Using Cu2O/ZnO as Two-Dimensional Hole/Electron Transport Nanolayers in Unleaded FASnI3 Perovskite Solar Cells

A Pb-free FASnI3 perovskite solar cell improved by using Cu2O/ZnO as two-dimensional-based hole/electron transport nanolayers has been proposed and studied by using a SCAPS-1D solar simulator. To calibrate our study, at first, an FTO/ZnO/MAPbI3/Cu2O/Au multilayer device was simulated, and the numerical results (including a conversion efficiency of 6.06%, an open circuit potential of 0.76 V, a fill factor parameter of 64.91%, and a short circuit electric current density of 12.26 mA/cm2) were compared with the experimental results in the literature. Then, the conversion efficiency of the proposed FASnI3-based solar cell was found to improve to 7.83%. The depth profile energy levels, charge carrier concentrations, recombination rate of electron/hole pair, and the FASnI3 thickness-dependent solar cell efficiency were studied and compared with the results obtained for the MAPbI3-containing device (as a benchmark). Interestingly, the FASnI3 material required to obtain an optimized solar cell is one-half of the material required for an optimized MAPbI3-based device, with a thickness of 200 nm. These results indicate that developing more environmentally friendly perovskite solar cells is possible if suitable electron/hole transport layers are selected along with the upcoming Pb-free perovskite absorber layers.


