Performance Enhancement of Lead-Free CsSnI3 Perovskite Solar Cell: Design and Simulation With Different Electron Transport Layers

CsSnI3 is a promising lead-free material that shows potential as a substitute for lead-based material in the development of ecologically benign perovskite solar cells (PSCs) due to its low cost, high efficiency, and excellent thermal stability. This research is intended to enhance the efficiency of CsSnI3-based PSCs by employing numerical simulation through the One Dimensional Solar Cell Capacitance Simulator (SCAPS-1D) to optimize the optoelectronic properties of the electron transport layer (ETL), absorber layer, hole transport layer (HTL), and different interface layers. An investigation was conducted to evaluate the influence of different ETLs namely WS2, ZnSe, C60 and PCBM on the performance of CsSnI3-based PSCs utilizing poly (3-hexylthiophene-2,5-diyl) (P3HT) as the HTL. Moreover, the influence of variations in the thickness, doping density, and defect density of the absorber layer, ETL, and HTL on key photovoltaic parameters including power conversion efficiency (PCE), short-circuit current density ( $\text{J}_{SC}$ ), open circuit voltage ( $\text{V}_{OC}$ ), and fill factor (FF) was observed. The findings revealed that depending on the ETL employed, the PCE varies between the structures. For structures using WS2, ZnSe, C60, and PCBM as ETL, the corresponding PCE values are 31.63%, 29.64%, 29.75%, and 29.62%. Additionally, for all PSC structures, the effects of interface defect, temperature, series-shunt resistance, capacitance-voltage characteristics, and Mott-Schottky plot have been observed, and the corresponding current density-voltage (J-V), quantum efficiency (QE), generation, and recombination rates have been computed. Finally, the outcomes of this analysis were compared with prior research conducted on CsSnI3-based PSCs, thereby offering significant perspectives for their advancement and commercial viability.


