## 2 Simulation Procedure  
In this study, as shown in Figure 1, ITO/ZnO/CsPbI\({}_{x}\)Br\({}_{3-x}\)/NiO\({}_{x}\)/Au PSC devices with an n-i-p structure were employed. The simulation was performed using SCAPS-1D simulation software ([PERSON] et al., 2000) [29]. This tool is based on [PERSON]'s equation (Equation (1)) and the continuity equation for each electron and hole (Equations (2) and (3)), and it helps to calculate the short-circuit current density (J\({}_{\rm sc}\)), open circuit voltage (V\({}_{\rm oc}\)), power conversion efficiency (PCE), fill factor (FF), quantum efficiency (QE), and recombination current, among others [30]. It is also based on the drift-diffusion equation (Equation (4)), which causes carrier transport in semiconductors [31]:  
\[d^{2}\varphi(x)/dx^{2}=e/\mathcal{E}_{0}\mathcal{E}_{\rm r}\big{(}p(x)-n(x)+ \mathrm{N}_{\rm D}-\mathrm{N}_{\rm A}+\rho_{\rm p}-\rho_{n}\big{)} \tag{1}\]  
In [PERSON]'s equation, we have the following:  
* \(\varphi=\) the electrostatic potential;
* \(e=\) electrical charge;
* \(\varepsilon_{\rm r}=\) relative permittivity;
* \(\varepsilon_{0}=\) vacuum permittivity;
* \(\mathrm{N}_{\rm D}=\) charged impurities of the donor;
* \(\mathrm{N}_{\rm A}=\) charged impurities of the acceptor;
* \(\rho_{\rm p}=\) distribution of holes;
* \(\rho_{n}=\) distribution of electrons. \[d\mathrm{J}_{n}/dx =\mathrm{G}-\mathrm{R} \tag{2}\] \[d\mathrm{J}_{\rm p}/dx =\mathrm{G}-\mathrm{R} \tag{3}\]  
In the continuity equation, we have the following:  
* R = the recombination rate;
* J\({}_{\rm n}\) = the electron current density;
* J\({}_{\rm p}\) = the hole current density;
* G = the generation rate. \[\mathrm{J}_{n}(x)=\mathrm{q}\mu_{n}n(x)\mathcal{E}(x)+\mathrm{q}\mathrm{D}_{n }\frac{dn(x)}{dx},\,\mathrm{J}_{\rm p}(x)=\mathrm{q}\mu_{p}p(x)\mathcal{E}(x)- \mathrm{q}\mathrm{D}_{p}\frac{dp(x)}{dx} \tag{4}\]  
In the above drift-diffusion model, we have the following:  
* q = the charge of the electron;
* \(\mu_{n}=\) the mobility of the electrons;
* \(\mu_{\rm p}=\) the mobility of the holes;
* \(n(x)=\) electron distribution;
* \(p(x)=\) hole distribution;
* \(\mathcal{E}(x)=\) the electric field;* D\({}_{n}\) = the electron diffusion coefficient;
* D\({}_{p}\) = the hole diffusion coefficient. Figure 1a shows the basic structure of the all-inorganic multi-layer perovskite solar cell (PSC) in the study. ITO, ZnO, CsPbI,Br3-x, NiO\({}_{x}\), and Au were used as the front metal contact, ETL, perovskite active layer (PAL), HTL, and back metal contact, respectively. The energy band alignment of this multi-layer perovskite solar cell (PSC) is shown in Figure 1b. The work functions of ITO (used as a front contact) and Au (used as a back contact) were \(-4.52\) eV and \(-5.1\) eV, respectively [32; 33]. The lowest unoccupied molecular orbital (LUMO) levels and the highest occupied molecular orbital (HOMO) levels of the ZnO, CsPbI\({}_{x}\)Br3-x, and NiO\({}_{x}\) were \(-4.1\) eV, \(-3.95\) eV, and \(-1.46\) eV and \(-7.4\) eV, \(-5.73\) eV, and \(-5.26\) eV, respectively [34; 35; 36; 37]. Through this optimized energy band alignment, it can be seen that the electrons and holes generated in the CsPbI\({}_{x}\)Br3-x absorption layer can efficiently move to the ITO electrode and the Au electrode, respectively. This device analysis was conducted under AM1.5G spectrum (1000 W/m, T = 300 K) conditions. Figure 1: (**a**) The basic structure of an all-inorganic multi-layer perovskite solar cell (PSC). (**b**) The energy band alignment of an all-inorganic perovskite solar cell (PSC). Table 1 summarizes the parameters of the PSC layers used in this study with SCAPS simulation software. For the numerical values of the ETL, HTL, and absorption layer, the previously published literature was referenced [34; 35; 36; 37]. For the initial PSC device configuration, the thickness of each layer of ZnO, CsPbIxBr3-x, and NiOx was 100 nm, 800 nm, and 50 nm, respectively. After that, optimization simulation was performed for each thickness to increase the power conversion efficiency first.