Introduction
Recently, perovskite-based solar cells (PSCs) have attracted significant attention because of their features, including excellent extinction coefficient, solution-processed synthesize methods, suitable band gap engineering, and remarkable growth in power conversion efficiency (PCE) up to ~25% [1,2] (which was initially only 3.8% in 2009 [3]).The appropriate selection of the hole transport layer (HTL) and electron transport layer (ETL) in a perovskite solar cell is a vital factor in the enhancement of the stability and performance of PSCs [4,5].The cost and efficiency of PSCs are usually determined by the kind of HTL that is employed.The HTL is a component that impacts the performance of PSCs by extracting the photo-induced holes from the perovskite region to the external electrode.
The most usual HTL used in PSCs is the spiro-OMeTAD, which, besides being very costly, faces various degradation issues.Therefore, it cannot be considered a proper choice for large-scale production.Hence, a low-cost and stable p-type metal oxide should be selected as the HTL in the next generation of PSCs.The optimum selection of metal oxide exhibits unique properties for electric charges to transport between layers by creating a cascade structure that is in better alignment with the energy bands in a solar cell.It was found that the application of metal oxides as effective charge transport layers can significantly improve the performance and stability of PSCs [6].Several inorganic materials, including NiO, Cu 2 O, and CuO, have been introduced as acceptable candidates for application as HTLs [7][8][9].Among these various materials, cuprous oxide (Cu 2 O) has been proposed as a superior HTL with a suitable band gap of ~2 eV [10], a high stability, and improved efficiency [11].For example, recently, Cu 2 O was used as an HTL for a ZnO-based PCS fabricated at room temperature [12].The fabricated device with an FTO/ZnO/CH 3 NH 3 PbI 3 /Cu 2 O/Au structure showed open circuit potential (V OC ), short circuit electric current density (J SC ), fill factor parameter (FF), and PCE values of 0.76 V, 12 mA/cm 2 , 63%, and 6.02%, respectively.
However, the presence of toxic lead metal in the structure of methylammonium Pbiodide (MAPbI 3 ) perovskite-based solar cells (as the material initiated the interest in lead halide perovskites for solar cell applications [13]) is a major concern from an environmental perspective.Therefore, introducing non-toxic elements into the perovskite structure is required.In this regard, different kinds of less toxic cations including Sn, Ge, Bi, Sb, and Ti could be substitute elements for lead in PSCs [14,15].Among them, Sn-based halide perovskites have received more attention because of their similar properties to Pb-halide PSCs and comparable PCE values [16].As a two-dimensional layer nanomaterial, graphene enhances the photoelectric conversion efficiency of dye-sensitized solar cells [17].
Recent investigations on formamidinium tin iodide (FASnI 3 ) with the chemical formula of CH 4 N 2 SnI 3 and a direct energy band gap of 1.41eV demonstrated that it can be considered a good candidate as an absorber layer in non-toxic PSCs.Moreover, FASnI 3based PSCs are distinguished as being more stable and efficient absorber layers than MASnI 3 -based ones due to their better thermal stability and greater band gap [18].Concerning this, Neol et al. proposed a Pb-free tin halide CH 3 NH 3 SnI 3 -based solar cell, which showed a PCE value of 6% [19].A Pb-free formamidinium tin-based PSC with optimized photovoltaic parameters of a V oc value of 1.81 V, a J sc value of 31.20 mA/cm 2 , an FF value of 33.72%, and a PCE of 19.08% was also examined by Kumar et al. [20].Koh et al. studied FASnI 3 -based perovskite solar cells with an FTO/TiO 2 /FASnI 3 /spiro-OMeTAD/gold multilayer structure.The results showed J SC , V OC , FF, and PCE values of 12.4 mA/cm 2 , 0.26 V, 44%, and 1.41%, respectively [21].Bian et al. found that the performance of Pb-free formamidinium Sn-triiodide PSC can be improved by Sn source purification, resulting in a maximum PCE of 6.75%, a V OC of 0.58 V, a J SC of 17.5 mA/cm 2 , and an FF of 66.3% in an indium tin oxide/PEDOT:PSS/CH 4 N 2 SnI 3 /C 60 /BCP/silver multilayer structure [22].López-Fernández et al. provided a comprehensive summary of the recent advancements, persistent challenges, and prospects regarding the synthesis, optical spectroscopy, and optoelectronic devices of Pb-free halide perovskite thin films and nanocrystals.This endeavor aims to offer guidance to both current researchers in this domain and future entrants.In general, they demonstrated the potential of Pb-free perovskites for applications in photovoltaics and optoelectronics [23].Ran et al. reported a perovskite precursor solution of FASnI 3 utilized to incorporate conjugated large-volume amines, specifically 3-phenyl-2propen-1-amine, thereby enhancing the photovoltaic performance of the resulting films.They also introduced a viable method to achieve equilibrium between stability and charge extraction in tin-based perovskites and underscored the significant prospects for enhancing efficiencies in tin-based PSCs [24].A functional additive based on pseudohalides in FASnI 3 was shown to be advantageous for enhancing the formation of the film, as well as for addressing imperfections both at the interface and within the bulk material, in a study by Dhruba B. Khadka et al. [25].Youssef El Arfaoui et al. proposed and analyzed an optimized tandem solar device consisting of lead-free perovskite CsGeI 3 /FASnI 3 with a high efficiency of 30.42%.This optimization was achieved through a numerical simulation using SCAPS [26].
However, there has not been an investigation concerning the performance of the FASnI 3 absorber layer in a perovskite-based solar cell structure containing Cu 2 O/ZnO as a two-dimensional (2D)-based HTL/ETL.Compared to 1D nanomaterials, 2D nanostructures, such as nanosheets or nanoplates, with large exposed surface areas and specific crystal facets, undoubtedly offer more space for adsorption and electron transmission.This could be a possible approach to enhance the photovoltaic properties of ZnO-based solar cells.
Therefore, in the present study, an FTO/ZnO/FASnI 3 /Cu 2 O/Au multilayer structure is proposed and studied by using a SCAPS-1D solar simulator.The findings, such as the depth profile energy levels, charge carrier concentrations, electron/hole pair recombination rates, and the FASnI 3 thickness-dependent solar cell efficiency, are compared with those obtained for the FTO/ZnO/MAPbI 3 /Cu 2 O/Au structure as benchmarks.

Simulation Parameters
A simulation of planar FTO/ZnO/Perovskite/Cu 2 O/Au heterojunction solar cells was performed for both MAPbI 3 and FASnI 3 perovskites by utilizing the Solar Cell Capacitance Program (SCAPS-1D) [27,28].The total thicknesses of the ZnO and Cu 2 O 2D multilayer structures were considered to be 50 (~95 monolayers) and 200 nm, respectively.These parameters were selected so that comparing the results with the experimental results reported in the literature was possible.The shallow donor and acceptor density in the ZnO and Cu 2 O nanolayers was set as 10 18 cm −3 [28,29].The solar cell architecture and corresponding energy level diagrams of the MAPbI 3 and FASnI 3 -based PSC devices are presented in Figure 1.The basic parameters of layers which are based on some previously published works are listed in Table 1 [30][31][32][33][34][35].
Materials 2024, 17, x FOR PEER REVIEW 3 of 11 crystal facets, undoubtedly offer more space for adsorption and electron transmission.This could be a possible approach to enhance the photovoltaic properties of ZnO-based solar cells.Therefore, in the present study, an FTO/ZnO/FASnI3/Cu2O/Au multilayer structure is proposed and studied by using a SCAPS-1D solar simulator.The findings, such as the depth profile energy levels, charge carrier concentrations, electron/hole pair recombination rates, and the FASnI3 thickness-dependent solar cell efficiency, are compared with those obtained for the FTO/ZnO/MAPbI3/Cu2O/Au structure as benchmarks.