I. INTRODUCTION
Conventional energy sources including gas, oil, and coal are being used to meet the enormous demand for Power, despite their contributions to environmental pollution and climate change [1].However, the supply of these sources is also limited [2].Therefore researchers are consistently The associate editor coordinating the review of this manuscript and approving it for publication was Xiaodong Liang .
working to find ways to migrate from conventional energy sources to renewable ones like hydropower, solar, wind, etc.Among them, solar energy can be a great alternative due to its free availability, excellent harvestability, and higher energy recovery rate [3].Solar cell (SC) exploits the fundamental principles of the photoelectric effect in order to transform solar energy into electrical energy [4].Currently, commercially viable silicon-based solar cells are available, yet their PCE is constrained to a maximum of 26% [5].
In order to alleviate this constraint, PSCs have emerged as a prominent candidate within a variety of diverse SC types for their advantageous characteristics such as tunable bandgaps, prolonged diffusion length, superior performance in carrier transfer, and simplified manufacturing processes [6].Lead-centered PSCs have greatly improved in efficiency since 2009 [7], increasing from 3.8% to 25.8% in 2022 [8].Although lead-centered PSCs have advantages in terms of efficiency, concerns regarding lead (Pb) toxicity and related environmental impacts persist.This has led to a noticeable increase in the interest of researchers in lead-free perovskites.However, tin (Sn) stands among a group of non-toxic or low-toxicity elements, including germanium, selenium, and antimony, that possess potential as substitutes for lead within perovskite compositions [9].Sn-based perovskites have similar optoelectronic characteristics to Pb but exhibit better carrier mobilities, smaller band gaps, and lower toxicity.CsSnI 3 , an entirely inorganic perovskite, possesses a band gap of 1.3 eV and exhibits optoelectronic properties that closely resemble those of its lead-based counterpart.
Numerous investigations have revealed the remarkable PV performance exhibited by CsSnI 3 [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].The presented PSC utilizes CsSnI 3 as the absorber layer within a Schottky SC configuration.This pioneering study achieved a PCE of 0.9% [11].An innovative photovoltaic quantum dot solar (PVQD) cell utilizing CsSnI 3 perovskite has demonstrated noteworthy PCE of 5.03% through experimental assessment [12].Similarly, the integration of a mixed ETL with a CsSnI 3 absorber yielded an enhanced efficiency of 6.08% [13].In a separate investigation, a CsSnI 3 PSC incorporating mesoporous metal oxide achieved a remarkable PCE of 7.50% [14].Notably, the utilization of Black-orthorhombic CsSnI 3 material in the SC led to a significant advancement, resulting in a PCE of 10.1% [15].Subsequently, lead-free CsSnI 3 quantum rods achieved a notable PCE of 12.96% in PV applications under AM 1.5G conditions.This technique improves light absorption and charge extraction in the perovskite phase.This lead-free CsSnI 3 perovskite is synthesized using a solvothermal process [16].Concurrently, a proposed solar cell based on CsSnI 3 with TiO 2 as the electron transport layer (ETL) demonstrated an enhanced efficiency of 20.13% [17].In another study, focusing on the realm of melt-synthesized CsSnI 3 -based PSCs, a projected PCE of 23% was anticipated [18].Moreover, CsSnI 3 with various charge transport layers was numerically simulated and discovered a maximum PCE of 24.73% [19].Most recently, a numerical analysis of β-γ -CsSnI 3 was conducted and found a PCE value of 28.76% [20].All of the aforementioned investigations unequivocally support the promise of CsSnI 3 PSCs in terms of PCE, while also demonstrating long-term thermal stability as well as illustrating the influence of various charge transport layers on photovoltaic (PV) key parameters.
In this study, lead-free CsSnI 3 -based optimized PSC having Al/FTO/ETL/CsSnI 3 /P3HT/Au architecture is presented that incorporates suitable ETLs to effectively enhance  its PV efficacy.P3HT has been selected as the HTL in the proposed PSC for its excellent hole mobility, costeffectiveness, and stability [21].While TiO 2 is a widely adopted ETL for PSCs, it exhibits limitations such as lower charge mobility and instability in the U-V region [22], prompting the exploration of alternative ETL materials.The device structure under investigation encompassed four different ETLs, namely WS 2 , ZnSe, C 60 , and PCBM.ETL in PSCs facilitates electron extraction and transport, preventing electrical shunts as well as potential reactions between transparent conducting oxide (TCO) and perovskite layer, and ideally possesses high electron mobility and hole-blocking capabilities.Subsequently, across four configurations, the effects of the absorber, HTL and ETL thickness, doping density, defect density, interface defects, series-shunt resistance, and temperature on key performance parameters of PSC were thoroughly analyzed.Furthermore, this study explores the properties of capacitance-voltage relationship (C-V), generation-recombination rate, J-V Curve and QE in order to improve the performance of PSCs based on CsSnI 3 by appropriate integration of the ETL.
its optical absorption characteristics.The PSC architecture employs P3HT and fluorine-doped tin oxide (FTO) as HTL and TCO, respectively, with Aluminum (Al) and Gold (Au) serving as front and rear contacts.Four different ETL materials, specifically WS 2 , ZnSe, C 60 , and PCBM were employed in the study.The interaction between ETL and HTL has a significant influence on the transportation and extraction of charges produced in the absorber layer, hence affecting key parameters such as V OC , PCE, FF, and J SC .The V OC is influenced by the disparity in Fermi energy levels between the ETL and HTL.Additionally, the enhanced charge mobility of the ETL leads to a commensurate increase in the FF and J SC .The successful extraction of holes is facilitated by the combination of high hole mobility and suitable ionization potential of the HTL.Consequently, the parameters of ETL and HTL play significant roles in enhancing the overall performance of PSCs.The band diagram of the PSC device configuration based on CsSnI 3 is depicted in Figure 2.

