## 3 Results and Discussion
The samples containing rGO in the structure exhibit a similar behavior regarding the position of the photoluminescence emission peak, with a slight shift to the left, centered  
Figure 5: FTIR spectroscopy of ZnO/ZnS, GO and ZnO/ZnSr:rGO samples. at 473 nm, as shown in the inset in Figure 6. It is noticeable that the intensity of the luminescence peaks decreases with the amount of rGO added to the system, and the sample with the highest rGO content demonstrates the lowest intensity. This effect indicates a decreased electron-hole recombination rate, as the photo-excited electrons in the ZnO/ZnS semiconductor are rapidly transferred to rGO sheets [43]. The transfer of electrons to the rGO after the conduction band excitation process is because rGO acts as an efficient electron acceptor due to its two-dimensional pi-conjugated structure, resulting in a longer average lifetime for the electrons and, consequently, an increase in the photocurrent of our solar cell [44]. Figure 7 shows the UV-Vis spectra of the ZnO/ZnS and ZnO/ZnSr:rGO samples. As seen in Figure 7, a bandgap of 3.14 eV was obtained for ZnO/ZnS, with an intermediate gap of 3.05 eV, indicating the type II band alignment process [30]. Additionally, a decrease in the bandgap of the samples was observed with the amount of rGO added. The sample demonstrating the lowest bandgap was ZnO/ZnS4% rGO, which reached 2.3 eV. The insertion of rGO in samples increased signal intensity in the post-absorption region, which indicates the insertion of intermediate states in the bandgap according to the Wood/Tauc model [45; 46]. The reduction in the bandgap of the samples implies a shift of the absorption range of the compound closer to the visible range, which has a higher emission intensity. This shift contributes to the increased photocurrent and cell efficiency due to the higher availability of photons within the solar spectrum's energy range. For the sample with a bandgap of 2.3 eV, absorption was obtained at approximately 540 nm, which is a value very close to the maximum peak emission of solar radiation. These practical implications highlight the relevance of our findings in the context of solar energy conversion. Figure 6: PL spectra of the ZnO/ZnS and ZnO/ZnS:rGO samples. The change in bandgap energy can be attributed to several factors, including electronic interactions between ZnO/ZnS and rGO, the turbulent synthesis method used, and the formation of Zn-O-C chemical bonds [47; 48]. The conductive properties of rGO can influence its interaction with ZnO/ZnS, leading to a redistribution of energy levels and introducing intermediate levels between the valence and conduction bands. This process is directly related to Urbach energy, which arises from structural, chemical, and thermal defects within the material [49]. These defects create new energy states that can reduce the bandgap in the heterostructure [49]. Figure 8 presents the dark curve for the proposed photovoltaic cells. The responses obtained for the ZnO/ZnS, ZnO/ZnS:0.5% rGO, and ZnO/ZnS:1% rGO cells show a nonlinear behavior in the J-V ratio, which is a typical characteristic of semiconductor materials. In contrast, as the percentage of rGO increases in the ZnO/ZnS:2% rGO and ZnO/ZnS4% rGO cells, they lose their semiconductor behavior, indicating an ohmic trend. This behavior is related to the amount of reduced graphene oxide (rGO) present in the system. The presence of rGO can significantly alter the system's behavior, as shown by the J-V curves obtained in the dark. Furthermore, the absence of characteristic ZnO rods in the samples containing ZnO/ZnS with 2% and 4% rGO, as demonstrated by structural and morphological characterizations, contributes to the observed effects. Figure 7: Bandgap energy of ZnO/ZnS and ZnO/ZnS:rGO samples. Typically, for small amounts of rGO, as seen in the ZnO/ZnS, ZnO/ZnS:0.5% rGO, and ZnO/ZnS:1% rGO cells, and as illustrated in Figure 9, the systems exhibit an interesting photovoltaic response compared to pure ZnO/ZnS.