Performance analysis of A3B2X9 (Cs3Bi2I9) based perovskite photovoltaic tandem structure with crystalline Silicon (c-Si)

The efficiency of solar cells with single-junction utilizing organic-inorganic hybrid perovskites have attained a value more than 25.5%. The device power conversion efficiency (PCE) can be improved further either by optimizing the absorber layer (Perovskite film) or by investigating the novel device structures such as tandem based solar cells with perovskite and silicon. This combination of top cell (Perovskite solar cell) and bottom cell (Silicon solar cell) can improve the PCE which surpasses the Shockley-Queisser limit of single-junction solar cells by utilizing a wider range of solar spectrum. This paper presents an optimization and simulation of standalone Cs3Bi2I9 perovskite solar cell which was later integrated with a c-Si solar cell to simulate a tandem structure using SCAPS-1D software. The aim is to investigate the performance enhancement of the perovskite solar cell by optimizing it and stacking it on top of a high-efficiency c-Si solar cell using a four-terminal (4T) structure. At a short-circuit current density of 16.165 mA/cm2 and an open-circuit voltage of 1.41 V, the simulation findings demonstrate that the Cs3Bi2I9 perovskite solar cell exhibits a high-power conversion efficiency of 20.37%. The tandem structure demonstrates an enhanced power conversion efficiency of 31.59% which is significantly higher than the efficiency of the individual cells. The Cs3Bi2I9 perovskite solar cell is a great choice for application in tandem systems with c-Si solar cells for high-efficiency, according to the simulation results. This study provides valuable insights for the development of efficient perovskite/c-Si tandem solar cells.


Introduction
Numerous attempts to create multi-junction cells (tandem cells), which are made up of solar cells with various band gaps, have been made to outperform solar cells with a single junction.The Shockley-Queisser power conversion efficiency (PCE) limit of 31% for single-junction devices is still being thoroughly researched for a variety of tandem solar device topologies (multijunction cells made up of various bandgaps).This is a workable strategy for addressing the projected rise in the world's energy demand from 15 TW in 2011 to 30 TW in the future (2050) [1]- [3] Wide bandgap (WBG) material for the top absorber and narrow bandgap (NBG) material for the bottom absorber are appropriate for tandem systems.When Si served as the bottom absorber and III-V compound semiconductor-based materials as the top absorber in the current generation of MJ-TSCs, the conversion efficiency was found to be 39% 1300 (2024) 012005 IOP Publishing doi:10.1088/1757-899X/1300/1/012005 2 under a 1 sun condition [4].Scaling up in the commercial sector is challenging due to the high cost and precision manufacturing required for these III-V compounds [5].Over the past ten years, research teams studying III-V materials have been impacted by the emergence of perovskite materials.
Due to their many beneficial features, including bandgap tunability, longer carrier diffusion length, smaller carrier effective mass, superior absorption coefficient, affordability, and ease of fabrication, perovskite materials have established themselves as a potential substitute.In order to improve conversion efficiency and lower the levelized cost of electricity (LCOE) of the solar cell (SC) technology, two-junction TSCs are being studied [5], [6].Mailoa et al. created the first two-terminal (2-T) perovskite/Si TSC in 2015, and it had a PCE of 13.7% [7].Zheng et al. improved it by using SnO2 as the top cell's electron transport layer (ETL) and interconnecting layer (ICL), and the resulting PCEs on 16cm 2 TSC were 17.10% [8].Due to its distinct characteristics, the A3B2X9 perovskite structure is a preferred option in perovskite solar cell technology more than the ABX3 structure [9].First off, the A3B2X9 perovskite structure has improved energy conversion efficiency due to its higher light absorption efficiency.Second, compared to the ABX3 structure, the A3B2X9 perovskite structure has a longer carrier lifetime, allowing it to maintain its energy conversion efficiency for a longer period of time.In comparison to the ABX3 structure, the A3B2X9 perovskite structure is also more stable and less likely to deteriorate.Additionally, compared to the ABX3 structure, the A3B2X9 perovskite structure has a higher defect tolerance, which leads to improved performance and increased efficiency.
Using SCAPS-1D software, we have simulated Pb-free PSCs with a device architecture of (FTO/Cd0.5Zn0.5S/Cs3Bi2I9/CuSbS2/Au). SCAPS (a Solar Cell Capacitance Simulator) is a 1-D solar cell simulation program developed at the Department of Electronics and Information Systems (ELIS) of the University of Gent, Belgium [10].Additionally, we used c-Si as the bottom sub-cell and the simulated perovskite structure as the top sub-cell to simulate a TPSC.The device architecture of Pb-free PSCs (FTO/Cd0.5Zn0.5S/Cs3Bi2I9/CuSbS2/Au)with experimental validation has not yet been reported in any publications, to our knowledge.