B. NUMERICAL SIMULATION APPROACH
In this research work, the designed PSC architectures were numerically analyzed by employing Solar Cell Capacitance Simulator (SCAPS-1D) software.SCAPS-1D is a program that can be used to model SCs in a single dimension.It was created at the Department of Electronics and Information Systems (ELIS) at the University of Gent in Belgium.It has the capability to accommodate a maximum of seven distinct layers.These layers consist of six interface layers and two electrodes [37].The software performs computations to determine PV parameters (PCE, FF, V OC and J SC ) by leveraging the Poisson equation, continuity equation, and current density equation.Equation (1) represents Poisson's equation that establishes the connection between the electrostatic potential and the distribution of charge density.The continuity equations for electrons and holes can be expressed mathematically as (2) and (3), respectively.The computation of electron and hole current densities was conducted utilizing (4) and (5), respectively.
where electric field, charge density, charge of electron, electrostatic potential, relative permittivity, electron/hole concentration, and rate of electron/hole generation is represented by E, ρ, q, ψ, ϵ, n/p, G n /G p respectively.Also, N D + /N A − represents ionized donor/acceptor density, D p / D n represents the coefficient of diffusion for hole/electron, U(n,p) denotes the overall recombination rate, µ p /µ n denotes hole/electron mobility [38], [39].The numerical simulation of the designed PSCs was performed under controlled parameters, including a solar radiation intensity of 100 mW/cm 2 , a radiation frequency of 1MHz and AM 1.5G solar spectrum.These conditions are maintained at an operating temperature of 300K.The incident light is postulated to originate from the top of TCO, specifically FTO, which has a work function of 4.4 eV.The rear contact is constructed from gold (Au) and exhibits a work function of 5.1 eV.In accordance with previous simulation outcomes, the thickness of the FTO layer was established at 0.5 µm [40].During the optimization of a specific parameter, the other parameters were maintained constant.Table 1 presents the optoelectronic properties of the materials employed in this study during SCAPS-1D simulation and Table 2 demonstrates the input parameters related to interface defect.At room temperature, the thermal velocity for electrons and holes within each layer is considered to be 10 7 cm/s, influencing mobility and recombination dynamics.The improvement of carrier collecting efficacy can be achieved through the enhancement of thermal velocities.However, it is important to note that this enhancement may also lead to an increase in recombination rates.Consequently, the diffusion length can be affected, which in turn has a negative impact on PCE [41].

A. IMPACT ON PV PARAMETERS FOR THE VARIATION OF ABSORBER DEFECT DENSITY AND THICKNESS
The performance of CsSnI 3 -based PSCs is greatly influenced by the variation in thickness and defect density of the absorber layer.This is because they directly impact light absorption and the generation of charge carriers within the absorber layer.Specifically, a thinner absorber layer usually results in reduced sunlight absorption, as it accommodates fewer photons, potentially diminishing PCE.On the contrary, PCE may rise as a result of improved light absorption from a thicker layer.However, excessive thick layer can also lead to greater electron-hole recombination and impede charge transport, lowering the efficiency.Additionally, thicker layers may also have an impact on mechanical stability and manufacturing consistency.Furthermore, defects are more prone to emerge in thicker absorber layer, reducing SC efficiency to a greater extent.High defect density results in low carrier diffusion length and a high recombination process, which lead to lower PV performance [42].Therefore, it is a must to investigate optimal defect density in relation to absorber layer thickness.For this reason, the thickness was varied from 0.4 µm to 1.2 µm while the total defect density ranged from 10 11 cm −3 to 10 18 cm −3 , concurrently.The contour plots in Figure (3)(4)(5)(6) show that the PV parameters varied in a very similar way across all of the ETL.The V OC of WS 2 , ZnSe, C 60 and PCBM has drastically decreased from 1.21 V to 0.87 V, 1.10 V to 0.78 V, 1.18 V to 0.87 V and 1.11 V to 0.802 V respectively.These structures showed that V OC values are highest when defect density is less than 10 17 cm −3 and thickness is between 0.4 µm to 0.9 µm.This variance in V OC is attributable to longer diffusion length and higher carrier lifespan within the specified range [43].However, the structure with WS 2 ETL had the highest V OC value, while the one with ZnSe ETL had the lowest.Similar trends in J SC values were seen across all four structures in Figure 4(a-d).J SC starts to increase when the thickness is beyond 0.8 µm and the defect density lies below 10 17 cm −3 .The rise in J SC happens because as the absorber layer thickens, its effective surface area increases, allowing more light to be absorbed and creating more charge carriers [44].Conversely, recombination driven by high defect density lowers J SC above that level of defect density [45].The J SC of structures consisting of WS  5(a-d).Since FF is influenced by the V OC and recombination rate at the depletion area, it follows the same trend as V OC [46].Further, intermediate defect densities exhibited favourable FF values for organic ETL.This may be a consequence of specific defects serving as passivators for more profound trap states [47].In addition, ZnSe exhibits significant deterioration in the Fill Factor.Figure 6(a-d) shows a drastic decrease in PCE of the PSCs for WS 2 , ZnSe, C 60 and PCBM from 37.40% to 21%, 32.35% to 16.75%, 32.20% to 20.15% and 32.20% to 18.90% respectively.Again, ZnSe demonstrated a higher PCE reduction rate (about 49%) than the other structures.PCE exhibits higher values for thickness over 0.6 µm and defect densities below 10 16 cm −3 .A thicker absorber layer (> 0.6 µm) increases carrier production through light absorption,     while defect density (< 10 16 cm −3 ) reduces non-radiative recombination.The notable PCE within the specified range of absorber layer thickness and defect density is due to the combined influence of those factors [48].Therefore, the PV cell's overall performance has been considerably impacted by the combined factors of thickness and total defect density.Based on the aforementioned assessment, it has been determined that the ideal thickness and total defect density of the absorber layer are decided to be 0.8 µm and 10 16 cm −3 , respectively, in order to provide viable properties of a proficient absorber layer.

