## 3 Results and Discussion
### Electrical Response  
The final structure to produce the diode was carried out using a substrate coated with ITO as a transparent conductor for frontal contact to produce the ITO/CdS/PbSe heterostructure. Subsequently, silver electrodes were deposited on the PbSe as rear contacts to perform the I vs. V characterization under darkness conditions (Figure 6). The first measurement presented an ohmic behavior, meaning that the p-n junction CdS/PbSe did not form a depletion zone, possibly because there was no full coupling among them (Figure 7a). Thus, an annealing treatment was performed to promote the layers' diffusion, with atmospheric conditions of 150 \({}^{\circ}\)C for 5 min. The result of the thermally annealed heterostructure was the forward bias response of a typical diode behavior (Figure 7b). It is important to mention that the effect of mild thermal annealing was to change the response from ohmic to diode behavior, since only 5 min at a relatively low temperature was needed to produce a better diffusion between the semiconductors to improve the p-n junction. Figure 5: Energy direct bandgap calculated by Tauc procedure for each involved material of the heterojunction: (**a**) CdS; and (**b**) PbSe. Figure 6: Photograph of ITO/CdS/PbSe heterostructure (real device). According to Figure 7b, the forward bias of the diode response was in the order of 0.8 V, and the current-voltage characteristics were analyzed by using the modified Shockley model [29; 30; 31]:  
\[I=I_{\rm s}\left[\exp\left(\frac{V-R_{\rm s}I}{\eta V_{T}}\right)-1\right] \tag{1}\]  
\[I_{\rm s}=A_{e}A*T^{2}\exp\left(-\frac{\phi_{B}}{V_{T}}\right) \tag{2}\]  
where \(I_{\rm s}\) is the saturation current, \(V_{T}=kT/q\) is the thermal voltage, k is the Boltzmann's constant, \(q=e\) is the elementary charge, T is the absolute temperature (300 K), \(\eta\) is the ideality factor, R\({}_{\rm s}\) is the series resistance, \(A_{e}\) is the effective area of the diode (625 mm\({}^{2}\)), \(A*\) is the effective Richardson's constant and \(\Phi_{B}\) is the barrier height. The diode parameters were extracted by fitting this model to the experimental curve, as shown in Table 1.  
The ideality factor obtained was 2.47 for the synthetized device, while [PERSON] et al. [18] reported an ideality factor of 1.79 for the same CdS/PbSe p-n heterojunction but placed on a Si wafer with ultrathin CaF\({}_{2}\) (2 nm). They grew CdS thin film via CBD in the method as used in this study, while PbSe film was set down via the MBE technique, which is a more expensive, complex, and non-scalable technique compared to our deposition method. The difference in the ideality factor may be attributed to the diffusion between ITO (500 nm) and CdS (100 nm) layers, which could form a deep combination among the metallic oxide and semiconductor behaving as a metal oxide semiconductor (MOS) hybrid layer, that together with PbSe produce a heterostructure with an ideality factor \(\eta>2\), that is categorized as a Schottky diode according to the literature. Also, ideality \(\eta>2\) can be attributed to defects and imperfections that affect the recombination current, which does not flux homogeneously at the junction and to the presence of trapping levels within the depletion region, interfacial states, or the existence of other conduction mechanisms [29; 31; 32; 33]. Furthermore, according to the literature, there are two types of contacts for a metal-semiconductor junction: ohmic and rectifier (Schottky) [34; 35]. With this in mind, the similarity of the ITO/CdS/PbSe heterojunction with a Schottky diode suggests that the ohmic behavior initially obtained is due to the formation of a tunnel ohmic contact. Figure 8a shows the suggested energy band diagram for this type of contact, which forms a very