## 5 Conclusions  
This study presents a novel method to fabricate CuFeS\({}_{2}\) chalcopyrite thin films using a three-step synthesis of ball-milling, thermal evaporation, and sulfurization. We can emphasize some points of the optimization process: (1) ball-milling the metallic precursors before evaporation is essential for effective Cu and Fe element mixing; (2) a two-step current of 105 A (for 10 min) and 140 A (for 1 min) is required due to the different melting points of Cu and Fe; (3) using a smaller quantity of mass can improve the energy-over-mass ratio during evaporation; (4) high vacuum (\(\sim\)10\({}^{-6}\) mbar) and proximity between source and target enhance film adhesion. Finally, the sulfurization requires vacuuming the furnace and sintering at 425 \({}^{\circ}\)C for 1 h under dynamic N\({}_{2}\) flow. The optimized thin film exhibits a plate-like surface morphology and demonstrates \(p\)-type semiconducting behavior with a purity of 93% for the CFS chalcopyrite phase. These features can be used for electrochemical applications, although further tests are needed to verify the stability of the material under an aqueous/electrolyte environment. Additionally, we observed the formation of bornite and talnakhite phases during the optimization process. This is likely because their energy phase formation is similar to that of CFS, as determined by DFT phase-diagram calculations. We suggest that this three-step approach can synthesize these ternary phases in pure form by making specific adjustments to the synthesis parameters. A proof-of-concept CFS-based in-plane TEG fabricated using the optimized thin film showed a power output per unit active area of \(\sim\)114 nW cm\({}^{-2}\) for a temperature gradient of \(\sim\)180 \({}^{\circ}\)C. The performance is superior to other sulfide-based materials fabricated using ball-milling and hot injection inks due to the smaller thickness and uniformity of the thin film. However, no CFS TEG performance was reported for a direct comparison. Further studies on the CFS material's properties, such as the Seebeck coefficient, electrical resistivity, and thermal conductivity, are required to understand the effect of thickness on the overall TE performance of the thin film. In short, this work provides a comprehensive step-by-step guide to fabricating CFS thin films and opens avenues to use this eco-friendly, stable, cost-effective, and sustainable material for diverse applications.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app131810172/s1](https://www.mdpi.com/article/10.3390/app131810172/s1)]([https://www.mdpi.com/article/10.3390/app131810172/s1](https://www.mdpi.com/article/10.3390/app131810172/s1)), Figure S1 to Figure S20: Rietveld refinements of the thin film XRD for all the samples; Figure S21 to Figure S31: pictures of the thin films for visual inspection; Figure S32: XRD patterns of the as-deposited CuFe thin films; Figure S33: SEM and EDX analysis of the as-deposited CuFe thin films; Figure S34: SEM micrograph of Sample 18; Table S1: Ball-milling parameters; Table S2: Thermal evaporation parameters.  
Conceptualization, N.A. and P.S.; Methodology, M.A.M., M.D. and N.A.; Software, H.N.; Validation, N.A.; Formal analysis, K.L.; Investigation, M.D., H.N. and N.A.; Resources, M.D. and N.A.; Writing--original draft, M.A.M.; Writing--review & editing, M.A.M., K.L. and H.N.; Supervision, N.A. and P.S.; Project administration, P.S.; Funding acquisition, P.S. All authors have read and agreed to the published version of the manuscript.  
University of Trento: Department Strategic Plan 2022-2024.  
Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author.  
Keywords:[PERSON] and [PERSON] are acknowledged for useful discussion regarding DFT calculations aided by the HAWK supercomputer within the Diskete Project framework.  
The authors declare no conflict of interest.