B. IMPACT OF ABSORBER LAYER DOPING DENSITY ON PV PERFORMANCE
The efficiency of the SC can be enhanced by employing precise doping on the absorbing material [49].However, excessive doping might be unfavorable due to its nonconventional nature, potential hindrance to the mechanism of carrier transport and increased fabrication complexity [50].Thus, the doping concentration of the absorber was changed from 10 11 cm −3 to 10 18 cm −3 to determine the optimum value during the simulation.Figure 7 demonstrates that the V OC of all ETL associated structure has stayed constant for N A values up to 10 15 cm −3 , and thereafter increases with the N A .An increase in N A lowers the Fermi energy of the holes, leading to a higher value of V OC [51].All ETLs have the same trend in PCE up to 10 14 cm −3 , but after that, the PCE increases as the N A becomes higher.The consistent J SC in WS 2 and ZnSe associated structures despite varying N A may be attributed to their limited band binding behavior.These findings are quite consistent with the SC outputs on altering the absorber doping reported in prior investigations [52], [53].For the organic ETL based structures, J SC remains constant up to 10 15 cm −3 and then increases as N A rises due to improved band alignment.FF stays the same up until 10 14 cm −3 for all of the ETLs, after which it starts to rise in a different manner.Since absorber doping has had a profound impact throughout the simulation, choosing an appropriate acceptor doping concentration is crucial for maximizing device performance [54].Optimal N A of the absorber layer is established at 10 18 cm −3 , 10 17 cm −3 , 10 16 cm −3 and 10 17 cm −3 for the WS 2 , ZnSe, C 60 and PCBM ETLs respectively, considering the superior performance in PCE and FF.

C. EFFECT OF ETL LAYER THICKNESS, DOPING, AND DEFECT ON PV PERFORMANCE 1) EFFECT OF ETL LAYER THICKNESS
ETL performs an indispensable role in electron extraction from the absorber layer [55].In the current study, the thickness of WS 2 , ZnSe, C 60 and PCBM as ETL layer was varied from 0.03 µm to 0.5 µm.This is evident from Figure 8(a) that thickness has a negligible effect on the inorganic ETL i.e.WS 2 and ZnSe due to their transparent nature.A similar pattern is observed in previous literature too [56], [57].On the contrary, there is a marginal diminution in the parameters using organic ETL i.e.C 60 and PCBM due to the formation of larger pinholes which is identical to the previous literature [58].There is only an exception in the FF of the C 60 -based one as it decreases first, then increased significantly.Yue et al. reported a similar pattern for C 60 as ETL-based structure [59].Thus, selecting a proper ETL with optimum thickness is important.In this study, 0.03 µm is taken as the optimum thickness for further simulation.

