# Effects of Adding Alkali Metals and Organic Cations to Cu-Based Perovskite Solar Cells
## 3 Results and Discussion
The total-energy calculation results in Table 1 indicate that the crystal structure was destabilized when Pb and MA were replaced with Cu and Na, respectively. The devices with Cu, Na, and EA exhibited higher durabilities than devices with just Cu and Na, which confirmed that the addition of EA stabilized the crystal structure. Therefore, the addition of organic cations more stable than MA to Cu-based perovskites enhanced the durability. It was also expected that the addition of EA was not a significant effect on the electronic structure. Figure 5: \(J\)-\(V\) characteristics of the present perovskite solar cells. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Devices** & \(J_{\text{SC}}\) **(mA cm\({}^{-2}\))** & \(V_{\text{OC}}\) **(V)** & **FF** & \(R_{\text{S}}\) **(\(\Omega\) cm\({}^{2}\))** & \(R_{\text{Sh}}\)**(\(\Omega\) cm\({}^{2}\))** & \(\eta\) **(\%)** & \(\eta_{\text{ave}}\) **(\%)** & \(E_{\text{g}}\) \\ \hline Standard & 22.2 & 0.739 & 0.497 & 4.18 & 383 & 8.17 & 5.82 & 1.55 \\ Cu 2\% + Na 2\% & 21.7 & 0.841 & 0.578 & 5.93 & 594 & 10.5 & 8.36 & 1.56 \\ Cu 2\% + Na 2\% + EA 2.5\% & 20.0 & 0.811 & 0.559 & 6.55 & 604 & 9.08 & 7.20 & 1.56 \\ Cu 2\% + Na 2\% + EA 5\% & 21.3 & 0.843 & 0.585 & 5.12 & 558 & 10.5 & 8.67 & 1.57 \\ Cu 2 + Na 2\% + EA 10\% & 16.1 & 0.824 & 0.586 & 5.29 & 445 & 7.78 & 5.77 & 1.57 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Measured device parameters of perovskite solar cells. that the device durability was highly dependent on the stability of the perovskite crystal, and that the durability could be estimated by the calculated total energy. Figure 6 shows X-ray diffraction (XRD) measurements for the MAPbI3 device and those doped with Cu, Na, and EA. Table 4 shows the measaured structural parameters calculated from the XRD data. A diffraction peak corresponding to tetragonal (211) planes was observed at 23.5\({}^{\circ}\). The peak intensity was slightly lower in the doped devices relative to that for MAPbI3, suggesting that elimination of lattice distortion by the substitution of Pb with Cu reduced the peak intensity. Although the ionic radii of Cu and Na are smaller than those of Pb and MA, their additions increased the lattice constant. The crystal lattice was expanded by the presence of Na in the interstitial position relative to that of MA. The lattice constants of devices doped with Cu, Na, and EA were the smallest, because of I substitution with Br, which had a smaller ionic radius. Crystallites of MAPbI3 were larger than those of the doped devices. Previously, CuBr2 addition to the perovskite precursor solution slowed the crystallization rate of the perovskite film, and decreased the crystallite size and diffraction-peak intensity. During annealing of the perovskite film, the color-changing rates with added Cu and Na were slower than those of MAPbI3, indicating that the addition of CuBr2 decreased the crystallization rate. Figure 7 shows optical microscopy images of perovskite crystals in actual devices. In MAPbI3 (Figure 7a), the surface morphology was non-uniform with less coverage. The addition of Cu and Na significantly increased the surface coverage and the homogeneity, as observed in Figure 7b. This suggests that the addition of CuBr2 reduced the crystallization rate, which, combined with the gradual increase of the annealing temperature, contributed to the formation of a homogeneous film. The dens  
As noted earlier in the calculated results of the partial-substitution structure model (Figure 4), Cu addition formed a shallow band of Cu d-orbitals slightly above the band of iodine p-orbitals, which reduced the energy gap and decreased hole mobility. However, if the Cu d-orbital band was an acceptor level, it would increase the device performance. Figure 8 shows an enlarged view of the band structure in Figure 4c. At the valence band maximum, the excitation of electrons from the I p-orbitals to the Cu d-orbitals increased the generation of carriers and increased \(J_{\text{SC}}\).