Device structure
While analyzing the performance of tandem solar cells, the top and bottom sub-cells are simulated separately.The tunnel junction is assumed to be flawless and both the optical and electrical losses at the interfaces are ignored in the standard multi-junction model [11]- [14].Perovskite solar cells are frequently used as the top cell and silicon solar cells as the bottom cell in tandem solar cell systems as shown in Figure 1.The device architecture shown in Figure 1. is the optimized architecture which provides a maximum efficiency.In this study, the top cell consists of Cs3Bi2I9 (Band gap: 1.9 eV, Thickness: 1.00 µm) as the absorber layer [15] [16].A hole-transport layer (HTL) and an electrontransport layer sandwich the perovskite layer (ETL).CuSbS2 (Band gap: 1.580 eV, Thickness: 0.100 µm), the material that makes up the HTL, has great stability and outstanding hole-transport characteristics [17].Cd0.5Zn0.5S(Band gap: 2.8 eV, Thickness: 0.150 µm), which has a narrow bandgap and high electron mobility, is the material used to make the ETL [18].Crystalline silicon (c-Si) cells, which have a high open-circuit voltage, low resistance, and long-term stability, make up the bottom cell [19].To guarantee the greatest amount of energy is captured, the bandgap and voltage of the c-Si cell are tuned to match those of the perovskite cell.It is a highly efficient photovoltaic device due to the combination of the perovskite cell's high absorption coefficient, the c-Si cell's high open-circuit voltage and low resistance, the low bandgap and high electron mobility of the ETL, the HTL's good holetransport properties, and the low bandgap and high electron mobility of the ETL.The back contact (Au) is only for the stand-alone cell.For the tandem structure ETL (CuSbS2) is in contact with n+Si.

Results and Discussion
This study utilized the SCAPS-1D software to simulate and analyse the efficiency of tandem solar cells, with a top cell made of Cs3Bi2I9 perovskite and a bottom cell made of crystalline silicon.A tandem device is a series of two-cell combinations that form a tandem structure with two terminals.Thus, the tandem cell's open-circuit voltage, Voc, is the summation of its sub-cell voltages.The lowest of the junction currents, however, will set a limit on the entire device's short-circuit current Jsc [20].The following subsections will include simulations of both the entire tandem device and individual cells.Table 1 gives the physical parameters used for simulation of the top cell and bottom cell.

Effect of the thickness and band-gap of the Absorber Layer
The overall cell's performance is considerably dependent on the perovskite layer's thickness.Using a numerical simulation, its impact on the overall performance of the cell is also examined.The consequences of varying the thickness in the range of 100 nm to 2000 nm are depicted in Figure 2. The Figure 2. shows that the Jsc and PCE values rise with the increase in the perovskite layer's thickness, while the Voc and FF fall.More photons are absorbed by the layer with the increase in the perovskite layer's thickness, raising the Jsc value.So, an increase in excess carrier concentration leads to an increase in Jsc.On the other hand, with the increase in the perovskite layer's thickness, the solar cell's internal power depletion and series resistance value also increases, causing FF and Voc to continuously decrease.As the thickness approaches and surpasses 1000 nm, the PCE hits its maximum and then starts to decline, as shown in Figure 2.
The band gap of Cs3Bi2I9 falls between 1.9 eV and 2.2 eV [15].We can see from the simulated results presented in Figure 3. that 1.9 eV is the ideal band gap for high efficiency as in the graph we can 1300 (2024) 012005 IOP Publishing doi:10.1088/1757-899X/1300/1/0120054 observe that with the increase in the band gap, the PCE decreases.Therefore, we set the absorber layer's band gap to 1.9 eV.

