Design and Simulation of an Environment-Friendly ZrS 2 /CuInS 2 Thin Film Solar Cell Using SCAPS 1D Software

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Introduction
Te world's rising energy demand is leading to an energy crisis and environmental damage virtually every day.Due to the scarcity, depletion, and ecological efects, fossil fuels still need to be a sustainable source of energy [1].A renewable resource can replace itself spontaneously over time, resulting in no greenhouse gas emissions from fossil fuels and lowering some air pollution.Solar energy is an excellent substitute for fossil fuels such as coal and natural gas since it produces pure, clean, and renewable energy.Silicon-based solar cells with a conversion efciency of less than 25% continue to dominate the solar cell market.Te main disadvantage of solar cells today is their low efciency and high manufacturing costs [2].However, research is still needed to enhance existing materials or create new ones, which could lead to new opportunities for developing highly efective, low-cost, toxic-element-free devices.Tin flm solar cells are introduced as second-generation solar cells.Tough highly efcient, frst-generation solar cells are highly cost-efective, whereas thin flms are low-cost.A thin flm tends to have efciencies of around 7% and up to 27%.Te future of thin flms looks strong because of the resources and endurance to overcome technological challenges.
Te last few decades have seen much work creating a high-quality light absorber layer.Some of the semiconductors used in solar cells have been created using various chemical and physical processes, including copper indium gallium sulfde and selenide (Cu 2 InGaS 4 /Se 4 -CCIGS/Se), copper indium sulfde and selenide (CuInS 2 / Se 2 -CIS/Se), and copper zinc tin sulfde and selenide (Cu 2 ZnSnS 4 -CZTS).Tis semiconductor is binary, ternary, and quaternary.It demonstrates nontoxic and earthabundant materials, a sufciently high light absorption coefcient in the visible and near-infrared IR areas, and a band gap close to the solar spectrum.CuInS 2 is the most suitable type of light-absorbing material.CuInS 2 is the most suitable type of light-absorbing material.CuInS 2 is nontoxic [2], has a direct band gap (1.3-1.5 eV) [3], and has a high optical absorption coefcient (10 5 cm −1 ) [4] with long-term stability in solar applications [5].Furthermore, toxic compounds such as Cd and Se are absent from CuInS 2 .CuInS 2 is thus safe for the environment and appropriate for solar cell applications [6].
In this simulation, we introduce nontoxic ZrS 2 as a bufer layer material, and aluminum (Al) and gold (Au) were utilized as the device's front and back metal contacts.Zirconium disulfde (ZrS 2 ), which belongs to group IV of TMDCs, is an n-type semiconductor that exhibits a low misft lattice with other absorber materials because of Van der Waals force [11].Te main beneft of TMDC over other materials is the absence of dangling bonds, which enables vertical stacking of several TMDC materials to create heterostructures without the need for lattice matching.ZrS 2 is considered a strong candidate for fabricating optoelectronics, particularly photovoltaic, due to its high absorption coefcient and readily manufactured band gap energy, which can be in the 1.2-2.2eV range.Combining n-ZrS 2 thin flms with other p-type semiconductors with appropriate energy level alignment, such as CuInS 2 , could be the future key to have solar cells with higher efciency.

Numerical Simulation and Parameters of Materials
In this study, SCAPS-1D simulation software has been used to model the heterojunction thin flm solar cells' device properties.At the Electronics and Information Systems (EIS) Department at the University of Gent in Belgium, the "Solar Cell Capacitance Simulator One-Dimensional" (SCAPS-1D) application was developed to model solar cells.[12].Te continuity equation and Poisson's equation are given for the free electrons and holes in the conduction and valence bands [13].Te electron and hole continuity equations are as follows: where G is the generation rate, Jn and Jp are the current densities for electrons and holes, respectively.Te Poisson formula is as follows: where e is the electrical charge, ε r is the relative, ε 0 is the vacuum permittivity, ψ is the electrostatic potential, p and n are the concentrations of holes and electrons, respectively, N A and N D are the charge impurities of the acceptor and donor types, respectively, and ρ p and ρ n are the distributions of holes and electrons, respectively.SCAPS-1D solves the above equations while considering boundary conditions using the steady-state response of the fundamental semiconductor equations in one dimension.Te suggested thin flm solar cell structure is shown schematically in Figure 1.
Te references specify the ZrS 2 and CuInS 2 thin flm parameters listed in Table 1 and are used to execute our numerical calculations.Te ZrS 2 /CuInS 2 -based thin flm solar cell's proposed energy band diagram was extracted using the SCAPS-1D program.Te energy band diagram, shown in Figure 2, describes the optical characteristics of solar cells.

