and (4). For Samples (5)-(8), an optimization of the target-to-source distance and current-step variation was performed. Fine details can be found in Supplementary Note S2. Overall, one can notice that the melting point of Fe (1538 \({}^{\circ}\)C) is higher than that of Cu (1085 \({}^{\circ}\)C), so clearly, compared to Cu (I = 105 A), the evaporation of Fe (I = 140 A) required a higher current. This also means that, during the evaporation process of Cu-Fe mixed powders, Cu evaporates first, forming the base layer of the film, followed by the deposition of Fe, which creates the surface layer (a bilayer film). Consequently, the majority of the film's surface exhibits the _bcc_ Fe phase. Among all these tests (Samples 1-8), Sample 8 is the most promising due to a better intermixing of Fe-Cu in the as-evaporated film. Sulfurization of Sample 8 under a static Ar atmosphere resulted in the formation of two phases, namely CFS chalcopyrite (I-42d, PDF # 04-002-0241) as the major phase (-70% weight proportion) and a minor Cu\({}_{18.32}\)Fe\({}_{15.9}\)S\({}_{4}\) talnakhite phase (I-43m, PDF # 01-071-2438) with \(\sim\)30% weight proportion. The talnakhite phase presents low thermal conductivity [10], although in this case, the  
Figure 2: **(a)** Diffractogram of Samples 1 to 8. The tick markers represent the Bragg position of the phases, where the corresponding labeling is in the top-right corner. Sample 8-S represents sulfurized Sample 8, where the process described in Section 2.3 was employed. The inset shows the crystal structure of CFS talnakhite. The Rietveld refinements are available in Supplementary Note S4. **(b)** Diagram with the main synthesis parameters used for the non-milled powder optimization and main results, such as element material used, current employed (A), evaporation time (h), target distance (cm), and mass of powder (g). Please refer to Table S2 for further details. mixing of phases turns the sample poor in adhesion, as can be visualized in Figure S22 of the Supplementary Materials. To gain an insight into the multiphase formation in the thin film, the DFT phase diagram for Cu-Fe-S is presented in Figure 3. The gradient scale shows the formation energy per atom, which is a measure of the energy required to form one unit of the compound from its atoms. The CFS chalcopyrite phase has a formation energy of \(-0.411\) eV/atom, with the talnakhite and bornit phases presenting values of \(-0.214\) eV/atom and \(-0.090\) eV/atom, respectively. Hence, chalcopyrite is the most stable phase among the ternary compounds; however, the proximity in the formation energies of other Cu-Fe-S phases makes it challenging to obtain a pure CFS. Moreover, the binary alloys of FeS\({}_{2}\) and CuS\({}_{2}\) also have low formation energies and are likely to form. In the literature, the Cu-Fe-S phase diagram was summarized in the work of [PERSON] [35] from experimental information.