Simulation Parameters
A simulation of planar FTO/ZnO/Perovskite/Cu2O/Au heterojunction solar cells was performed for both MAPbI3 and FASnI3 perovskites by utilizing the Solar Cell Capacitance Program (SCAPS-1D) [27,28].The total thicknesses of the ZnO and Cu2O 2D multilayer structures were considered to be 50 (~95 monolayers) and 200 nm, respectively.These parameters were selected so that comparing the results with the experimental results reported in the literature was possible.The shallow donor and acceptor density in the ZnO and Cu2O nanolayers was set as 10 18 cm −3 [28,29].The solar cell architecture and corresponding energy level diagrams of the MAPbI3 and FASnI3-based PSC devices are presented in Figure 1.The basic parameters of layers which are based on some previously published works are listed in Table 1 [30][31][32][33][34][35].   Figure 2a exhibits the short circuit electric current density-voltage (J-V) curves of the experimental [12] and simulation results for solar cells with FTO/ZnO/MAPbI3/Cu2O/Au Figure 2a exhibits the short circuit electric current density-voltage (J-V) curves of the experimental [12] and simulation results for solar cells with FTO/ZnO/MAPbI 3 /Cu 2 O/Au structures.A comparison between the photovoltaic parameters of the simulated device and the reported measurements is shown in Table 2.The consistency of the results for the MAPbI 3 -based structure provided a justification for the simulation method.Then, the justified model was extended to the new FASnI 3 -based cells for further evaluation.In Figure 2b, it is seen that the device with a MAPbI 3 absorber has a larger V OC but a smaller J SC than the device with a FASnI 3 layer.This decrease in the V OC of a device with a FASnI 3 absorber refers to the location of the highest occupied molecular orbital (HOMO) level of the absorber layer, which is higher than the valance band level of Cu 2 O.In such a situation, the extraction of holes becomes slower, which results in an increase in the electron/hole recombination rate at the perovskite/HTL interface.Therefore, the V OC was reduced from 0.76 to 0.54 V.In fact, it can be seen in Figure 1c,d that the C.B level of MAPbI 3 is located at −3.93 eV, while that of FASnI 3 is located at −3.52 eV.Therefore, the difference between C.B of Cu 2 O and the absorber layer in the device with MAPbI 3 is 0.73 eV, while it is 0.32 eV in the device with FASnI 3 .As a result, the recombination rate in the device with MAPbI 3 is lower than that of the device with FASnI 3 .That is why the V oc in the device with MAPbI 3 (0.76 V) is higher than that of the device with FASnI 3 (0.54 V). structures.A comparison between the photovoltaic parameters of the simulated device and the reported measurements is shown in Table 2.The consistency of the results for the MAPbI3-based structure provided a justification for the simulation method.Then, the justified model was extended to the new FASnI3-based cells for further evaluation.In Figure 2b, it is seen that the device with a MAPbI3 absorber has a larger VOC but a smaller JSC than the device with a FASnI3 layer.This decrease in the VOC of a device with a FASnI3 absorber refers to the location of the highest occupied molecular orbital (HOMO) level of the absorber layer, which is higher than the valance band level of Cu2O.In such a situation, the extraction of holes becomes slower, which results in an increase in the electron/hole recombination rate at the perovskite/HTL interface.Therefore, the VOC was reduced from 0.76 to 0.54 V.In fact, it can be seen in Figure 1c,d that the C.B level of MAPbI3 is located at −3.93 eV, while that of FASnI3 is located at −3.52 eV.Therefore, the difference between C.B of Cu2O and the absorber layer in the device with MAPbI3 is 0.73 eV, while it is 0.32 eV in the device with FASnI3.As a result, the recombination rate in the device with MAPbI3 is lower than that of the device with FASnI3.That is why the Voc in the device with MAPbI3 (0.76 V) is higher than that of the device with FASnI3 (0.54 V).

