Identifying ETL is essential for optimizing charge extraction and improving overall performance in PVSCs. With Cu₂O as HTL, six ETL materials ZnO, TiO₂, SnO₂, SnS₂, IGZO, and WS₂, were systematically evaluated. A series of simulations were performed to examine the impact of each ETL on the photovoltaic performance of the device. The energy band structure of ETLs is shown in Figure 2.

The typical features of several ETLs are shown in Table 3, whereas Table 4 shows the PVSC performance metrics for various ETLs together with the associated CBOs.

To find the CBOs for the ETLs, the following Equation 10 is applied (Qaid et al., 2024): C B O = X A − X E T L (10)

Here,   X E T L indicates the electron affinity of the ETL, whereas X A indicates the electron affinity of the ZnSnN₂-based absorber. The CBOs for ZnO, TiO₂, SnO₂, SnS₂, IGZO, and WS₂ are 0.1eV, 0.2eV, 0.1eV, −0.16eV, −0.06eV, and 0.15eV, respectively, according to the data in Table 4. The motion of electrons produced by photons is impeded by materials with positive CBO, whereas it is facilitated by materials with negative CBO (Qaid et al., 2024). However, an extremely negative CBO may cause electrons to accumulate at the absorber/ETL junction, which would encourage recombination (Qaid et al., 2024). Thus, it is essential to take into account materials with a slightly negative or zero CBO in order for optimal band alignment and ultimately improve the device’s performance (Qaid et al., 2024) (Chabri et al., 2023). SnS₂ and IGZO perform marginally worse than ZnO in our investigation due to their larger negative CBO. Consequently, ZnO exhibits better performance than other ETLs. Due of ZnO’s 3.3 eV band gap, its light transmittance is higher than that of other ETLs. A larger band gap increases the creation of electron-hole pairs by letting more light into the absorber layer. ZnO as ETL exhibits a moderate CBO of +0.1 eV with ZnSnN₂, forming a gentle spike that maintains favorable electron transport while preventing backflow (Khan et al., 2023). As a result, the ZnO material has been chosen as the ETL in the PVC design because to its superior electrical performance.

PVCs device efficiency is influenced by the HTL, which blocks electrons, lowers the electron-hole recombination rate, and facilitates hole migration from the MASnBr₃/ZnSnN₂ (absorber) layers to the back contact layer. A suitable bandgap, positive charge carrier mobility, ideal thickness, and an ideal doping charge concentration are all requirements for the HTL (Saikia et al., 2022). In this section, we use multiple hole transport layers (NiO, MoO₃, Cu₂O, CZTS, CuI, P3H, and Sprio-OMeTAD) to replicate the solar device efficiency in the original device configuration FTO/TiO₂/MASnBr₃/ZnSnN₂/HTL/Au. The energy band structure of HTLs is shown in Figure 3. ZnO was initially kept as the ETL, but the HTLs were changed in order to evaluate the PVSC device’s electrical properties.

The typical features of several HTLs are shown in Table 5, whereas Table 6 shows the PVSC performance parameters for various HTLs together with the associated VBO.

The VBOs for different HTLs are calculated using the following Equation 11 (Qaid et al., 2024) as: V B O = X H T L + E g , H T L − X a b s + E g , a b s (11)

The symbols X H T L and X a b s in the equation represent the electron affinities of the HTL and the MASnBr₃ absorbers, respectively. Furthermore, the bandgaps of the absorber layer and the HTL are expressed as E g , H T L and E g , a b s respectively.

Table 6 shows that CNTS is more effective than other HTLs. The CNTS HTL provides the highest PCE (30.50%) among the PVC layers, however the Spiro-OMeTED layer has the lowest PCE (18.75%). CNTS was chosen as the HTL based on its excellent energy level alignment with MASnBr₃, exhibiting a VBO of +0.05 eV, which enables efficient hole extraction while minimizing recombination (Kumar et al., 2024). On the other hand, a tiny negative spike value increases the device’s PCE by lowering photon recombination at the junction interface. A significant negative VBO value may be an impediment to the passage of holes from the active layer to the HTL in the case of P3HT and Spiro-OMeTAD HTLs, which lowers the PCE of the PVSC. Out of all the HTL’s, the CNTS material had the highest values for J_SC, V_OC, and FF, measuring 32.88 mA/cm2, 1.09 V, and 84.39%, respectively. The CNTS is a promising option for the hole transport layer in perovskite solar cells because to its border bandgap energy of 1.74 eV (Khan et al., 2023). CNTSs have a variety of produced procedures, such as dip-coating from molecular ink, electrodeposition (Jariwala et al., 2018), (Patil et al., 2024), direct-coating from nanoparticle inks (Jariwala et al., 2017), (Rondiya et al., 2017), and the sol-gel approach (Ghosh et al., 2016). Together, CNTS’s favorable energy levels, earth-abundant composition, chemical stability, high hole mobility, easy synthesis, tunable properties, and reduced environmental impact (lead-free) suggest that it could help improve the sustainability and performance of PVSCs technologies (Uddin et al., 2024). Consequently, the CNTS material has been chosen as the HTL in our PVC design following an assessment of the electrical performance.

