1 Introduction

A photovoltaic solar cell has received much attention as an essential device for generating electricity. In this context, crystalline silicon is the highly commonly used material, accounting for more than 90% of installed solar cells (Pathi et al. 2017; Okil et al. 2021). Recently, Yoshikawa et al. have recorded a c-Si solar cell efficiency (η) of more than 26% (Green et al. 2022) which is near its theoretical efficiency limit of 29.56% (Schafer and Brendel 2018). However, the bandgap (Eg) of crystalline silicon is 1.12 eV, which is less than the optimal Eg (1.35 eV) preferred for single-junction cell applications (Shockley and Queisser 1961). To attain higher efficiencies while keeping a relatively low cost, several thin-film solar cell (TFSC) materials, like Cu(In, Ga)Se2, CdTe, and perovskite (Huang 2020; Gamal et al. 2021; Farooq et al. 2021), have drawn interest due to their greater optical absorption coefficient (α) and larger Eg compared with crystalline silicon. Still, several of the mentioned materials include toxic and/or rare elements.

Notably, an ultimate alternate absorbing material ought to have a high α, high minority-carrier lifetime, and suitable bandgap for use as a single junction or in tandem cells. These characteristics are shared by orthorhombic barium di-silicide, which is stable, environmentally friendly, and has abundant constituent elements (Suemasu and Usami 2017). BaSi2 has a 1.3 eV bandgap, and its α value is 3 × 104 cm−1 at 1.5 eV (Toh et al. 2011), almost forty times that of crystalline silicon, regardless of the indirect bandgap of BaSi2 (Kumar et al. 2014). The undoped BaSi2 is an n-type semiconductor that has an approximate electron density of about 5 × 1015 cm−3 (Khan et al. 2012), roughly 10 μm minority-carrier diffusion length (Baba et al. 2012), and about 10 μs minority-carrier lifetime (Hara et al. 2013) that is appropriate for TFSC applications. Due to the reported values of L and α, an efficiency that is higher than 25% could be estimated from the BaSi2 p–n junction diode having a thickness of 2 μm (Suemasu and Usami 2017). An efficiency of 23.17% has been achieved from p-type Zn3P2/n-type BaSi2 heterojunction thin-film cell (Takahashi et al. 2017). In 2018, Tian et al. (2018) fabricated polycrystalline BaSi2 thin films via an industrially applicable sputtering process and uncovered the process of structure transformation. Recently, Tian et al. (2020) deposited a Si/BaSi2/Si heterostructure by magnetron sputtering with different Si layer thicknesses and it was found that the thickness of the Si layer has a great effect on oxide layer growth, Ba depletion, and multiphase Si layer formation at the heterojunction interfaces. These findings contribute to the development of BaSi2/Si heterojunction solar cells and may allow the introduction of BaSi2 to be used in conjunction with other materials thanks to the progress in material quality.

Notably, a conventional single-junction cell has some drawbacks, including thermalization and sub-bandgap losses. On the other hand, tandem solar cells have theoretically attained an efficiency greater than the S–Q limit by a combination of a wide bandgap top sub-cell along with a lower bandgap bottom sub-cell (Zhang et al. 2014). In this regard, the resultant tandem cells can be divided into several architectures based on their electrical connections and fabrication techniques (Werner et al. 2018; Wang et al. 2019). These include 2T, 3T and 4T tandem structures. The two sub-cells of the 2T monolithic solar cells are electrically coupled by a recombination film; thus, the current flowing into the two sub-cells is the same. Also, the open circuit voltage is given by the sum of the two sub-cells' open circuit voltages of this series connection. To increase the viability of BaSi2 to be used in tandem cells, bandgap engineering should be performed to increase the Eg from its initial value of 1.3 eV. By substituting part of Ba with Sr atoms to create Ba1−xSrxSi2, the Eg of the compound can be adjusted to 1.4 eV (Morita et al. 2007). In the meantime, when replacing some Si atoms with isoelectric C, the corresponding bandgap of Ba(CxSi1−x)2 can steadily increase from 1.3 to 3 eV (Imai and Watanabe 2010). Eventually, BaSi2-based TSCs have been demonstrated in association with perovskite and crystalline silicon (Vismara et al. 2016; Tianguo 2019).

In this paper, we investigate a 2T monolithic tandem cell comprising barium di-silicide materials in the top cell as well as the bottom cell. Devices based on barium di-silicide material are regarded as environmentally friendly solar cells besides their low processing cost. A wide bandgap absorber layer is crucial for the top cell to permit a part of visible light and the near-infrared to pass through it and be readily absorbed by the bottom one. Thus, the presented designed TSC is based on combining BaSi2 (1.3 eV) and Ba(CxSi1−x)2 (1.6–1.9 eV) materials for the bottom and top sub-cells, respectively (Imai and Watanabe 2010; Eperon et al. 2017). Firstly, we investigate the effect of the top cell bandgap on the tandem performance, showing that the best performance is attained at a 1.8 eV bandgap (x = 0.78). Then, the performance of Ba(C0.78Si0.22)2/BaSi2 TSC is optimized through various steps including the effect of doping and the thickness of the absorber layers. Further, the current matching point is monitored whilst altering the thickness of the top cell. Finally, the impact of changing the absorber defect density of both cells and the work function of both contacts on the tandem performance is also discussed to highlight the possible routes for boosting the tandem efficiency.

