A Numerical Approach to Analysis of an Environment-Friendly Sn-Based Perovskite Solar Cell with SnO 2 Buffer Layer Using SCAPS-1D

In this research, we have proposed a Sn-based perovskite solar cell using solar cell capacitance software. Te main aim of this study is to develop an environment-friendly and highly efcient structure that can be used as an alternative to other toxic lead-based perovskite solar cells. Tis work performed a numerical analysis for the proposed (Al/ZnO/SnO 2 /CH 3 NH 3 SnI 3 /Ni) device structure. Te absorber layer CH 3 NH 3 SnI 3 , bufer layer SnO 2 , and electron transport layer (ETL) ZnO, with aluminium as the front contact and nickel as the back contact, have been used in this simulation. Several analyses have been conducted for the proposed structure, such as the impact of the absorber layer thickness, acceptor density, defect density, series and shunt re-sistances, back contact work function, and operating temperature. Te device simulation revealed that the optimum thickness of the absorber layer is 1.5 μ m and 0.05 μ m for the bufer layer. Te proposed Sn-based perovskite structure has obtained a conversion efciency of 28.19% along with FF of 84.63%, Jsc of 34.634mA/cm 2 , and Voc of 0.961V. Tis study shows the upcoming lead-free perovskite solar cell’s enormous potential.


Introduction
Energy usage is expected to increase signifcantly in the future. Most energy utilized today comes from fossil fuels, which have a large carbon footprint and will decay quickly. As a result of industrialization and population increase, energy prices are growing. Te most challenging task is developing a renewable energy source because there is a huge demand for green energy. Solar energy is a promising future technology since it is clean and good for the environment [1,2]. Tere are many diferent types of solar cells, including amorphous silicon, cadmium telluride, copper indium gallium selenide, dye-sensitized, hybrid, monocrystalline, nanocrystal, and perovskite solar cells [3]. Tird-generation (emerging technology) perovskite solar cell is the one that was chosen. Te light-harvesting active layer of a perovskite solar cell is made of a material with a perovskite structure; generally, this material is a hybrid organic-inorganic lead-or tin-based compound [4,5]. CH 3 NH 3 PbI 3 is the most popular perovskite absorber and has an optical band gap that varies from 1.6 eV to 2.3 eV [6][7][8][9][10][11][12]. Lead perovskite halide shows the highest solar energy conversion efciency at 23%. It however sufers from toxicity issues. Lead-free perovskites have been shown as a viable candidate for potential use as light harvesters to ensure renewable PV technologies, where the lead can be replaced with Sn, Ge, Bi, Sb, and Cu. Te candidates reported efciencies of up to 9%; however, their efciency and stability in the air are still urgently needed to be enhanced [13].
To date, researchers have made eforts in contributing to the development of perovskite semiconducting materials which have led to the rise of low cost and high efciency concerning the simulation of the solar cell capacitance simulator-one dimension (SCAPS).
In this case study, we introduce a Sn-based perovskite structure that has high efciency compared to previously submitted cells and is ecofriendly. Currently, tin-based PSCs demonstrate a maximum PCE over 13% together with excellent device stability, making them a very attractive photovoltaic technology to be further developed in the near future [14]. Since tin-based perovskite solar cells are afected by the oxidation process they tend not to be stable in the surrounding atmosphere. Hence, the measured efciency of the perovskite solar cells decreases drastically with time. Researchers attempted to solve this issue by adding SnF 2 to the perovskite structure. Several studies have been conducted in recent years, both simulation and experimental, with CH 3 NH 3 SnI 3 -based solar cells. A simulated research work published in 2021 achieved a J SC of 31.66 mA/cm 2 , V OC of 0.96 V, FF of 67%, and a conversion efciency of 20.28% [15]. Ten, another work published in 2016 has a J SC of 31.59 mA/cm 2 , V OC of 0.92 V, FF of 79.99%, and conversion efciency of 23.36% [16]. In 2019, the work had a conversion efciency of 23.76% [17]. For the proposed (Zn 2 SnO 4 / CH 3 NH 3 SnI 3 /Spiro-MeOTAD) structure, the obtained conversion efciency in 2021 is 24.73% [18]. In 2017, it had a conversion efciency of 24.82%, J SC of 25.67 mA/cm 2 , FF of 78.14% and V OC of 1.0413 V [19]. Te conversion efciency of 26.33%, along with the V OC of 0.98 V, J SC of 31.93 mA/ cm 2 , and FF of 84.34%, have been reported for 2021 [20]. In 2021, another study obtained a conversion efciency of 26.92% V OC of 0.99 V, J SC of 36.81 mA/cm 2 , and FF of 73.80% [21]. In experimental works, two structures have been mentioned that obtained conversion efciency of 17.1 and 15.1%. Te structures are ITO/PEDOT: PSS/ CH 3 NH 3 SnI 3 /C 60 /BCP/Ag and FTO/TiO 2 /Perovskite/Spi-roMeOTAD/Ag [22,23]. After studying several valuable works, a novel structure (Al/ZnO/SnO 2 /CH 3 NH 3 SnI 3 /Ni) has been proposed in this research.
In this study, SnO 2 : F (fuorine-doped tin oxide) is used as a bufer layer because of the high transparency of the tin oxide semiconductor. F-doped SnO 2 is the least expensive transparent conductive oxide material with the highest work function, the best contact with p-Si, thermal stability, chemical durability, a wide band gap, and negligible toxicity [24]. Due to its high conductivity, electron mobility, photocorrosion resistance, and low cost, zinc oxide (ZnO) has been recognized as a helpful electron transport material for solar cell applications [25].
Te Sn-based perovskite solar cell is an HTL-free perovskite solar cell. HTL-free devices can greatly save the cost of materials. On the other hand, devices based on HTL-free confgurations can reduce the fabrication cost, interface defects without afecting cell performance, and the complexity of processing because fewer layers are used [14].