Once the appropriate HTL for PVC was determined, we adjusted the ZnO ETL’s depth between 10 and 50 nm to examine its impact on PVC efficiency. Additionally, the donor concentration (N_D) of the ETL is varied between 10¹⁴ cm⁻³ and 10²⁰ cm⁻³, and the PVSC efficiency is calculated in order to examine the impact of the doping charge density on the PVSC efficiency. The performance of PVSC with respect to the thickness and N_D of the ETL is depicted in Figure 4.

The ETL’s thickness affects the charge transport’s velocity and efficiency. An optimal ETL thickness ensures efficient charge collection, increasing photo-generated current and overall device efficiency. A broad ETL acts as a barrier, preventing or lessening charge carrier recombination inside the active absorber material. Furthermore, the effectiveness of PVSC is significantly impacted by the electron carrier level in the ETL. On the other hand, a deeper ETL may absorb more incident photons, which could reduce the amount of light that reaches the photoactive perovskite layer. Conversely, a thin ETL might make it more likely that charge carriers will recombine inside the photoactive layer or at the ETL-perovskite interface. The efficient transfer of electrons to the electrode may be hampered by an inadequately thick ETL layer, which would lower device performance (Chabri et al., 2023). The device’s PCE first increases from roughly 30.72% to roughly 30.77% until the thickness reaches 30 nm, as seen in Figure 4. Additionally, thicker ETL causes FF and V_OC to rise and then stabilize. But as thickness increases, J_SC decreases. As a result, the ETL’s ideal thickness, taking into account all factors, is 30 nm.

On the other hand, as seen in Figure 6, the N_D is increased to 10¹⁹ cm⁻³, and the PVSC device characteristics’ FF and PCE exhibit a notable increase followed by a decline. Conversely, J_SC increases until N_D reaches 10¹⁹ cm⁻³, beyond which it decreases. When the N_D of ETL is raised to 10²⁰ cm⁻³, the PVSC device properties show a significant decrease. Additionally, as the N_D rises, V_OC gradually decreases. The relationship between the increase in doping density and the concomitant rise in the ETL’s internal electric field is responsible for the growth in device PCE. This ultimately results in improved conductivity and charge transport motion (Qaid et al., 2024). However, too much doping results in the formation of Coulomb traps, which lowers electron mobility (Qaid et al., 2024). Therefore, for upcoming simulations, we will maintain the ETL’s ideal N_D at 10¹⁹ cm⁻³.

Since CNTS performs better than the other HTLs previously described, we have chosen it as an HTL. We are presently optimizing the CNTS layer thickness by altering it between 0.1 and 0.5 μm. Additionally, The HTL layers have a significant impact on the PVSC device’s efficacy in respect to acceptor concentration (N_A). To investigate the effect of HTL, N_A on PVSC device efficiency, we measured the PVSC device’s efficiency by altering the HTL layer’s N_A between 10¹⁴ cm⁻³ and 10²⁰ cm⁻³. Note that we do not alter any other materials, i.e., ZnO is utilized as the ETL and no interfacial layer is applied. Figure 5 shows how well the PVSC performs for varying N_A and HTL (CNTS) thicknesses.

