# Effects of Adding Alkali Metals and Organic Cations to Cu-Based Perovskite Solar Cells
## 3 Results and Discussion
Furthermore, comparing Figure 4b,c, the partial substitution of Na for some of the MA resulted in a band of Na s-orbitals slightly above the conduction band minimum and a slight increase in electron mobility. In Figure 4d, the energy gap was increased by substituting some of the I with Br, which was in good agreement with the calculated results for the total-substitution structure model. An increased effective mass for holes was attributed to 3d-orbital localization, as shown in Table 2.  
Figure 3: Calculated band structures and DOS of (**a**) NaPb\({}_{3}\), (**b**) KPb\({}_{3}\), (**c**) RbPb\({}_{3}\) and (**d**) CsPb\({}_{3}\). Figure 5 shows experimental current-voltage curves obtained from perovskite photovoltaic devices. Detailed parameters in Table 3 indicated the effects of additives on device performances. From the calculated results on the partial substitution structure models in Figure 4, it was expected that the addition of Cu would decrease the energy gap and reduce hole mobility. However, the results in Table 3 indicated that co-addition of Cu and Na slightly increased the energy gap, the short-circuit current density (\(J_{\text{SC}}\)), the open-circuit voltage (\(V_{\text{OC}}\)), and the fill factor (\(FF\)), resulting in a higher conversion efficiency. Figure 1a,b shows that the energy gap increased when I was replaced by Br, and the \(V_{\text{OC}}\) enhancement from the increased energy gap could be considered an effect of Br addition. The reason for the slight \(J_{\text{SC}}\) increase was attributed to increased carrier generation resulting from the acceptor-level Cu d-orbital band. The additional excitations from the Cu d-orbital to the Na s-orbital would also promote carrier generation. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Total Energy (keV)** & **Energy Gap (eV)** & \(m_{\text{e}}\)**\textasci{}/\(m_{\text{0}}\)** & \(m_{\text{n}}\)**\textasci{}/\(m_{\text{0}}\)** \\ \hline MA\({}_{0.875}\)EA\({}_{0.125}\)PbI\({}_{3}\) & \(-3.52\) & \(1.49\) & \(0.228\) & \(0.199\) \\ MA\({}_{0.875}\)EA\({}_{0.125}\)Pb\({}_{0.875}\)Cu\({}_{0.125}\)I\({}_{3}\) & \(-3.39\) & \(1.00\) & \(0.266\) & \(0.352\) \\ MA\({}_{0.750}\)EA\({}_{0.125}\)Na\({}_{0.125}\)Pb\({}_{0.875}\)Cu\({}_{0.125}\)I\({}_{3}\) & \(-3.32\) & \(1.04\) & \(0.241\) & \(0.355\) \\ MA\({}_{0.750}\)EA\({}_{0.125}\)Na\({}_{0.125}\)Pb\({}_{0.875}\)Cu\({}_{0.125}\)I\({}_{2.25}\)Br\({}_{0.75}\) & \(-3.36\) & \(1.30\) & \(0.250\) & \(0.490\) \\ \hline \end{tabular}
\end{table} 
Table 2: Calculated parameters of partially substituted perovskite compounds. Previously [29], the co-addition of 2% Cu and 2% Na exhibited the highest conversion efficiency. When the amount of EA was varied, in addition to 2% Cu and 2% Na, the highest conversion efficiency was obtained with 5% EA. For the device with the lowest amount of EABr, the effect of increasing the energy gap and \(V_{\text{OC}}\) was small because of the low bromine content. A comparatively small value of \(f_{\text{SC}}\) was obtained for the device with the highest amount of EABr. In Table 1, the calculated effective mass ratio for the electrons increased when I was replaced with Br. Hence, the electron mobility decreased with the addition of Br, decreasing \(f_{\text{SC}}\). The devices doped with Cu-Na and Cu-Na-EA exhibited the same conversion efficiencies. From the density of states in Figure 2b, no peaks corresponding to C or N were observed near the conduction-band minimum or the valence-band maximum. Thus, the substitution of MA with EA was estimated to have no special effect on the electronic structures. Furthermore, because there was almost no difference in the effective mass ratios of carriers between MAPbI3 and EAPbI3 (Table 1), the two devices had similar conversion efficiencies. However, there was a difference in the durability of the devices. The conversion efficiency of the device doped with Cu and Na decreased to 79.1% of the initial value after four weeks, while that of the device containing 2% Cu, 2% Na, and 5% EABr remained at 96.2% of its initial value.