*2.2. Device Model and Parameters*

The conventional undoped absorber and proposed hybrid lead-free PCS structures are illustrated in Figure 1a,b, respectively. In Figure 1b, the p-CH3NH3SnI3 and n-CH3NH3SnI3 are perovskite layers having two different doping types, namely the p-type and n-type regions. In these devices, TCO and Au are front and back electrodes, TiO2 is used as the electron transporting layer (ETL), and Spiro-OMeTAD is utilized as a hole transporting layer (HTL). Further, Figure 1c presents the energy level diagram of the conventional pin structure and how carriers are transported across the layers.

**Figure 1.** (**a**) A schematic representation of the conventional pin lead-free PSC, (**b**) the proposed hybrid hetero-homo junctionbased structure, (**c**) the energy level diagram of the conventional pin structure showing how carriers are transported across the layers.

In order to validate the SCAPS simulation model, the conventional pin lead-free PSC with the structure of TCO/TiO2/CH3NH3SnI3/Spiro-OMeTAD/Au [47], illustrated in Figure 1a, is simulated. The material parameters for the used layers are given in Table 1, which are derived from some reported experimental and simulated works [27,47–51]. The thickness is denoted by *t*, the band gap energy is termed *Eg* while the electron affinity is denoted by χ. The relative dielectric permittivity is *εr***,** and conduction and valence band effective density of states are *Nc* and *Nv*, respectively. *μ<sup>n</sup>* and *μ<sup>p</sup>* are the electron and hole mobility, respectively. The donor concentration, acceptor concentration and trap density are denoted as *ND*, *NA* and *Nt*, respectively. The front and back electrode work functions are 4.4 eV (corresponding to TCO) and 5.4 eV (corresponding to Au), respectively. Other parameters of the front and back metal contacts utilized in SCPAS simulation are presented in Table S1 in the Supplementary Material.


**Table 1.** Simulation parameters of materials used in simulation of PSC devices.

In this simulation study, the defects are situated above the valence band by 0.65 eV (which coincides with the mid gap of the perovskite material under consideration) and put as neutral Gaussian distribution, having a characteristic energy of 0.1 eV. The trap density was found to be 5 × 1017 cm−<sup>3</sup> for the best fit between experimental results and simulation. The capture cross-section of the electron (σn) and hole (σp) is 1 × <sup>10</sup>−<sup>15</sup> cm2. Very thin interface defect layers (IDLs) are inserted at the TiO2/CH3NH3SnI3 and CH3NH3SnI3/Spiro-OMeTAD interfaces to represent the interface carrier recombination.

The physical parameters of the IDLs are as follows. The defect energy (*Et*) is located at the mid gap of the perovskite and the defect type is set as neutral single defect with a total density of 10<sup>17</sup> cm−3. The capture cross-section of the electron and hole is also <sup>1</sup> × <sup>10</sup>−<sup>15</sup> cm2, like the bulk traps. A summary of the main defect parameters is presented in Table 2.

**Table 2.** Defect density parameters in the absorber and at the interfaces.


Further, Equation (6) is used for the calculations of the absorption coefficients (*α*) of TCO, ETL, CH3NH3SnI3 and HTL materials with a pre-factor (*Aα*), which is selected to be 105 cm<sup>−</sup>1eV−1/2 [52],

$$a(E) = A\_a \sqrt{h\mathbf{v} - E\_\mathcal{g}} \tag{6}$$

The simulation results are compared vs the reported experimental results [47]. Figure 2 illustrates the illuminated *J*–*V* curve of the simulated device vs measurements showing a good accuracy. The main parameters are listed in Table 3, indicating an absolute error (Δξ) of less than 8% for all parameters. Therefore, the reliability of our simulation model is validated, and further inspections can be done based on this simulation model.

**Figure 2.** The illuminated *J*–*V* curve of the simulated device vs measurements.

**Table 3.** Main performance parameters and absolute error between simulation results compared with measurements.