It is evident from the graph that the variation in HTL thickness has minimal impact on our PVSC’s performance characteristics, specifically FF, J_SC, V_OC, and PCE. This is due to the fact that the depth of the HTL layer does not directly affect the VBO at the intersection of the perovskite layer film and the nearby HTL layer (Qaid et al., 2024). The energy levels of the materials in concern and the interface’s characteristics have the biggest effects on the VBO. However, longer lengths for holes to traverse from the perovskite layer to the interface may arise from a thicker HTL. Despite its potential advantages for charge transfer, excessive thickness might result in slower charge carriers, more resistive losses, and worse overall device efficiency. It may also increase the probability of charge carrier recombination. The quantity of photocurrent generated by the perovskite layer to the interface decreases as a result of holes’ increased propensity to recombine with electrons in the layer as they pass through a dense HTL. Despite its potential advantages for charge transfer, excessive thickness might result in slower charge carriers, more resistive losses, and worse overall device efficiency (Qaid et al., 2024). As a result, 0.5 μm has been chosen as the HTL’s thickness. On the other hand, As the N_A is raised to 10²⁰ cm⁻³, the PVSC device’s properties FF, J_SC, V_OC, and PCE show a discernible increase. The performance parameters of the PVSC gadget rapidly rise with increasing HTL doping concentration. The relationship between the rise in the electric field inside the HTL and the increase in doping charge concentration is responsible for the improvement in the device’s performance. As a result, conductivity and carrier transport velocity both rise (Haneef et al., 2024b). Therefore, we maintain the N_A of the HTL at 10²⁰ cm⁻³ for the simulations that follow.

To optimize the thickness of the dual absorber layers MASnBr₃ and ZnSnN₂ a contour mapping approach was employed as illustrated in Figure 6. The performance parameters analyzed include PCE, V_OC, J_SC, and FF. Figure 6a shows that as the thicknesses of both MASnBr₃ and ZnSnN₂ increase, PCE also increases significantly, ranging from approximately 19% to over 31%. A similar trend is observed in J_SC from Figure 6c, with values increasing up to 34.90 mA/cm², indicating enhanced light absorption and carrier generation with thicker absorbers. On the other hand, Figure 6b illustrates that V_OC reaches its peak of around 1.2 V at lower MASnBr₃ thicknesses (<0.2 µm), but the variation across the range is minimal (1.10–1.21 V), and thus not critical in this optimization. Figure 6d reveals that FF is slightly higher at lower absorber thicknesses, varying from 80.48% to 84.42%. Despite the inverse behavior of FF with thickness, the overall efficiency gain justifies the selection of a thicker configuration. Therefore, the optimum thicknesses are chosen as MASnBr₃ = 0.8 µm and ZnSnN₂ = 0.1 µm. At this configuration, the corresponding performance metrics are V_OC = 1.1135 V, J_SC = 34.43 mA/cm², FF = 81.79%, and PCE = 31.35%, demonstrating a balanced and efficient design for further device optimization.

The effect of acceptor density variation in both MASnBr₃ and ZnSnN₂ absorber layers on the performance of the solar cell was analyzed using contour plots, as shown in Figure 7. Figure 7a indicates that increasing the acceptor density of MASnBr₃ significantly enhances PCE, while variation in ZnSnN₂ acceptor density has a relatively minor effect. The highest efficiency is observed at lower ZnSnN₂ acceptor density levels. This improvement in PCE with higher MASnBr₃ doping can be attributed to the enhancement of built-in electric field strength, which improves charge carrier separation and reduces recombination losses. A similar trend is observed in the variation of V_OC and FF, as shown in Figures 7b,d, where higher MASnBr₃ acceptor density results in consistent performance gains. Figure 6c also confirms that high J_SC values are obtained under these optimized conditions. Based on these findings, the optimal acceptor densities were selected as MASnBr₃ = 1 × 10¹⁸ cm^-3 and ZnSnN₂ = 1 × 10¹³ cm^-3. At this configuration, the device achieved a V_OC of 1.1716 V, a J_SC of 34.39 mA/cm², an FF of 89.02%, and a PCE of 35.87%, confirming a substantial improvement in overall solar cell performance under these doping conditions.

The influence of total defect density in the absorber layers MASnBr₃ and ZnSnN₂ was examined by varying their respective values from 1 × 10¹³ cm^-3 to 1 × 10¹⁸ cm^-3, as presented in Figure 8. Figure 8a depicts the effect of MASnBr₃ defect density on device performance. As the defect density increases, a pronounced decline in all key parameters PCE, V_OC, J_SC, and FF is observed. Specifically, PCE drops steeply from 35.88% at 1 × 10¹³ cm^-3 to just 11.63% at 1 × 10¹⁸ cm^-3. This degradation is primarily attributed to enhanced non-radiative recombination pathways that arise from the increased density of trap states, severely limiting charge carrier lifetimes (Eperon G. E. et al., 2014). In contrast, Figure 8b shows the effect of ZnSnN₂ defect density. While all parameters exhibit a downward trend, the decline is less severe compared to MASnBr₃. The PCE decreases from 35.94% at 1 × 10¹³ cm^-3–25.30% at 1 × 10¹⁸ cm^-3. Notably, J_SC remains relatively stable across the range, indicating that light absorption and charge generation are less sensitive to defect density in the ZnSnN₂ layer, although increased recombination still impacts V_OC and FF. In our study, we have employed a defect density of 1 × 10¹³ cm^-3 for MASnBr₃ and 1 × 10¹⁴ cm^-3 for ZnSnN₂. At these conditions, the device maintains strong PV performance with a V_OC of 1.1716 V, a J_SC of 34.39 mA/cm², an FF of 89.02%, and a PCE of 35.87%.