Results and Discussion
In Figure 1c, the HOMO level of MAPbI3 is located at −5.54 eV, which is lower than that of the Cu2O (−5.37 eV), making sure a significant driving force is applied to the holes and resulting in their extraction from the perovskite layer and transfer into the HTL.Similarly, the lowest unoccupied molecular orbital (LUMO) levels of the perovskite and ZnO (electron transport layer, ETL) are located at −3.93 and −4.4 eV, respectively.This can provide a suitable path for electrons toward the ZnO nanolayer.In a device with a FASnI3 perovskite, for the best calibration, the initial carrier density of holes was assumed to be 7 × 10 16 cm −3 [34].The deficiencies at the ETL/perovskite and the perovskite/HTM interfaces  structures.A comparison between the photovoltaic parameters of the and the reported measurements is shown in Table 2.The consistency o MAPbI3-based structure provided a justification for the simulation met tified model was extended to the new FASnI3-based cells for further ev 2b, it is seen that the device with a MAPbI3 absorber has a larger VOC bu the device with a FASnI3 layer.This decrease in the VOC of a device with refers to the location of the highest occupied molecular orbital (HOM sorber layer, which is higher than the valance band level of Cu2O.In su extraction of holes becomes slower, which results in an increase in th combination rate at the perovskite/HTL interface.Therefore, the VOC 0.76 to 0.54 V.In fact, it can be seen in Figure 1c,d that the C.B level of at −3.93 eV, while that of FASnI3 is located at −3.52 eV.Therefore, the d C.B of Cu2O and the absorber layer in the device with MAPbI3 is 0.73 eV in the device with FASnI3.As a result, the recombination rate in the d is lower than that of the device with FASnI3.That is why the Voc in the d (0.76 V) is higher than that of the device with FASnI3 (0.54 V).

Results and Discussion
In Figure 1c, the HOMO level of MAPbI3 is located at −5.54 eV, w that of the Cu2O (−5.37 eV), making sure a significant driving force is a and resulting in their extraction from the perovskite layer and transfer ilarly, the lowest unoccupied molecular orbital (LUMO) levels of the p (electron transport layer, ETL) are located at −3.93 and −4.4 eV, respecti vide a suitable path for electrons toward the ZnO nanolayer.In a de perovskite, for the best calibration, the initial carrier density of holes w × 10 16 cm −3 [34].The deficiencies at the ETL/perovskite and the perovsk (%) 6.02 6.06 7.83

Results and Discussion
In Figure 1c, the HOMO level of MAPbI 3 is located at −5.54 eV, which is lower than that of the Cu 2 O (−5.37 eV), making sure a significant driving force is applied to the holes and resulting in their extraction from the perovskite layer and transfer into the HTL.Similarly, the lowest unoccupied molecular orbital (LUMO) levels of the perovskite and ZnO (electron transport layer, ETL) are located at −3.93 and −4.4 eV, respectively.This can provide a suitable path for electrons toward the ZnO nanolayer.In a device with a FASnI 3 perovskite, for the best calibration, the initial carrier density of holes was assumed to be 7 × 10 16 cm −3 [34].The deficiencies at the ETL/perovskite and the perovskite/HTM interfaces are assumed to be single and/or neutral.The energy level diagram and carrier concentrations across the devices with MAPbI 3 and FASnI 3 perovskites under the AM 1.5 solar spectrum are presented in Figure 3.In the FASnI 3 -based PSCs, the HOMO level of FASnI 3 is located at −4.93 eV, which is higher than that of Cu 2 O (−5.37 eV).This barrier for hole transporting increases the carrier recombination possibility at the FASnI 3 /Cu 2 O interface.Therefore, the V OC decreases from 0.76 V to 0.54 V.
Materials 2024, 17, x FOR PEER REVIEW 5 of 11 are assumed to be single and/or neutral.The energy level diagram and carrier concentrations across the devices with MAPbI3 and FASnI3 perovskites under the AM 1.5 solar spectrum are presented in Figure 3.In the FASnI3-based PSCs, the HOMO level of FASnI3 is located at −4.93 eV, which is higher than that of Cu2O (−5.37 eV).This barrier for hole transporting increases the carrier recombination possibility at the FASnI3/Cu2O interface.Therefore, the VOC decreases from 0.76 V to 0.54 V.It can be seen from Figure 3a,b that the Fermi level of electrons (Fn) is positioned under (~0.4 eV) the conduction band (EC) of the Cu2O, while it is very close to the EC of ZnO (with only ~0.05 eV at the lower level).On the other hand, the hole Fermi level (Fp) is located above the valence band (EV) of the ZnO and close to the EV of Cu2O.Hence, it can be concluded that there is a remarkable minority carrier concentration in the contacts/interfaces.
The electron (n) and hole (p) concentrations in the simulated devices with MAPbI3 and FASnI3 perovskites are depicted in Figure 3c and 3d, respectively.It can be observed that, near the ZnO/perovskite interface, the electron concentration (n) is high.Similarly, near the perovskite/Cu2O interface, the hole concentration (p) is high.It can be seen that the carrier concentrations in both devices are constant across the absorber layer.However, a sudden reduction in carriers near the contacts/interfaces could be because of the higher density of defects in these regions.