2) IMPACT OF DOPING DENSITY OF ETL LAYER
Small amounts of impurity atoms can enhance solar cell performance [60].The effect of changing the doping concentration (N D ) from 10 15 cm −3 to 10 20 cm −3 on the PV characteristics of several ETLs including WS 2 , ZnSe, C 60 and PCBM is shown in Figure 8(b).It is found that an increase in ETL doping results in a marginal enhancement in J SC for all configurations which reaches saturation at 10 17 cm −3 .In contrast, it exhibits almost constant progress for open-circuit voltage, Fill Factor, and PCE as doping gets higher.Therefore, it can be concluded that raising the doping level did not make any substantial improvement across all the structures in this study.A very similar pattern has been documented in prior research studies [61].For the purpose of further optimization, the value of N D was set as 10 17 cm −3 .

3) IMPACT OF DEFECT DENSITY OF ETL LAYER
Figure 8(c) demonstrated that all performance parameters exhibited almost constant values across all ETL layers as defect density (N t ) increased from 10 11 cm −3 to 10 17 cm −3 .Thus, the repercussion of higher defect density of ETL on device configurations is negligible.The previously published work also exhibits resembling behavior [62].The simulation disclosed that the WS 2 associated structure did not exhibit a better J SC value, but it performed exceptionally well in every other parameter.With a V OC of 1.0527 V, a J SC of 33.923 mA/cm 2 and an FF of 88.570%, the WS 2 -related structure achieved the greatest efficiency of 31.629%.On the contrary, the structure using PCBM as the ETL demonstrated the worst performance in terms of both V OC and PCE, despite showing the greatest values of J SC (34.529 mA/cm 2 ).Furthermore, it is found that the structure associated with C 60 had the lowest fill factor of 85.04%.In this study, the optimized N t is set to 10 15 cm −3 for all four device structures.

D. EFFECT OF HTL LAYER THICKNESS, DOPING AND DEFECT DENSITY ON PV PERFORMANCE 1) EFFECT OF HTL LAYER THICKNESS
Figure 9(a) shows the output parameters with increasing HTL thickness for different combinations of ETL.The thickness was varied from 0.05 µm to 0.5 µm to obtain an optimum value.It is noticed from the figure that, V OC and J SC remained constant whereas there was a negligible decrease in FF and PCE.It has been previously documented by Hossain et al. [63].This is because CsSnI 3 has high absorption coefficient and it is less susceptible to the thickness of the HTL layer [27].Yet, the perovskite layer is better able to collect photons when the HTL is thicker [64].Therefore, the thickness of HTL layer was kept at 0.1 µm for furthur optimization.individual architectures.N A of HTL layer was ranged from 10 14 cm −3 to 10 20 cm −3 .It is found from the figure that V OC initially increased up to 10 16 cm −3 with variation in N A and then remained constant.A similar trend has been observed in the case of FF and PCE.Both the parameters increased decisively up to 10 17 cm −3 and after that remained constant.The increase in the parameters is owing to the improvement in the electric field because it facilitates the charge carrier extraction [65].But there was no significant change in J SC .This means N A variation of the HTLs has a negligible impact on J SC .Overall, WS 2 showed the best performance.Previous studies have already mentioned a similar pattern in the SC parameters [66].The ideal concentration of the hole transport layer was taken as 10 17 cm −3 for further optimization.

3) IMPACT OF VARIATION IN DEFECT DENSITY AT HTL LAYER
Figure 9(c) shows the influence of PV parameters with increasing defect density of HTL.The density of neutral type defect was varied from 10 12 cm −3 to 10 18 cm −3 , while the remaining parameters were maintained at their optimized values.The value of V OC , J SC , FF, and PCE was almost constant throughout this variation.This may be related to the high absorption coefficient of CsSnI 3 or the high carrier mobility of P3HT.Previously, the same behavior was observed when the effect of HTL defect density was investigated [61], [67].However, the optimum density was determined to be 10 12 cm −3 for further simulation.