The impact of series resistance R_S and shunt resistance R_Sh on the PV performance of the dual-absorber solar cell was examined, as illustrated in Figure 9. The values of R_S were varied from 0 to 6 Ω cm^-2, while R_Sh was swept from 10 to 10⁶ Ω cm^-2. An increase in R_S shows a clear detrimental effect on all performance metrics. PCE drops from 35.99% at R_S = 0 Ω cm^-2–29.30% at R_S = 6 Ω cm^-2. This decline is driven primarily by the drop in FF, while J_SC and V_OC remain largely unchanged. In contrast, increasing R_Sh significantly improves device performance. At low R_Sh = 10 Ω cm^-2, the device shows poor metrics with PCE = 2.93%, V_OC = 0.344 V, and FF = 25.00%. As R_Sh increases, leakage currents are suppressed, leading to a rapid enhancement in all parameters. At R_Sh ≥ 10⁵ Ω cm^-2, performance saturates. According to previous studies (Karthick et al., 2020), maintaining a low series resistance is crucial to minimize internal power losses and ensure efficient power transfer to the external load. Meanwhile, a high shunt resistance is necessary to suppress leakage currents and uphold high V_OC and FF (Karthick et al., 2020). In alignment with these insights and supported by other simulation-based investigations (Sunny et al., 2021), we selected a realistic R_S of 0.1 Ω cm^-2 and R_Sh of 1 × 10⁶ Ω cm^-2 in our simulations to emulate practical conditions and optimize device efficiency.

The influence of temperature on the performance of the dual-absorber solar cell was examined over a range from 300 K to 440 K, as shown in Figure 10. Figure 10a presents the variation of key PV parameters PCE, V_OC, J_SC, and FF with temperature. As temperature increases, a notable decline in both V_OC and FF is observed, leading to a corresponding reduction in PCE. Specifically, efficiency drops from 35.88% at 300 K to 30.57% at 440 K. The decline in V_OC with increasing temperature is attributed to the enhanced intrinsic carrier concentration and increased recombination activity at higher temperatures, which reduces the quasi-Fermi level splitting. Similarly, FF degrades due to elevated series resistance and reduced built-in electric field strength. In contrast, J_SC remains relatively stable throughout the temperature range, indicating that photogeneration is not significantly affected. Figure 10b further confirms this trend through the J–V characteristics at various temperatures, where the FF and V_OC both diminish as temperature rises. This inverse relationship between temperature and solar cell performance is well-documented in literature, where it has been shown that elevated temperatures adversely affect charge carrier recombination and material interface stability (”31. Numerical simulation on an).

Figure 11a shows the quantum efficiency (QE) graph of the solar cell. The device exhibits excellent spectral response, maintaining nearly 100% QE in the visible range, indicating efficient light harvesting and charge collection throughout the working wavelength range. The QE declines at higher wavelengths beyond 850 nm, consistent with the optical absorption limits of the absorber materials. Figure 11b presents the current voltage (J–V) characteristics before and after optimization. Initially, based on the calculated IV-curve, the solar cell exhibited the following performance: V_OC = 1.0974 V, J_SC = 32.70 mA/cm², FF = 83.86%, and PCE = 30.10%. To improve performance, a step-by-step optimization procedure was employed. Initially, the ETL and HTL were selected based on favorable band alignment and their contribution to the overall device metrics. In this process, ZnO was selected as the ETL and CNTS as the HTL due to their suitable energy levels and compatibility. Following the material selection, the thicknesses of the ETL, HTL, and both absorber layers (ZnSnN₂ and MASnBr₃) were optimized. The optimum ETL thickness was determined to be 30 nm, and the donor density of ZnO was set to 1 × 10¹⁹ cm^-3. The MASnBr₃ absorber thickness was optimized to 0.8 µm and its acceptor density to 1 × 10¹⁸ cm^-3. For the ZnSnN₂ absorber, a thickness of 0.1 µm and acceptor density of 1 × 10¹³ cm^-3 were found to be optimal. The HTL thickness was adjusted to 500 nm, with an optimized acceptor density of 1 × 10²⁰ cm^-3. After completing these optimizations, the solar cell demonstrated significant improvements across all performance indicators. The final device performance reached V_OC = 1.1716 V, J_SC = 34.40 mA/cm², FF = 89.02%, and a PCE of 35.88%, confirming the effectiveness of the comprehensive optimization strategy and material selections in achieving a high-efficiency dual-absorber solar cell.