Impact of Absorber Layer (CuInS 2 ) Tickness and Band
Gap. Figure 3 illustrates the infuence of the band gap and absorber layer thickness on solar cell performance metrics.Te band gap and bufer layer thickness in this work were fxed at 1.7 eV and 0.3 µm, respectively.One of the essential factors in improving solar cells' efciency is the absorber layer's thickness [14,15].Figure 3(a) shows that all solar cell output parameters signifcantly increased as the absorber layer thickness increased.Te Voc rises from 0.746 V to 0.812 V.While varying the absorber thickness from 0.5 to 5.0 µm, Jsc increases from 25.33 mA/cm 2 to 30.72 mA/cm 2 , fll factor changes from 83.37% to 85.86%, and efciency rises from 15.75% to 21.43%.Tis may be due to the fact that when the absorber layer thickens, more short-wavelength photons are absorbed, which boosts the photogeneration of more free carriers [16].
Te band gap of the active layer is signifcant in achieving high-efciency solar cells.Te band gap of the ideal photovoltaic material is between 1 eV and 1.8 eV [17].Figure 3(b) shows the impact of the band gap of the absorber layer on solar output parameters.Te absorber layer CuInS 2 band gap varies from 1.3 eV to 1.5 eV.When the band gap of the absorber layer is 1.3 eV, the value of Voc is 0.8058 V, Jsc 34.67 mA/cm 2 , FF 85.72%, and efciency 23.95%.When the band gap of the absorber layer is 1.5 eV, the value of Voc is 0.8065 V, Jsc 27.87 mA/cm 2 , FF 85.82%, and efciency 19.29%.Equation (4) could explain these fndings: Te local collection efciency of light absorption improves at the interface between the thin flms of p-CuInS2 and n-ZrS2 as the band gap increases, accelerating the rate of carrier production and increasing the open circuit voltage [16].

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Advances in Materials Science and Engineering From Figure 3(b), efciency is decreased from 25.43% to 14% when the band gap of the absorber layer increases from 1 eV to 1.8 eV.Efciency is declining because fewer electrons can enter the conduction band when the gap between the valence and conduction bands increases [18,19].

Impact of Bufer Layer (ZrS 2 ) Tickness.
For optimal solar cell performance, optimizing the thickness of the bufer layer is essential [19][20][21].When sunlight reaches the absorber layer, the bufer layer's thickness must be decreased.Terefore, activating the light spectrum's absorption process is important [22].Figure 4 illustrates the infuence of bufer layer thickness on solar cell output parameters.Te thickness varies from 0.1 µm to 0.5 µm.Here, the Voc increases from 0.8062 to 0.805 V.
Increased bufer thickness results in more photons being absorbed outside the hole difusion length region, lowering the recombination rate and raising Voc [23].Tough it is observed that after 300 nm, Voc has reached to saturation point, the short circuit current increased slightly from 30.37 to 30.56 mA/cm 2 , and the fll factor changes from 85.6 to 85.85%.Efciency has risen slightly from 20.96 to 21.16%.Trough the simulation, it is noticed that the efciency is becoming saturated after 300 nm.Our solar cell optimum bufer layer thickness was 300 nm for the above results.

Current Density-Voltage (J-V) Characteristics.
Figure 5 illustrates the current density-voltage characteristics of the proposed solar cell.Te thickness of the absorber layer varies from 0.5 to 5 µm.Te Voc rises from 0.74 V to 0.81 V as the thickness rises.Te Jsc is increased from 25.33 to 30.72 mA/cm 2 .Te fll factor rises from 83.37% to 85.8%.Efciency also increases from 15.75% to 21.43%.Te current density-voltage curve demonstrates that efciency rises as thickness rises simultaneously.Te reason behind that is the addition of electron-hole pairs with increasing active layer thickness [24].

Impact of Donor Concentration and Acceptor
Concentration. Figure 6(a) shows the impact of donor density on solar performance parameters.For donor density variation, acceptor density is fxed at 2 × 10 17 cm −3 ; for acceptor density variation, donor density is constant at 1 × 10 19 cm −3 .Here, donor density varies from 1 × 10 15 to 1 × 10 20 cm −3 .As the donor density increased, both Voc and FF decreased from 0.8067 V to 0.8065 V and 85.99% to 85.81%.Jsc is nearly constant.Efciency rises from 21.02% to 21.11%.
Figure 6(b) demonstrates the solar cell output parameters Voc, Jsc, FF, and η with variations of acceptor density.Here, density is varied from 2 × 10 13 to 2 × 10 18 cm −3 .Voc increases dramatically from 0.5 V to 0.86 V. Te fll factor increased by about 11%, from 75.09% to 86.78%.Jsc is decreased from 31.5 to 30.35 mA/cm 2 .Efciency increases signifcantly from 12.04% to 22.82%.As the combination's depletion zone approaches, the photogenerated minority carriers are separated by an existing electrical feld [25].Higher acceptor density would decrease the device's shunt resistance, which would lower the device's efciency.3.5.Quantum Efciency (Q-E) Curve.Figure 7 illustrates the quantum efciency curve of the proposed solar cell.Te thickness of the CuInS 2 layer ranges from 0.5 µm to 5 µm.Te ratio of carriers collected by the solar cell to photons incident on the solar cell at a specifc energy is known as quantum efciency.A wavelength function or an energy value might be used to express it [26].Figure 6 demonstrates that quantum efciency increases at longer wavelengths when the absorber layer thickness increases.Tis is due to the lack of photons to produce sufcient electron-hole pairs inside the absorber layer [27].Quantum efciency falls to zero for wavelengths longer than 860 nm because light is not captured below band gaps for longer wavelengths of low energy.