Effect of Perovskite Defect Density (Nt) on the Device's Performance
In this study, the perovskite defect density (Nt) of the MAPbI3 and FASnI3 layers were set to 2.5 × 10 14 and 2 × 10 15 cm −3 , respectively.This high value of Nt in the FASnI3 perovskite layer results in the trap-assisted Shockley-Read-Hall (SRH) recombination effect [35], which could be expressed as the following equation: It can be seen from Figure 3a,b that the Fermi level of electrons (F n ) is positioned under (~0.4 eV) the conduction band (E C ) of the Cu 2 O, while it is very close to the E C of ZnO (with only ~0.05 eV at the lower level).On the other hand, the hole Fermi level (F p ) is located above the valence band (E V ) of the ZnO and close to the E V of Cu 2 O. Hence, it can be concluded that there is a remarkable minority carrier concentration in the contacts/interfaces.
The electron (n) and hole (p) concentrations in the simulated devices with MAPbI 3 and FASnI 3 perovskites are depicted in Figure 3c and 3d, respectively.It can be observed that, near the ZnO/perovskite interface, the electron concentration (n) is high.Similarly, near the perovskite/Cu 2 O interface, the hole concentration (p) is high.It can be seen that the carrier concentrations in both devices are constant across the absorber layer.However, a sudden reduction in carriers near the contacts/interfaces could be because of the higher density of defects in these regions.

Effect of Perovskite Defect Density (N t ) on the Device's Performance
In this study, the perovskite defect density (N t ) of the MAPbI 3 and FASnI 3 layers were set to 2.5 × 10 14 and 2 × 10 15 cm −3 , respectively.This high value of N t in the FASnI 3 perovskite layer results in the trap-assisted Shockley-Read-Hall (SRH) recombination effect [35], which could be expressed as the following equation: where n and p are the electron and hole concentrations, τ n, and τ p are the electron and hole lifetimes, and n i is the intrinsic carrier concentration.The n 1 and p 1 are defined by the following: where N C and N V show the carrier densities, and E C and E V indicate the energy levels of the conduction and valence bands, respectively.In the SRH recombination model, the electrons in the CB (or the holes in VB) can recombine through trap sites.The open circuit electric potential is expressed by the following equation: where n, V T , I G , and I 0 are the diode ideality factors, thermal electric potential, photogenerated current, and dark saturated current, respectively.Figure 4a presents the dependence of the PCE on the defect density (N t ).variation in the perovskite layers (MAPbI 3 and FASnI 3 ).The recombination rate inside the perovskite layers is depicted in Figure 4b.Table 3 also shows the numerical results obtained for the PCE of the devices at various defect densities. =  −   ( +  ) +  ( +  ) (1) where n and p are the electron and hole concentrations, τn, and τp are the electron and hole lifetimes, and ni is the intrinsic carrier concentration.The n1 and p1 are defined by the following: where NC and NV show the carrier densities, and EC and EV indicate the energy levels of the conduction and valence bands, respectively.In the SRH recombination model, the electrons in the CB (or the holes in VB) can recombine through trap sites.The open circuit electric potential is expressed by the following equation: where n, VT, IG, and I0 are the diode ideality factors, thermal electric potential, photogenerated current, and dark saturated current, respectively.Figure 4a presents the dependence of the PCE on the defect density (Nt).variation in the perovskite layers (MAPbI3 and FASnI3).The recombination rate inside the perovskite layers is depicted in Figure 4b.Table 3 also shows the numerical results obtained for the PCE of the devices at various defect densities.Table 3.The PCE variation due to the defect density of the absorber layer.
Nt (cm −3 ) PCE (MAPbI3) PCE (FASnI3) 1 × 10 11  7.18 10.55 1 × 10 12  7.18 10.55 1 × 10 13  7.13 10.52 1 × 10 14  6.68 10.32 2.5 × 10 14  6.06 10.12 1 × 10 15  4.39 8.85 2 × 10 15  3.45 7.83 1 × 10 16  4 5.03  By increasing the defect density in the absorber layer, the PCE of the cells, at first, decreased sharply and then showed a nearly saturated trend in both devices.Increasing the defects results in the enlargement of the series resistance of the device and, correspondingly, the reduction in the efficiency.In Figure 4b, it can be observed that the carrier recombination process through the FASnI 3 absorber layer is 4.1 × 10 21 cm −3 s −1 , which is higher than that of the MAPbI 3 layer (1.54 × 10 21 cm −3 s −1 ).This higher recombination rate in FASnI 3 decreases the V OC of the cell.Thus, the V OC of a cell with a FASnI 3 absorber (0.54 V) is lower than that of a device with a MAPbI 3 absorber layer (0.76 V).It should be noted that in Figure 2b, the J-V characteristic was found for the defect density of 2.5 × 10 14 cm −3 for MAPbI 3 and 2 × 10 15 cm −3 for MAPbI 3 , resulting in efficiencies of 6.06% and 7.83%, respectively.Meanwhile, the thickness of the absorber layer was set to 200 nm in both devices.

