## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**PV** & \(V_{oc}\) **(V)** & \(J_{sc}\) **(mA/cm\({}^{2}\))** & **FF** & \(PCE\) **(\%)** & \(P_{max}\) **(mW)** \\ \hline ZnO/ZnS & 0.73 & 0.64 & 0.45 & 0.21 & 0.212 \\ ZnO/ZnS:0.5\% rGO & 0.75 & 0.60 & 0.50 & 0.22 & 0.223 \\ ZnO/ZnS:1\% rGO & 0.70 & 0.60 & 0.57 & 0.24 & 0.238 \\ ZnO/ZnS:2\% rGO & 0.37 & 0.47 & 0.32 & 0.05 & 0.055 \\ ZnO/ZnS:4\% rGO & 0.41 & 0.43 & 0.33 & 0.06 & 0.058 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Photovoltaic parameters of the cells: (a) ZnO/ZnS; (b) ZnO/ZnS:0.5% rGO; (c) ZnO/ZnS:1% rGO; (d) ZnO/ZnS:2% rGO; and (e) ZnO/ZnS:4% rGO. Figure 9: Photovoltaic J-V response of the photovoltaic cells for the ZnO/ZnS and ZnO/ZnSr:GO samples. EIS [56; 57; 58] was performed to investigate recombination processes based on the Nyquist plot and the corresponding equivalent circuit. In Figure 11, it is observed that the ZnO/ZnS0.5% rGO PV cell exhibits the highest recombination resistance (\(R_{rec}\) = 274.70 \(\Omega\)/cm\({}^{2}\)), which was followed by the ZnO/ZnS1% rGO cell (\(R_{rec}\) = 118.60 \(\Omega\)/cm\({}^{2}\)), which aligns with the J-V curve (Figure 9), showing that the 0.5% and 1% rGO cells have the best photovoltaic performance. Table 2 summarizes the related EIS data acquired that were fitted using Z-view 3.1 software. Figure 11: Nyquist plots of ZnO/ZnS and ZnO/ZnS:rGO samples. Figure 10: Schematic diagram of the electrons flow in the DSSC. Furthermore the Bode phase plot (Figure 12) allowed us to investigate the effective lifetime of the electrons (\(\tau_{eff}\)) [59]. Using Equation (1), the values of (\(\tau_{eff}\)) were determined and are shown in Table 2.  
\[\tau_{eff}=\frac{1}{2\pi f}\, \tag{1}\]  
where the value of \(f\) is related to the highest frequency analyzed in the Bode phase plot. The analysis shows that the ZnO/ZnS:0.5% rGO DSSC exhibits a longer electron lifetime than the other cells (\(\tau_{eff}\) = 0.200 ms). Additionally, a decrease in electron lifetime was noted in cells with a higher concentration of rGO, which aligns with the lower \(J_{sc}\) values observed. These findings again underscore the importance of our research in understanding the role of rGO in photovoltaic systems. While small amounts of rGO can enhance the system's capacity, an increase in rGO content can reduce the energy conversion capability due to electron recombination and potential trapping. Therefore, a delicate balance between ZnO/ZnS and rGO content is crucial for achieving optimal efficiency with small amounts of rGO in combination with ZnO/ZnS likely to be the most effective approach. \begin{table}
\begin{tabular}{c c c c c} \hline
**PV** & \(R_{s}\) **(\(\Omega\)/cm\({}^{2}\))** & \(R_{rec}\) **(\(\Omega\)/cm\({}^{2}\))** & **\(CPE\) (\(\mu\)F/cm\({}^{2}\))** & \(n\)\({}^{1}\)** & \(\tau_{eff}\) **(ms)** \\ \hline ZnO/ZnS & 28.76 & 101.60 & 19.98 & 0.795 & 0.159 \\ ZnO/ZnS:0.5\% rGO & 21.83 & 274.70 & 14.73 & 0.744 & 0.200 \\ ZnO/ZnS:1\% rGO & 20.66 & 118.60 & 17.57 & 0.770 & 0.126 \\ ZnO/ZnS:2\% rGO & 22.81 & 93.65 & 15.80 & 0.815 & 0.116 \\ ZnO/ZnS:4\% rGO & 22.37 & 99.50 & 19.15 & 0.767 & 0.116 \\ \hline \end{tabular} \({}^{1}\)\(n\): Capacitance ideality index.  
\end{table} 
Table 2: EIS parameters of cells with photoelectrodes of: (a) ZnO/ZnS; (b) ZnO/ZnS:0.5% rGO; (c) ZnO/ZnS:1% rGO; (d) ZnO/ZnS:2% rGO; and (e) ZnO/ZnS:4% rGO. Figure 12: Bode phase plot of ZnO/ZnS and ZnO/ZnSr:rGO samples.