## 3 Results
### Optimized CFS Thin Film
(**b**) Cross section of the thin film. reducing the valence of the cations or by increasing the valence of the anions [45]. The Hall measurement reveals that the carrier concentration does not show significant temperature variation, which is consistent with previous reports on this material (\(1.8\times 10^{19}\) cm\({}^{-3}\)) [45]. Furthermore, the carrier concentration is in the same order of magnitude and the mobility is reduced from the literature values, which can be credited to the morphology of the sample. This affects the overall conductivity, which tends to be lower than the literature-reported values for \(p\)-type CFS [26,45]. An in-plane TEG was fabricated with two couples of CFS and Ag legs. The two CFS legs were deposited following the optimized evaporation and sulfurization parameters of Sample 18, as illustrated in Figure 1a. The current and voltage output data, shown in Figure 10a, were collected using a homemade setup (Figure 1b) by varying the hot side temperature from 50 \({}^{\circ}\)C to 200 \({}^{\circ}\)C. In this temperature span, the TEG shows an open circuit voltage (VOC) varying from 3 to 80 mV and a short circuit (SC) current from 1 to 7.7 \(\upmu\)A. The current-voltage-power (I-V-P) curves are shown in Figure 10b. For a temperature difference (\(\Delta T\)) of 180 \({}^{\circ}\)C, a maximum power of 171 nW was obtained by applying a load resistance of 5 k\(\Omega\). More convenient ways to express the performance rely on dividing the power output by units of total, active, or cross-sectional areas of the generator [19]. Here, the total denotes the entire surface area of the TEG, and the actual area refers to the specific region where the CFS material is present. The thickness of the material is measured using a profilometer, and the cross-sectional area is obtained by multiplying it by the width of the film. Per unit of total area, the performance reads 27 nW cm\({}^{-2}\); in units of active area, 114 nW cm\({}^{-2}\); and in cross-sectional area, 4 mW cm\({}^{-2}\). The power output results are higher compared to some of the eco-friendly TEGs fabricated using hot injection and ball-milled inks. Cu\({}_{2}\)SnS\({}_{3}\), Cu\({}_{2}\)ZnSnS\({}_{4}\), Cu\({}_{2.124}\)Zn\({}_{0.875}\)SnS\({}_{3}\)Se TEGs, for example, presented a power output per unit of active area of 59 nW cm\({}^{-2}\), 43 nW cm\({}^{-2}\), 23 nW cm\({}^{-2}\), respectively, at a \(\Delta T=160\)\({}^{\circ}\)C [20]. The better homogeneity of the film provided by the three-step method can be the reason for the superior performance. However, a fair comparison can only be made between similar materials, and, to the best of our knowledge, no device made of CFS was reported in the literature. The values  
Figure 9: Electronic characterization of the CFS-18 samples with increasing temperature. The electrical conductivity is displayed in black squares (left axis), charge carrier concentration (CCC) in red, and electron mobility in blue, both in the right axis. reported here are still lower than the optimized TEGs for Cu\({}_{2+x}\)Zn\({}_{1-x}\)SnS\({}_{4}\) (152 nW cm\({}^{-2}\)) and Cu\({}_{2+x}\)Zn\({}_{1-x}\)SnSe\({}_{4}\) (279 nW cm\({}^{-2}\)) at a \(\Delta T=150\)\({}^{\circ}\)C, produced by sputtering [19]. It is important to notice that the TEG presented in this work is a proof-of-concept device fabricated using a noble approach where the CFS legs were deposited using the three-step method. The optimization of the CFS TEG can lead to a further enhancement in overall power output.