## 4 Discussion  
In this study, we present a comprehensive analysis of the three-step synthesis of CFS, examining its physical and chemical properties. Moreover, we demonstrate its successful application as a thermoelectric generator, yielding a power output comparable to similar chalcogenide-based materials [20; 46; 47; 48; 49; 50]. Comparing our proposed three-step method with existing literature can offer valuable insights into the potential applications of CFS chalcopyrite beyond thermoelectricity.  
The predominant technique for producing bulk polycrystalline CFS involves wet-chemical reactions [13; 14; 51; 52; 53; 54; 55; 56; 57]. This route provides control over its nanoscale dimensions, enabling the tunning of its optical properties and exploring the quantum confinement effect. The polycrystalline nature also results in a higher surface-area-to-volume ratio for the material, promoting additional active sites for electrocatalysis applications [58]. Another notable attribute of CFS is its robust electrochemical stability, enabling it to endure repeated recharging cycles in battery systems [59; 60]. Furthermore, this stability has been harnessed in various applications, including its role as an electrode material for electrocatalytic oxygen reduction in fuel cells and water electrolyzers [58]. It also emerges as a promising alternative to platinum as counter electrodes in dye-sensitized solar cells [61].  
The synthesis method provided here does not achieve precise control over the microstructure, although a nano flower-like morphology is observed (see Figure 8a). Examples of similar materials with a plate-like structure have been demonstrated by [PERSON] et al. [62], [PERSON] et al. [59], and [PERSON] et al. [63] as advantageous for electrochemical, Li-ion battery cathode and magnetic applications due to its high surface-area-to-volume ratio. The difference between the three-step method compared to the above-mentioned wet-chemical process is that it is a dry route, with no need for washing processes and the complete absence of wet chemical contaminations. However, further studies must be performed to check the stability under the aqueous (electrolyte solution) environment from our samples for electrocatalytic (battery) applications.  
For thermal evaporation, [PERSON] et al. [27] adopted a similar synthesis route compared to the present work. They heated the substrate during the evaporation to increase the  
Figure 10: (**a**) I-V plots and (**b**) I-V-P output measurements for the CFS TEG. The temperature depicted in the figure corresponds to the hot side, while a passive cooler was employed on the cold side to maintain a temperature of approximately 20 \({}^{\circ}\)C.  
diffusion of Fe into Cu instead of ball-milling. The control of stoichiometry is similar to what is reported here, although the plate-like morphology was absent. Therefore, the microstructure of the CFS film can be extremely diverse if milling is employed before powder evaporation and offers a simple alternative for improving Fe-Cu diffusion. Moreover, the inclusion of a ball-milling step before evaporation showed the formation of three distinct Cu-Fe-S phases. We put forward that by fine-tuning specific synthesis parameters, these phases can be obtained separately. Additionally, the parameters of milling can be tuned to regulate the Cu:Fe proportion in CFS, especially if aiming to modify its electrical properties from \(p\) to \(n\)-type nature.