Device Architecture and Numerical Simulation
Te proposed Sn bead solar cell structure, which has the composition Al/ZnO/SnO 2 /CH 3 NH 3 SnI 3 /Ni, is shown schematically in Figure 1. Figure 1 shows that Ni was used as the back contact, CH 3 NH 3 SnI 3 was used as the absorber layer, SnO 2 was chosen as the bufer layer, and ZnO was used as the ETL for this perovskite solar cell.
Researchers from the University of Gent's Electronics and Information Systems (ELIS) Department created the numerical simulation tool known as SCAPS [26]. SCAPS-1D uses two essential semiconductor equations: the continuity equation for electrons and holes under steady-state circumstances and the Poisson equations (27)- (34). Poisson's and continuity equations are applied to free electrons and holes in the conduction and valence bands.
Te following are the continuity equations for electrons and holes: where J n and J p represent electron and hole current densities, R represents the recombination rate, and G represents the generation rate. Te Poisson equation is as follows: where Ψ is the electrostatic potential, e is the electrical charge, ϵ r is the relative permittivity, ϵ o is the vacuum permittivity, p and n are the hole and electron concentrations, N D is the donor type, N A is the acceptor type charged impurities, and ρ p and ρ n are the hole and electron distributions, respectively [19,35]. Te physical parameters needed for the simulation are shown in Table 1. rise with increasing absorber thickness, as shown by the J-V curve in Figure 2. Improved current and voltage result from adding the electron-hole pair while thickening the absorber. Figure 3 shows the wavelength versus quantum efciency. Quantum efciency is defned as the ability of a solar cell to absorb carriers from incident photons of a specifc energy. As can be seen, the thinner the absorber, the fewer photons at longer wavelengths are absorbed. Tis is because photons form fewer electron-hole pairs inside the absorber layer [20]. With a thick absorber layer, solar cells operate more efciently because the likelihood of back recombination is reduced, which enhances the quantum efciency [38]. Moreover, light is not absorbed below band gaps for longer low-energy wavelengths, resulting in a quantum efciency of zero for wavelengths greater than 950 nm. Te light-absorbing layer's thickness considerably infuences perovskite solar cell performance [16,[39][40][41].