Effect of the Perovskite Thickness on the Device's Performance
The influence of the perovskite thickness (varying in the range of 100-800 nm) on the photovoltaic properties of the cells was investigated as shown in Figure 5.
By increasing the defect density in the absorber layer, the PCE of the cells, at first, decreased sharply and then showed a nearly saturated trend in both devices.Increasing the defects results in the enlargement of the series resistance of the device and, correspondingly, the reduction in the efficiency.In Figure 4b, it can be observed that the carrier recombination process through the FASnI3 absorber layer is 4.1 × 10 21 cm −3 s −1 , which is higher than that of the MAPbI3 layer (1.54 × 10 21 cm −3 s −1 ).This higher recombination rate in FASnI3 decreases the VOC of the cell.Thus, the VOC of a cell with a FASnI3 absorber (0.54 V) is lower than that of a device with a MAPbI3 absorber layer (0.76 V).It should be noted that in Figure 2b, the J-V characteristic was found for the defect density of 2.5 × 10 14 cm −3 for MAPbI3 and 2 × 10 15 cm −3 for MAPbI3, resulting in efficiencies of 6.06% and 7.83%, respectively.Meanwhile, the thickness of the absorber layer was set to 200 nm in both devices.

Effect of the Perovskite Thickness on the Device's Performance
The influence of the perovskite thickness (varying in the range of 100-800 nm) on the photovoltaic properties of the cells was investigated as shown in Figure 5. Figure 5a indicates that the variation in the perovskite thickness cannot affect the open circuit potential of the devices.This implies that maximum charge separation, as well as charge recombination, simultaneously occurred in the active layers.
In the device with FASnI3, the short circuit current density significantly increased with the increasing perovskite thickness and reached the maximum value of 22.29 mA/cm 2 at the thickness of 300 nm.Then, it showed a slight decrease to reach the value of 21.55 mA/cm 2 at the thickness of 800 nm, as exhibited in Figure 5b.
It was found that the FF drops continuously from 64.91% and 74.78% to 43.78% and 61.99% upon an increase in the MAPbI3 and FASnI3 thickness from 100 nm to 800 nm, respectively, as presented in Figure 5c.This decrease in the FF with an increasing thickness can be attributed to the increased recombination rate of the charge carriers inside the active layers.In the case of thicker absorber layers, the charge carriers do not have sufficient time to reach the desired energy band within the perovskite layer, resulting in premature recombination [20].Figure 5a indicates that the variation in the perovskite thickness cannot affect the open circuit potential of the devices.This implies that maximum charge separation, as well as charge recombination, simultaneously occurred in the active layers.
In the device with FASnI 3 , the short circuit current density significantly increased with the increasing perovskite thickness and reached the maximum value of 22.29 mA/cm 2 at the thickness of 300 nm.Then, it showed a slight decrease to reach the value of 21.55 mA/cm 2 at the thickness of 800 nm, as exhibited in Figure 5b.
It was found that the FF drops continuously from 64.91% and 74.78% to 43.78% and 61.99% upon an increase in the MAPbI 3 and FASnI 3 thickness from 100 nm to 800 nm, respectively, as presented in Figure 5c.This decrease in the FF with an increasing thickness can be attributed to the increased recombination rate of the charge carriers inside the active layers.In the case of thicker absorber layers, the charge carriers do not have sufficient time to reach the desired energy band within the perovskite layer, resulting in premature recombination [20].
It can be seen from Figure 5d that the PCE is raised to 6.38% and 7.83% due to the increases in the thicknesses of the MAPbI 3 and FASnI 3 layers to 400 and 200 nm, respectively.The observed improvement in the PCE with an increase in the thickness of the perovskite layer can be attributed to the greater light absorption that occurs within the active layer.The PCE is unchanged after the thickness of the FASnI 3 is increased 400 nm, while it is decreased slightly for the MAPbI 3 after reaching the maximum value (6.38% at 400 nm).However, after reaching the thicknesses of 400 nm (of MAPbI 3 ) and 200 nm (of FASnI 3 ), the PCE decreased because of an increase in the recombination rate in the solar cells, which was induced by increasing the thickness.Therefore, the thicknesses of 400 and 200 nm were determined as optimum values for the MAPbI 3 and FASnI 3 layers, respectively.The decrease in the PCE after reaching the maximum value could be attributed to the growth in the recombination rate in a solar cell with a higher thickness.