2 Simulation methodology and device structure

2.1 Silvaco Atlas simulation methodology

The simulation study is performed by using the Silvaco Atlas simulation package which is a physics based 2- and 3-dimensional device simulator. The operating basis of this simulator is based on solving transport equations and Poisson’s equation through a defined grid, allowing for depicting the physical processes related to device operation and giving reliable estimates about the device operation (Atlas User’s Manual 2022). The physical models embedded in the simulation must be appropriately chosen to optimize the TSC. In this study, the principal physical models utilized for the design of a given solar cell structure are included. SRH recombination, the primary recombination mechanism in BaSi2 (Hara et al. 2012), and the Fermi–Dirac statistics have been considered. Carriers’ mobility and lifetime values within cell layers have been taken according to available experimental values (Takabe et al. 2014; Deng et al. 2018). In addition, Auger recombination (AUGER), optical recombination (OPTR), and concentration-dependent low-field mobility (CONMOB) models are enabled. Regarding the TSC, a lumped resistance was specified for the tunnel junction such that the current flows through the cell without considerable limitation (Elbar et al. 2015).

Firstly, our simulation was calibrated against an experimental BaSi2/Si heterojunction cell with a structure of indium-doped tin oxide (ITO)/p-BaSi2/n-Si/Aluminum (Al) metal back contact (Yachi et al. 2017). Figure 1a, b depict the device design and energy band diagram of the BaSi2/Si heterojunction cell. The essential parameters of cell layers derived from previously published studies are summarized in Table 1 (Deng et al. 2018, 2020; Yachi et al. 2017). The work function of the ITO (transparent conducting front contact) is taken as 4.4 eV while that of the back Al metal is set to 4.28 eV. The absorption coefficients of the cell layers are derived from practical data (Toh et al. 2011; Vismara et al. 2016; Green 2022). After applying the listed parameters, the illuminated current density–voltage (J–V) curves for both experimental and simulated cells are presented in Fig. 1c. The simulated cell achieves the following PV parameters: Jsc = 35.52 mA/cm2, Voc = 0.462 V, FF = 59.40%, and η = 9.74%. These parameters are quite close to the reported parameters, as shown in the inset of Fig. 1c. As a result, an acceptable match between both cells is observed, implying that the simulation model employed in Silvaco Atlas software is validated.

Fig. 1
figure 1

a structure indicating the design parameters and b Energy band diagram after contact at the dark condition of the p-BaSi2/n-Si heterojunction solar cell. c Comparison of the simulated J–V characteristics with the measured data (Yachi et al. 2017) under AM 1.5 illumination condition

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Table 1 Basic parameters of the BaSi2/Si heterojunction cell layers (Deng et al. 2018, 2020; Yachi et al. 2017)
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2.2 Structures of two sub-cells

A homojunction solar cell with the n+-p-p+ architecture is introduced based on the practical structure of a BaSi2 cell (Kodama et al. 2018). Figure 2 depicts the device design and energy band diagrams of the two sub-cells, containing an n+pp+-BaSi2 bottom cell and an n+pp+-Ba(CxSi1−x)2 top cell. To create a homojunction solar cell, heavily doped n+-and p+-layers were positioned on opposite sides of a lightly doped p-absorber layer to extract photoexcited electron–hole pairs. The thickness of these layers was maintained at 20 nm, which was found experimentally to be the minimum thickness required to cover the entire surface adequately (Yachi et al. 2017). Table 2 summarizes the primary required parameters of the sub-cells layers used to design the tandem solar cell (Tianguo 2019; Yachi et al. 2017; Deng et al. 2020). To model the optical properties, the refractive index n' and extinction coefficient κ as functions of a wavelength of barium di-silicide were extracted from experimental data (Toh et al. 2011; Vismara et al. 2016; Tianguo 2019), where κ was calculated from the relationship α = 4πκ/λ.