Effect of Defect Density of Absorber Layer
Defect density measurements from this investigation vary from 10 9 to 10 13 cm -3 [21].It is clear from Figure 4. that the PCE and FF decrease as the absorber layer defect density increases.A nonradiative Shockley-Read-Hall recombination center is a deep energy level defect.As a result, as the number of defects in the absorber layer increases, charge recombination increases, leading to an increase in Voc and Jsc while the short minority carrier lifespan decreases.Furthermore, the essential p-n junction does not form as the PSC transitions to semi-insulation, which results in poor cell performance when the absorber's defect density is lower than or equal to the doping density [20], [21].The density of defects in the absorber layer must be decreased, if not eliminated, to achieve the greatest PSC performance.A substantial reduction in defect density using current product innovations is also still extremely difficult.According to our study, for maximum cell effectiveness, the perovskite layer's defect density shouldn't be higher than 10 15 cm -3 .The thicknesses of the active layer (also known as the perovskite layer) is 1000 nm, the ETL (Cd0.5Zn0.5S) is 150 nm, the HTL (CuSbS2) is 100 nm, and the ITO layer (FTO) is 500 nm.Table 1.provides a list of other parameters that were used in this simulation.SCAPS -1D software simulates the stand-alone top perovskite solar cell using these parameters.An optimum efficiency of 20.37% is obtained for the top cell in our simulation.The results of this simulation and the optimum values of various parameters are listed in Table 2.
The bottom cell for the tandem device in this work is a c-Si solar cell.Here, n+ Si of 0.5 nm, p Si of 300 nm, and p+ Si of 10 nm have been used [19].The electrical properties of the separate cells must match in order to determine how efficient tandem solar cells will be.This synchronization makes sure that each layer's absorption of light is effectively turned into electrical energy.The electrical resistance and open circuit voltage between cells can be matched by modifying the layer thickness, doping levels, and material composition of each layer to achieve the current matching condition in tandem solar cells.Tandem solar cells are a potential technique for the production of renewable energy because they can reach high conversion efficiencies by preserving this current matching state [22].An optimum efficiency of 22.78% for the bottom cell is observed as indicated in Table 2.

Simulation of Tandem Solar cell
The tandem cell was simulated using the SCAPS-1-D simulation program using a fundamental approach.In our research, we have used mechanically stacked two-terminal tandem cells, which may be understood as a series connection of two diodes.Additionally, we have implemented the current matching condition between the top and bottom cells, as described in reference [22].The cell with the lower Jsc dominates the current-limiting criterion of the entire tandem arrangement, even if the voltage is derived by summing the voltages of individual cells.Either maximum power current density (JMP) variation or Jsc variation is used to match the current.The Cs3Bi2I9/c-Si tandem structure's current matching profile is depicted in Figure 5.We adjust the bottom cell's thickness to determine the current matching point.A maximum efficiency of 31.59% is observed for the tandem structure.The results tabulated in Table 3. in comparison with the reference PSC [21] shows an improved efficiency of 31.59% from a reported efficiency of 11.54%.

Conclusion
In this work, non-lead PSCs with absorber layer made of Cs3Bi2I9 perovskite are investigated using SCAPS modelling.First, it is decided which materials are best for the HTL and ETL, which are CuSbS2 and Cd0.5Zn0.5S,respectively.The effects of thickness of absorber layer, its density of defect, and bandgap are then investigated with regard to PSC performance.PSC setup is glass substrate/FTO/Cd0.5Zn0.5S/Cs3Bi2I9/CuSbS2/Au.According to modelling studies, the ideal thickness of absorber layer is 1000 nm.The simulation also concludes that the absorber layer's ideal defect density is 1x10 13 cm -3 , and that any higher defect density will result in solar cell performance reduction because additional recombination sites will occur.The research's findings will contribute to the creation of effective, non-lead PSCs and the expansion of solar wind wave and other forms of renewable energy.

Figure 1 .
Figure 1.Structure of Proposed Standalone and Tandem Solar Cell

Figure 2 .Figure 3 .
Figure 2. Effect of Thickness of Absorber Layer

Figure 4 .
Figure 4. Effect of Defect Density 3.3.Simulation of Top cell (FTO/Cd0.5Zn0.5S/Cs3Bi2I9/CuSbS2/Au)and Bottom Cell (c-Si) Using SCAPS -1D, standalone devices with active layers of Cs3Bi2I9 have been simulated.The top device's layer thicknesses are as follows:The thicknesses of the active layer (also known as the perovskite layer) is 1000 nm, the ETL (Cd0.5Zn0.5S) is 150 nm, the HTL (CuSbS2) is 100 nm, and the ITO layer (FTO) is 500 nm.Table1.provides a list of other parameters that were used in this simulation.SCAPS -1D software simulates the stand-alone top perovskite solar cell using these parameters.An optimum efficiency of 20.37% is obtained for the top cell in our simulation.The results of this simulation and the optimum values of various parameters are listed in Table2.

Table 1 .
Input parameters for different layers used for SCAPS -1D simulation

Table 3 .
Results of Simulations