E. EFFECT OF INTERFACE DEFECT DENSITY ON PV PERFORMANCE 1) IMPACT OF DEFECT DENSITY AT HTL/CsSnI 3 INTERFACE
The effectiveness of solar cell is greatly influenced because of the recombination losses in the interface [68].To investigate the influence of interface recombination occurring at interfaces between HTL and CsSnI 3 , the defect density (N t ) had ranged from 10 11 cm −3 to 10 20 cm −3 .Figure 10(a) shows that the V OC is almost constant up to 10 13 cm −3 .Beyond this concentration, V OC drops significantly for all the structures.WS 2 had the highest V OC of 1.052 V, while PCBM had the lowest of 0.881 V for varying N t .For all structures, there were no appreciable changes in J SC when the defect concentration was increased up to 10 15 cm −3 .J SC significantly declines over this concentration.The PCE, as well as FF values, showed a similar downward trend as V OC when the defect density (N t ) across HTL/CsSnI 3 Interface increased.Due to higher trap and recombination centers because of higher defect concentrations at the interfaces, the solar cell parameter has dramatically dropped, lowering SC performance [66].Therefore, a value of 10 12 cm −3 was set for further simulation.demonstrates the impact of defect density across CsSnI 3 /ETL interface when N t was raised from 10 10 cm −3 to 10 18 cm −3 .The study found that the value of V OC , J SC , FF, and PCE declines as N t is raised.The V OC , FF and PCE remained constant till 10 11 cm −3 and then starts declining with increment in N t .J SC almost remained constant till the defect of 10 15 cm −3 and then varies in a different manner.The C 60 -associated structure had the lowest FF, J SC and PCE value of 56.87%, 2.283 mA/cm −2 and 0.83%, respectively.In the case of V OC , PCBM demonstrated the lowest value, which was less than 0.35 V. Overall, the worst tolerance against the interface defect was shown by the C 60 -associated structure whereas ZnSe showed the best.The results also show that the V OC is highly susceptible to changes in N t compared to J SC .Again, it has been found that defects in the performance of SC is affected by the ETL/CsSnI 3 interface more so than the CsSnI 3 /HTL interface.Light entering the solar cell by ETL causes the front side to have a higher carrier generation and recombination rate than the back side [69].Tan et al. observed a similar pattern [70].However, the ideal defect density value was determined to be 10 11 cm −3 , which is identical to the result obtained in prior research [71].

F. IMPACT OF PARASITIC RESISTANCES AND TEMPERATURE 1) IMPACT OF SERIES RESISTANCE (R S )
Series resistance is a result of the PSC's internal resistance, terminal resistance and metal electrode resistance [72].Figure 11(a) illustrates the influence of R s affecting the performance parameters of WS 2 , ZnSe, C 60 and PCBM-based perovskite solar cells.Keeping R sh set at 10 5 -cm 2 , R s is ranged from 0 -cm 2 to 6 -cm 2 .It has been observed that the FF and PCE have decreased by an average of 20% from their maximum value due to increasing power dissipation for all four perovskite device structures [73].The lowest PCE and FF values were shown by PCBM ETL-based structure, whilst the highest PCE and FF values were shown by WS 2 ETL-based structure for the variation of R s .The V OC and J SC remained almost unchanged with increasing series resistance for all configurations.That means the variation in series resistance can't influence the V OC as well as J SC for the investigated device configurations.A similar pattern of results where series resistance had no impact on the current was reported by Sunny et al. [74].Another study by Pooja et al. reported that series resistance could not alter the open circuit voltage [75].However, it can be stated that series resistance needs to be minimized to a minimum value so that the best performance can be attained.

2) IMPACT OF SHUNT RESISTANCE (R SH )
Shunt resistance is a parallel resistance that allows current to bypass the absorbing layer and decreases efficiency due to fabrication defects [72].Figure 11(b) shows the impact of R sh on the key parameters of WS 2 , ZnSe, C 60 and PCBMbased PSC.R sh was varied from 10 1 -cm 2 to 10 7 -cm 2 and the Series Resistance (R s ) was kept fixed at 0.5 -cm 2 .The values of performance parameters followed the same trend as R sh is increased.From 10 1 -cm 2 , all the parameters started to enhance decisively.This trend continued till 10 3 -cm 2 .After that, they hold the same value though R sh increases.It can be observed from the figure that, SC with WS 2 ETL exhibits the highest PCE, while the SC with PCBM ETL demonstrates the lowest PCE value within the specified range.Lower R sh creates the alternating path for the photogenerated current which results in output power loss [76].So, higher R sh is better for device performance.