The external quantum efficiency (EQE) of the MAPbI 3 and FASnI 3 -based solar cells exposed to the illumination of a solar simulator is shown in Figure 6a.The EQE curve in the visible wavelength region has a soft slope.A broad spectral response was obtained up to ~830 and 870 nm wavelengths, which correspond to 1.52 and 1.41 eV, i.e., the band gap values of the MAPbI 3 and FASnI 3 perovskites, respectively.The photons with energies lower than those of the band gaps of the absorber layers cannot be absorbed, causing vanished EQEs. Figure 6a shows that there is no significant difference in the EQE behaviors of the two perovskite layers at the short wavelength region (below 400 nm).However, for the MAGeI 3 -based device, the cutoff in the EQE trend presented a blue shift due to its larger band gap.Meanwhile, the integrated J SC of the FASnI 3 -based cell was found to be ~35% higher than that of the MAPbI 3 -based one.A Cambridge Structural Database (CSD) [36] survey was searched to explore the structures of MAPbI 3 and FASnI 3 perovskites.The MAPbI 3 and FASnI 3 structures with CSD codes of FOLLIB [37] and WUFYEE [38] are shown in Figure 6b,c.
tively.The observed improvement in the PCE with an increase in the thickness of the perovskite layer can be attributed to the greater light absorption that occurs within the active layer.The PCE is unchanged after the thickness of the FASnI3 is increased 400 nm, while it is decreased slightly for the MAPbI3 after reaching the maximum value (6.38% at 400 nm).However, after reaching the thicknesses of 400 nm (of MAPbI3) and 200 nm (of FASnI3), the PCE decreased because of an increase in the recombination rate in the solar cells, which was induced by increasing the thickness.Therefore, the thicknesses of 400 and 200 nm were determined as optimum values for the MAPbI3 and FASnI3 layers, respectively.The decrease in the PCE after reaching the maximum value could be attributed to the growth in the recombination rate in a solar cell with a higher thickness.
The external quantum efficiency (EQE) of the MAPbI3 and FASnI3-based solar cells exposed to the illumination of a solar simulator is shown in Figure 6a.The EQE curve in the visible wavelength region has a soft slope.A broad spectral response was obtained up to ~830 and 870 nm wavelengths, which correspond to 1.52 and 1.41 eV, i.e., the band gap values of the MAPbI3 and FASnI3 perovskites, respectively.The photons with energies lower than those of the band gaps of the absorber layers cannot be absorbed, causing vanished EQEs. Figure 6a shows that there is no significant difference in the EQE behaviors of the two perovskite layers at the short wavelength region (below 400 nm).However, for the MAGeI3-based device, the cutoff in the EQE trend presented a blue shift due to its larger band gap.Meanwhile, the integrated JSC of the FASnI3-based cell was found to be ~35% higher than that of the MAPbI3-based one.A Cambridge Structural Database (CSD) [36] survey was searched to explore the structures of MAPbI3 and FASnI3 perovskites.The MAPbI3 and FASnI3 structures with CSD codes of FOLLIB [37] and WUFYEE [38] are shown in Figure 6b,c.Devices with non-toxic absorbers (Pb-free perovskites) are friendly environment materials that have the potential for mass production.Although they show lower PV parameters, by improving the light absorption, this challenge may be solved.In addition, the low PV parameters observed can be attributed to a limited collection of carrier charges.Consequently, to enhance the collection of charge carriers, it is possible to decrease the thickness of the absorber layer [39].Furthermore, low PV parameters reflect inadequate performance, as the performance of photovoltaic cells relies on the recombination kinetics Devices with non-toxic absorbers (Pb-free perovskites) are friendly environment materials that have the potential for mass production.Although they show lower PV parameters, by improving the light absorption, this challenge may be solved.In addition, the low PV parameters observed can be attributed to a limited collection of carrier charges.Consequently, to enhance the collection of charge carriers, it is possible to decrease the thickness of the absorber layer [39].Furthermore, low PV parameters reflect inadequate performance, as the performance of photovoltaic cells relies on the recombination kinetics of carriers within the device [40,41].When the absorber thickness is too low, the absorption of light is also low, resulting in low PV parameters.Conversely, by increasing the thickness, there is a significant improvement in the power conversion efficiencies (PCEs) of the cells.However, once the thickness exceeds a specific amount, the growth of the PCE slows down.This is because if the absorber layer becomes too thick, the photogenerated carriers cannot be collected efficiently, as they must traverse the absorber before reaching the carrier collecting layers, leading to the quenching of charge carriers [42].