J-V and Q-E Curve of Sn-Based Perovskite Solar Cell.
To evaluate the efectiveness of the suggested CH 3 NH 3 SnI 3 -based perovskite solar cell, the absorber's thickness is changed from 0.5 to 3.0 μm. As the absorber layer thickness increased, all solar cell output metrics signifcantly increased, as shown in Figure 4. Te value of V OC rose from 0.920 to 0.982 V, J SC 32.150 to 35.123 mA/cm 2 , FF 80.76 to 86.02%, and efciency from 23.89 to 29.68% likewise increased while varying the absorber thickness from 0.5 to 3.0 μm.
It is proposed that the photogenerated electrons and holes will be signifcantly boosted at thicker absorber layers, improving the overall performance of the solar cell. As a result, considering the device manufacturing cost, the CH 3 NH 3 SnI 3 absorber thickness is set at 1.5 μm in this study, which is chosen as the best thickness for further research.

Impact of Acceptor Density and Defect
Density of the Absorber Layer. Figure 5 displays the solar cell output parameters V OC , J SC , FF, and efciency. In this study, we  discovered that as acceptor density increased, so did the V OC , FF, and efciency. When the acceptor density is increased, the J SC decreases. Te acceptor density has been changed from 10 11 to 10 17 cm −3 . Te V OC varies from 0.913 to 1.081 V, J SC 35.178 to 31.559 mA/cm 2 , FF 84.41 to 86.66%, and efciency 27.12 to 29.58%, while the acceptor densities have changed from 10 11 to 10 17 cm −3 , respectively. Te absorber layer acceptor density infuences the performance of photovoltaic systems. Tis computational study looks at how the proposed CH 3 NH 3 SnI 3 -based perovskite solar cell reacts to absorber layer acceptor density changes. Te defect density in the absorber signifcantly impacts the performance of photovoltaic systems. Another critical factor that might afect the device's performance is the active layer's total defect density. Figure 6 illustrates the numerical study's investigation of the proposed Sn-based perovskite solar cell reaction to changing defect densities in the absorber layer related to the intended perovskite solar cell V OC , J SC , FF, and efciency values. Te absorber layer's defect density varied from 10 14 to 10 18 cm −3 . Te V OC decreased from 0.957 to 0.625 V, J SC 34.604 to 10.863 mA/cm 2 , FF 83.88 to 39.96%, and efciency 27.79 to 2.71% when defect density varied from 10 14 to 10 18 cm −3 . More signifcant pinhole production and recombination due to increased flm deterioration decreased stability and reduced device performance are all efects of higher defect concentrations in the absorber layer [42]. We conclude that an increase in defects results in a reduction in the charge carriers' difusion length and an increase in recombination carriers in the absorber layer, which directly impacts efciency [43].

