## 3 Results and Discussion
### Optimization of HTL Thickness  
The \(PCE\) of PSCs is affected by the properties of ETL and HTL materials. Furthermore, the thickness of the thin film is closely related to its electrical properties, which determine the transport of electrons and holes. Thickness is also directly related to \(PCE\). Thus, as shown in Figure 3, we first investigated the effects of thickness on the \(V_{oc}\), \(J_{sc}\), \(FF\), and \(PCE\) of the Cu\({}_{2}\)O HTL layer. Extremely thin HTL film forms micro-pinholes, resulting in direct contact between the absorption layer and the rear electrode, and an extremely thick HTL can increase the series resistance in the interface between the metal electrode and the HTL [61].  
Notably, as shown in Figure 3, the \(PCE\) value does not change with an increasing HTL thickness at the initial acceptor density of the HTL layer from Table 1. To understand such behavior from the \(PCE\), the effects of HTL thickness on \(V_{oc}\), \(J_{sc}\), and \(FF\) were examined. Similar to \(PCE\), all these parameters remain almost constant with an increasing HTL thickness. In general, as the thickness of the charge transport layer increases, the physical distance that the charge can migrate across increases. However, the results in Figure 3 demonstrate that the \(PCE\) remains almost constant with an increasing HTL thickness. To clarify these results further, we examined the factors that had the most significant effect by changing the level of each parameter specified in Table 1. Among them, the impact of the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \(V_{oc}\) & \(J_{sc}\) & \(FF\) & \(PCE\) \\ \hline
0.83 V & 21.54 mA/cm\({}^{2}\) & 76.68\% & 13.79\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Solar cell main parameters for the ITO/TiO\({}_{2}\)/MAPbI\({}_{3}\)/Cu\({}_{2}\)O/Au PSC structure.  
Figure 2: Current Density-Voltage (\(J\)–\(V\)) graph for the ITO/TiO\({}_{2}\)/MAPbI\({}_{3}\)/Cu\({}_{2}\)O/Au PSC structure.  
Figure 4 shows the _PCE_ values according to the thickness and acceptor density of Cu\({}_{2}\)O. Figure 4 confirms that the efficiency decreases with an increasing thickness when the acceptor density of HTL is 10\({}^{15}\) cm\({}^{-3}\) or less. In contrast, when the acceptor density is 10\({}^{16}\) cm\({}^{-3}\) or higher, the _PCE_ changes insignificantly due to the increase in thickness. To investigate the cause of this effect, the relationship between the _PCE_ and acceptor density was examined for a 100 nm HTL thickness, where we obtained a remarkable change. Figure 5 shows the _PCE_ as a function of the acceptor density at a Cu\({}_{2}\)O thickness of 100 nm. This result indicates that the higher the acceptor density, the greater the efficiency. In contrast, the _PCE_ and _CCE_ are more sensitive to the acceptor density, and the _CCE_ changes are more sensitive to the acceptor density.  
Figure 4: HTL thickness vs. _PCE_ curves for various HTL acceptor densities.  
Figure 3: _PCE_ curve for various HTL (Cu\({}_{2}\)O) thicknesses.  
particular, the rapid increase in the values of \(10^{16}\) cm\({}^{-3}\) or higher appears to be due to the improvement of sheet resistance and conductivity at a high doping rate [62; 63].