3) IMPACT OF TEMPERATURE
To observe the impact of the environment in PSC, the temperature is ranged between 275 K to 450 K. Figure 11(c) demonstrates the effect of temperature on cell parameters.The simulation found that for all structures, a rise in temperature resulted in a drop in the V OC , FF, and PCE, while the J SC almost remained constant.The V OC value has reduced as the temperature is increased due to its inverse relationship with the saturation current density [5].A similar downtrend can be observed in the case of FF as well as PCE for all structures.The rise in temperature causes an improvement in series resistance, which in turn reduces the FF as well as the efficiency of the device [77], [78].The pattern of the constant behavior of current throughout the rise in temperature was reported by Sunny et al. [74].

G. EFFECT OF CAPACITANCE AND MOTT-SCHOTTKY LINES
Figure 12 and Figure 13 illustrates the impact of capacitance along with M-S with the voltage variation from -0.8 volt to +0.8 volt for four individual configurations.The simulation had been performed at a higher frequency of 1 MHz to exclude the effect of traps situated internally at a deep level [79].In Figure 12, an increment in supply voltage causes the capacitance to rise exponentially.As there are several junctions, the plots are curved and not linear in characteristics.The C 60 ETL-based structure had the highest capacitance, while the WS 2 ETL had the lowest at 0.8 V.It can be assumed that when the bias is off, the device is in the depletion region.When the depletion width is nearly the same as the thickness of the absorber, the forward bias induces the capacitance to increase considerably [80].The capacitance decreases significantly when a reverse bias is applied because the saturation current increases [81].On the other side, the diffusion potential (V bi ) is observed from the Mott-Schottky plot.The slope between 1/C 2 and V curve represents the defect density [82].The MS values decreased for every structure in this work with rising voltage, which is consistent with previous research study [79].

H. EFFECT OF GENERATION RATE AND RECOMBINATION RATE
Electron-hole pairs are formed when an electron is stimulated from the valence band towards the conduction band, creating a hole [83].Figure 14 shows that WS 2 , C 60 , ZnSe, and PCBM-based structures had a greater rate of generation at depths of 0.8 µm, 1 µm, 0.9 µm, and 1 µm, respectively.The recombination rate inversely combines and annihilates the generated electrons and holes [84].According to Figure 15, the recombination has taken place in the 0.1µm -1µm range.The recombination rate distribution is not uniform due to the presence of grain boundaries and defects in different layers [85].However, the structures demonstrated significant recombination occur at 0.1 µm and 0.9 µm.The depth of 0.1 µm represents the HTL/Absorber interface and the depth of 0.9 µm represents the Absorber/ETL interface.Recombination tends to be more apparent at interfaces due to energy level mismatch, defect or trap States.The differences in energy levels between different materials at interfaces can result in increased carrier recombination.Furthermore, interfaces frequently possess defects or trap states which can function as recombination centers, facilitating electron and hole recombination [86].

I. J-V AND QE CHARACTERISTICS
Under a 1.5 AM lighting condition (100mW/cm 2 ), the J-V characteristics of the four configurations are shown in Figure 16.It was discovered that C 60 ETL has shown the worst J-V characteristics compared to WS 2 , ZnSe and PCBM.Despite its high V OC , C 60 ETL-associated PSC has a current density of 33.3 mA/cm 2 .The value of current density was around 34.5 mA/cm 2 in PCBM and ZnSe despite the V OC being less than 1.0 V.Meanwhile, the value of the current density in WS 2 was 33.92 mA/cm 2 .Figure 17 displays the wavelength-quantum efficiency (QE) relationship for these structures.A similar pattern is found for all the structures.It is observed that at wavelengths between the 0.36 µm and 0.6 µm, QE for WS 2 , ZnSe, and PCBM is almost 100%, whereas C 60 showed the worst QE (90%) and it rises to 100% beyond 0.7µm wavelength.Solar cells having QE 100% means that it has absorbed almost incident photons in order to create electron-hole pairs and minimize all the recombination [87].Furthermore, beyond this wavelength range, photons with higher wavelengths cause lower QE.This is due to the inability of those photons to generate high enough energy to create electron-hole pairs.Finally, the quantum efficiency falls to zero at the wavelength of 0.96 µm.Therefore, the configurations mostly cover the visible light spectrum.