Figure 1 .
Figure 1.Architecture of the perovskite solar cells designed based on (a) the MAPbI3 and (b) the FASnI3 absorber layers and the energy level diagrams of (c) the MAPbI3 and (d) the FASnI3-based PSCs.

Figure 1 .
Figure 1.Architecture of the perovskite solar cells designed based on (a) the MAPbI 3 and (b) the FASnI 3 absorber layers and the energy level diagrams of (c) the MAPbI 3 and (d) the FASnI 3 -based PSCs.

Figure 2 .
Figure 2. Comparison of the J-V curves obtained from (a) the experimental and simulation results for the FTO/ZnO/MAPbI3/Cu2O/Au solar cell and (b) the simulated results for the MAPbI3 and FASnI3-based solar cells.

Figure 2 .
Figure 2. Comparison of the J-V curves obtained from (a) the experimental and simulation results for the FTO/ZnO/MAPbI 3 /Cu 2 O/Au solar cell and (b) the simulated results for the MAPbI 3 and FASnI 3 -based solar cells.

Figure 2 .
Figure 2. Comparison of the J-V curves obtained from (a) the experimental an for the FTO/ZnO/MAPbI3/Cu2O/Au solar cell and (b) the simulated results FASnI3-based solar cells.

Figure 4 .
Figure 4. (a) Variation in the PCE of the cells due to the defect density of the absorber layer and (b) recombination rates along the depth of the multilayer structure of the MAPbI3-and FASnI3-based devices.

Figure 4 .
Figure 4. (a) Variation in the PCE of the cells due to the defect density of the absorber layer and (b) recombination rates along the depth of the multilayer structure of the MAPbI 3 -and FASnI 3 -based devices.

Figure 5 .
Figure 5.The effect of perovskite thickness on the various cell parameters, including (a) VOC, (b) JSC, (c) FF, and (d) PCE.

Figure 5 .
Figure 5.The effect of perovskite thickness on the various cell parameters, including (a) V OC , (b) J SC , (c) FF, and (d) PCE.

Figure 6 .
Figure 6.(a) The EQE outcomes of solar cells with the different perovskite absorber layers and partial views of the (b) MAPbI3 and (c) FASnI3 structures.

Figure 6 .
Figure 6.(a) The EQE outcomes of solar cells with the different perovskite absorber layers and partial views of the (b) MAPbI 3 and (c) FASnI 3 structures.

Table 1 .
The parameters considered for the simulation of the perovskite solar cells.

Table 2 .
Comparison of the simulation results with experimental results reported in the literature.

Table 2 .
Comparison of the simulation results with experimental results reported in the literature.

Table 2 .
Comparison of the simulation results with experimental results repor

Table 3 .
The PCE variation due to the defect density of the absorber layer.