The search for high-efficiency and eco-friendly solar tech has led to extensive development of Pb-free perovskite and nitride-based absorbers. Table 7 presents a detailed comparison of the PV performance of our proposed n-ZnO/p-MASnBr₃/p-⁺ZnSnN₂/p-⁺⁺CNTS heterostructure with previously reported devices. Arca et al. (Arca et al., 2017) explored ZnSnN₂ based structures incorporating Mg:CuCrO₂ as HTL and achieved a PCE of 23.5%, limited by moderate Jsc. With improvements in absorber quality and interface tuning, Laidouci et al. (Laidouci et al., 2020) reported 22.32% efficiency using CuCrO₂ as HTL. In a subsequent study (Laidouci et al., 2023), integrating a Si BSF pushed the PCE to 29.5%.

Meanwhile, MASnBr₃ based devices have shown notable promise. Mushtaq et al. (Haneef et al., 2024a) demonstrated a planar FTO/SnO₂/MASnBr₃/NiO/Au stack with 34.52% PCE. Rawat et al. (Rawat et al., 2024) investigated temperature-dependent MASnBr₃ performance and achieved 26.01% using CuSbS₂ as HTL. Haneef et al. (Haneef et al., 2024b) introduced graphene and 3C–SiC interface layers, reaching 31.97%.

In comparison, the dual-absorber structure proposed here combining MASnBr₃ and ZnSnN₂ achieved a PCE of 35.87%, outperforming all previous reports. The graded absorber strategy leverages the complementary bandgap of MASnBr₃ (1.3eV) and ZnSnN₂ (1.5 eV), enhancing carrier collection and reducing losses. Optimized thickness, doping, and defect further improved performance. This highlights the importance of absorber engineering and interface tuning for next-gen, high efficiency, Pb-free PVSCs.

A future experimental validation of the proposed MASnBr₃/ZnSnN₂-based photovoltaic structure can follow scalable, low-cost fabrication approaches already explored for similar materials. MASnBr₃ thin films can be synthesized using solution-based methods such as spin coating or ultrasonic spray pyrolysis (USP), or vacuum-based methods like sequential thermal evaporation or co-evaporation of SnBr₂ and methylammonium bromide (MABr) precursors (Kaleli et al., 2022), (Wang et al., 2025). Among these, sequential evaporation has yielded better film quality and higher efficiencies (up to 1.12%) than co-evaporation due to better phase control and stoichiometry. Additionally, fully USP-fabricated structures have been demonstrated using FTO/TiO₂/MASnBr₃/P3HT/Ag configurations.

For ZnSnN₂, RF magnetron co-sputtering is a well-established technique that provides high purity and stoichiometric control (Laidouci et al., 2020). This method allows co-deposition from Zn and Sn targets in a nitrogen plasma environment. Stoichiometry can be verified by XRF, while phase purity and crystallinity can be confirmed via XRD. Hall effect measurements can assess carrier mobility and type.

MASnBr₃ faces major challenges with Sn²⁺ oxidation to Sn⁴⁺ in ambient conditions, leading to rapid degradation. This can be addressed through encapsulation techniques such as ALD-deposited Al₂O₃, parylene coatings, or by introducing reducing additives during synthesis to stabilize Sn²⁺ (Noel et al., 2014), (Li et al., 2016), (Raiford, 2021). On the other hand, ZnSnN₂ requires precise stoichiometry control and annealing optimization to minimize defects and maintain phase purity (Bui, 2022). Its metastable wurtzite structure can be sensitive to temperature and growth conditions. Additionally, interface engineering is crucial. Proper alignment and passivation of the MASnBr₃/ZnSnN₂ interface can suppress recombination and enhance charge transfer. Experimental deposition order and interlayer treatment (e.g., surface treatments or insertion of ultra-thin buffer layers) may play a key role. Together, the integration of sequential evaporation for MASnBr₃ and RF co-sputtering for ZnSnN₂, followed by post-deposition annealing and encapsulation, represents a viable experimental roadmap to replicate the high-efficiency dual-absorber structure proposed in this simulation study.