J. OPTIMIZED STRUCTURE
Different ETL materials were incorporated into the device structure using P3HT as the HTL and the parameters such as the thickness, doping level, defect density of different layers,temperature, parasitic resistance etc. are optimized.
Among the several ETL candidates implemented, including WS 2 , ZnSe, C 60 , and PCBM, it's evident that WS 2 can be recognized as a potential and efficient ETL material for leadfree Sn-based perovskite solar cells with an improved V OC of 1.053 V, the device architecture demonstrated an efficiency of 31.63% that is comparable to previously reported studies mentioned in Table 3.

IV. CONCLUSION
In this study, a highly efficient, lead-free, and sustainable PSC based on CsSnI 3 has been recommended with a new combination of FTO/WS 2 /CsSnI 3 /P3HT/Au by evaluating the impacts of various ETL materials on device performance and improving various solar cell parameters along with the operating conditions of the solar cell.Additionally, the performance is compared against previously published literature.The proposed structure features a FF of 88.57%, a short circuit current density of 33.92 mA/cm 2 , an efficiency of 31.63% and an open-circuit voltage of 1.053V.The structure also exhibits excellent quantum efficiency in the near IR and visible spectrum.Hopefully, in the future, this research will aid in the development of lead-free solar cells that are more stable and can serve as a foundation for flexible technology.

FIGURE 1 .
FIGURE 1. Schematic structure of the designed PSC.

FIGURE 3 .
FIGURE 3. Contour plot demonstrating V OC for the variation of absorber layer thickness and defect concentration using (a) WS 2 , (b) ZnSe, (c) C 60 and (d) PCBM as ETLs.

FIGURE 4 .
FIGURE 4. Contour plot demonstrating J SC for the variation of absorber layer thickness and defect concentration using (a) WS 2 , (b) ZnSe, (c) C 60 , and (d) PCBM as ETLs.

FIGURE 5 .
FIGURE 5. Contour plot demonstrating FF for the variation of absorber layer thickness and defect concentration using (a) WS 2 , (b) ZnSe, (c) C 60 and (d) PCBM as ETLs.

FIGURE 7 .
FIGURE 7. Demonstrating PV parameters for the variation of absorber layer acceptor density.

FIGURE 8 .
FIGURE 8. Demonstrating PV parameters for the variation of (a) ETL thickness, (b) ETL donor doping density, and (c) ETL defect density.

Figure 9 (
Figure 9(b) demonstrates the influence of PV parameters with increasing acceptor concentration (N A ) of HTL for four

FIGURE 9 .
FIGURE 9. Demonstrating PV parameters for the variation of (a) HTL thickness, (b) HTL Acceptor doping density and (c) HTL defect density.

FIGURE 10 .
FIGURE 10.Impact of variation in interface defect density of the (a) HTL/CsSnI 3 and (b) CsSnI 3 /ETL on Cell Parameters.

2 )
Figure 10(b)  demonstrates the impact of defect density across CsSnI 3 /ETL interface when N t was raised from 10 10 cm −3 to 10 18 cm −3 .The study found that the value of V OC , J SC , FF, and PCE declines as N t is raised.The V OC , FF and PCE remained constant till 10 11 cm −3 and then starts declining with increment in N t .J SC almost remained constant till the defect of 10 15 cm −3 and then varies in a different manner.The C 60 -associated structure had the lowest FF, J SC and PCE value of 56.87%, 2.283 mA/cm −2 and 0.83%, respectively.In the case of V OC , PCBM demonstrated the lowest value,

FIGURE 11 .
FIGURE 11.Impact of (a) series resistance (R s ), (b) shunt resistance (R sh ), and (c) temperature on Cell Performance.

FIGURE 14 .
FIGURE 14.Generation rate curve for four PSCs.

FIGURE 16 .
FIGURE 16.J-V characteristic curve for all four optimized designs.

TABLE 1 .
Input parameters of various layers used in simulation.

TABLE 2 .
Input parameters of interface and bulk defect

TABLE 3 .
Comparison of PV parameters of CsSnI 3 based PSC with existing research work.
FIGURE 15.Recombination rate for four PSCs.