Impact of Series (R S ) and Shunt (R Sh ) Resistances.
Te performance of Sn-based perovskite solar cell structures is signifcantly infuenced by series (R S ) and shunt (R Sh ) resistances. Te resistance R S between various terminals on the front and back contacts of the cell is the sum of these resistances. Te series resistance (R Sh ) is produced by the active junction's reverse saturation current caused by manufacturing faults. Using the SCAPS-1D simulator, the impact of R S and R Sh on solar cell performance, such as V OC , J SC , FF, and efciency, has been assessed as a function of R S , as shown in Figure 7. Te performance is examined by changing R S between 0 and 5 Ω-cm 2 while R Sh is constant at 10 5 Ω-cm 2 . Figure 7 demonstrates that the efciency and FF dramatically dropped when the R S increased. When R S was increased from 0 to 5 Ω-cm 2 , the efciency decreased from 28.18 to 22.93% and the FF from 84.60 to 68.97%. Te results achieved in this simulation are identical to prior research   Advances in Materials Science and Engineering conclusions [44,45]. At the same time, J SC and V OC are almost constant, as shown in Figure 7. J SC obtained 34.63 to 34.55 mA/cm 2 , V OC increased 0.961 to 0.962 V while R S varied from 0 to 5 Ω-cm 2 . Figure 8 shows the output parameters as a function of R Sh for the proposed perovskite solar cell. With R Sh being changed from 10 1 to Ω-cm 2 and R S remaining constant at 0.5 Ω-cm 2 , the impact of R Sh is examined. Te V OC increases from 0.344 to 0.961 V, J SC 32.978 to 34.627 mA/cm 2 , FF 25.01 to 83.02%, and efciency 2.84 to 27.65% along with the increase of R Sh .

Impact of back Contact Work Function and Operating
Temperature. To generate a moderate built-in potential there, a material with a suitable work function at the back contact properties of the recommended cell is investigated, as illustrated in Figure 9. Te back contact work function varied from 4.4 to 5.4 eV. Figure 9 shows that V OC , FF, J SC , and efciency rise as the work function is raised. Te V OC is increased by 0.143 to 0.966 V, J SC 33.455 to 34.728 mA/cm 2 , FF 54.55 to 84.89%, and efciency 2.61 to 28.48%. Te performance of solar cells is observed to be signifcantly impacted by the back contact work function. According to the current modelling fndings, a work function more signifcant than 5.1 eV is required for adequate PV performance. Because of its reasonable cost and essential work function, nickel (Ni � 5.35 eV) has been utilized as the back contact in this numerical study to achieve the high performance of a CH 3 NH 3 SnI 3 -based solar cell [46]. Te device simulations were conducted at an operating temperature of 300 K. Under steady light of 1000 Wm −2 , the device's operating temperature was changed from 295 to 425 K to examine the efects of temperature on perovskite solar cells. Figure 10 shows the device parameters as a function of temperature. Figure 10 shows that the output parameters of V OC , FF, and efciency are decreasing, and J SC is almost stable in this investigation. For perovskite solar cells, working temperature is an important consideration. Mainly, there is a high correlation between operating temperature and metrics like J SC and V OC [47].   [48]. Because there are fewer free carriers in the cell due to the increase in the recombination rate of photogenerated carriers, such as electrons and holes, the performance of the cell as a whole decrease at higher temperatures [49].

Conclusions
In this study, Sn-based perovskite solar cell structure (Al/ ZnO/SnO 2 /CH 3 NH 3 SnI 3 /Ni) has been investigated and simulated using SCAPS-1D software. Te absorber layer thickness varied from 0.5 to 3.0 μm, acceptor density 10 11 to 10 17 cm −3 , defect density 10 14 to 10 18 cm −3 , series (R S ) 0 to 5 Ω-cm 2 and shunt (R Sh ) 10 1 to 10 6 Ω-cm 2 , back contact work function 4.4 to 5.4 eV, and the operating temperature 295 to 425 K as shown on the output parameters of the solar cell. Te optimum thickness of the absorber layer is 1.5 μm and 0.05 μm for the bufer layer. Te proposed Sn-based perovskite solar cell structure has obtained a conversion efciency of 28.19% along with FF of 84.63%, J SC of 34.634 mA/cm 2 , and V OC of 0.961 V. Moreover, the conversion efciency decreases with an increase in operating temperature. Due to the absence of harmful components, this structure is also excellent for the environment. So, we hope it will be an efcient and ecofriendly perovskite solar cell for fulflling the upcoming generation's demands.

Data Availability
Te data supporting the investigation's results are available upon reasonable request from the relevant author.