Impact of Temperature and Defect Density.
Te operating temperature has signifcant efects on a solar cell's efciency [28].Te performance of the solar cell is afected by temperature changes.Temperature afects efciency, as seen in Figure 8(a).Here, the temperature is varied from 250 K to 400 K.With the increase in temperature, efciency is decreased.At 250 K, efciency is 21.84% which is reduced at 400 K to 19.33%.When the temperature rises, charged particles velocities increase [29].Te rate of electron and hole recombination rises as temperature rises because there are fewer free electrons and holes accessible [30].

Impact of Series and Shunt
Resistance. Figure 9 illustrates the series and shunt resistance impact on the suggested solar cell.As series resistance ranged from 0 to 4 Ω-cm 2 , shunt resistance remains constant at 10 6 Ω-cm 2 , and for shunt   Acceptor Density (cm -3 ) 6 Advances in Materials Science and Engineering resistance, variation series is selected at 0.5 Ω-cm 2 .Figure 9(a) shows the efect of series resistance varied from 0 to 4 Ω-cm 2 .Te variation does not impact Voc, which is constant at 0.807 V. Jsc slightly changes from 30.5 to 30.48 mA/cm 2 .But both fll factor and efciency decrease.Te variation reduces the fll factor by 14%, from 85.78% to 71.72%.Figure 9(b) shows the efect of shunt resistance varied from 10 to 10 6 Ω-cm 2 .As Rsh increases, Voc dramatically increases from 0.3 V to 0.8065 V, Jsc increases from 29.04 to 30.5 mA/cm 2 , and the efciency changes from 2.21% to 20.16%.

Capacitance-Voltage (C-V) and Mott-Schottky
Characteristics.Te Mott-Schottky is a well-known and efcient tool for determining a device's built-in potential (Vbi) and doping level [32].Te 1/C 2 (V) slope in the Mott-Schottky plot indicates a concentration of active trapping centers.[33][34][35].Figure 10(a) shows that the capacitance gradually rises with applied voltage, peaks at higher voltages.According to Figure 10(a), it has been observed that this structure is depleted at zero bias.When the forward bias is applied at a voltage of about 0.5 V, the depletion width falls to a value that is approximately comparable to the thickness of the absorber layer.Terefore, as the forward bias voltage increases, the capacitance also increases as well.Te charge develops at the interface when doping rises, and the capacitance value will increase [36].According to the Mott-Schottky relation, the built-in potential (Vbi) value is found at 1/C 2 � 0 on the corresponding potential axis [37,38].Te 1/C 2 -V curve in Figure 10(b) is obtained by Mott-Schottky equation (2) [39,40].

Conclusion
Tis research studies a noble structure (Al/ZrS 2 /CuInS 2 /Au) by simulation in SCAPS-1D software.Te absorber layer's variation in thickness, band gap, and concentration during the simulation ranges from 0.5 to 5 μm, 1 to 1.8 eV, 2 × 10 13 to 2 × 10 18 cm −3 and for the bufer layer, 0.1 to 0.5 μm, 1.4 to 1.8 eV, 10 15 to 10 20 cm −3 .Temperature is varied from 250 to 400 K. Te frequency is fxed to 1 MHz.After the successful simulation process, CuInS 2 thin flms were found to have an optimal thickness, carrier concentration, and band gap of 4 μm, 2 × 10 17 cm −3 , and 1.43 eV, respectively, and for ZrS 2 to be 0.3 μm, 10 20 cm −3 , and 1.7 eV, respectively, for have higher solar cell efciency.Te highest efciency was gained, 21.1%, with Jsc at 30.5 mA/cm 2 , Voc at 0.806 V, and FF at 85.78%.Te performance of the solar cells is observed to be reduced by high temperatures.Due to its nontoxic components, this solar cell is highly recommended for thin flm solar cell technology.
Figure 10 demonstrates the C-V characteristics and Mott-Schottky plot analysis of the proposed solar cells as a function of the shallow uniform donor density (N d ) of the ZrS 2 structure.Te donor density (N d ) concentration varied from 10 16 cm −3 to 10 19 cm −3 .

Figure 8 :
Figure 8: Impact of temperature and defect density of absorber layer on Voc, Jsc, FF, and efciency.(a) Impact of temperature on Voc, Jsc, FF, and efciency.(b) Impact of defect density on Voc, Jsc, FF, and efciency.

Figure 9 :Figure 10 :
Figure 9: Impact of series and shunt resistance on Voc, Jsc, FF, and efciency.(a) Impact of series resistance on Voc, Jsc, FF, and efciency.(b) Impact of shunt resistance on Voc, Jsc, FF, and efciency.
VB for valence band and CB stands for conduction band.