Fig. 2
figure 2

The main configuration of a Ba(CxSi1−x)2 top cell, and b BaSi2 bottom cell, indicating the design parameters. The energy band diagram at the short-circuit condition of c Ba(CxSi1−x)2 top cell, and d BaSi2 bottom cell, where Fn and Fp represent the electron and hole quasi-Fermi levels, respectively

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Table 2 Primary parameters of the sub-cells’ layers used for the design of Ba(CxSi1−x)2/BaSi2 tandem solar cell (Tianguo 2019; Yachi et al. 2017; Deng et al. 2020)
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2.3 Proposed tandem cell

In this subsection, the suggested structure of the tandem cell design (Ba(CxSi1−x)2/BaSi2) is displayed in Fig. 3. In this tandem cell, the two cells are coupled through a tunnel junction that performs as a recombination film (Mailoa et al. 2015). The practical design of the tunnel junction is a critical task in order to maintain proper functionality leading to the enhancement of the tandem performance (Shen et al. 2018). The junction has to be transparent, and it should not absorb any part of the input radiation spectrum reaching the bottom cell. Such requirements have been met by fulfilling two design conditions: (1) the tunnel junction has to be heavily doped, and (2) both sides of the junction should be very thin in the nanometer range (Wang et al. 2012). Furthermore, the tandem cell current is controlled by the smaller current transporting through either the top or the bottom sub-cell. Once stacking the two sub-cells, the current matching condition should be maintained to minimize the current loss.

Fig. 3
figure 3

Schematic representation of a Ba(CxSi1−x)2/BaSi2 tandem cell comprising of an n+pp+-Ba(CxSi1−x)2 top sub-cell and an n+pp+-BaSi2 bottom sub-cell (ETL and HTL represent electron transport and hole transport layer, respectively)

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3 Results and discussion

First, we investigate the effect of the top cell bandgap on the tandem performance to design the tandem device for possible maximum efficiency. Then, the tandem performance of the cell is optimized through various steps including the effect of doping and the thickness of the absorber layers, changing the absorber defect density of both cells, and the work function of both contacts to obtain the maximum available η.

3.1 Tandem cell with different bandgap top cell

In this subsection, we investigate the effect of the top cell bandgap on the tandem performance where the compound Ba(CxSi1−x)2 as a top absorber is adopted. The variation of the top cell absorber bandgap depends on the composition (x) (Imai and Watanabe 2010). Here, the Ba(CxSi1−x)2top cell bandgap is varied from 1.6 to 1.9 eV, which corresponds to x from 0.73 to 0.80. So, the Ba(CxSi1−x)2/BaSi2 tandem cells are simulated using Atlas device simulator under 1-sun of AM1.5G spectrum to evaluate its performance parameters. The absorber thickness of the top cell was set to 500 nm, while the bottom BaSi2 was fixed at 1 μm. Figure 4a, b depict a comparison between the illuminated J–V and EQE curves concerning the Ba(CxSi1−x)2/BaSi2 tandem cells. The corresponding performance metrics are listed in Table 3. The results show that the best performance is obtained with a Ba(C0.78Si0.22)2 cell with a 1.8 eV bandgap. These complementary bandgap values agree with those reported in Eperon et al. (2017) to achieve the maximum theoretical η. In the coming simulations, this value will be used for further optimization of the TSC under investigation.

Fig. 4
figure 4

a Illuminated J–V, and b EQE curves of a Ba(CxSi1−x)2/BaSi2 tandem cell

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Table 3 A comparison between the performance metrics for Ba(CxSi1−x)2/BaSi2 tandem cells
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3.2 Tandem cell optimization

This subsection presents the following optimization steps for a 2T monolithic Ba(CxSi1−x)2/BaSi2 TSC. First, the impact of the absorber layers doping on the tandem performance is studied. Then, we investigate the influence of top and bottom absorber thicknesses on TSC working metrics. Furthermore, we inspect the current matching point to get the maximum available η. The effect of top and bottom absorber defect densities on TSC working metrics is then investigated. Finally, we study the impact of front and rear contact work functions on the TSC performance.

3.2.1 Doping of absorber layers

The absorber layer doping is crucial in regulating bulk recombination. Its role in controlling the fill factor is also significant as increasing the doping level results in decreasing the series resistance which, in turn, increases the fill factor. However, increasing the doping level beyond a certain value may trigger some other undesirable effects like band gap narrowing and mobility degradation that causes an overall decline in performance. Herein, a parametric analysis study is done by changing the absorber layers doping from 1 × 1015 to 1 × 1019 cm−3 to investigate its impact on the tandem performance. Figure 5a, b demonstrate that the η attains its maximum value when the doping density is 1 × 1017 cm−3 for both cells. To inspect the influence of changing the absorber layers doping on the tandem performance, the electric field under light illumination before and after optimization is depicted in Fig. 5c, d. The electric field in a depletion region is responsible for separating photogenerated carriers and, in addition, depends on the carrier’s doping on both junction sides. As shown in Fig. 5c, d, increasing absorber doping increases the depletion region width and thus increases its electric field for both sub-cells and, consequently, improves their conversion efficiency.

Fig. 5
figure 5

Impact of the doping density of the absorber on η of a Ba(C0.78Si0.22)2 top cell, and b BaSi2 bottom cell. The electric field before and after optimization of c Ba(C0.78Si0.22)2 top cell, and d BaSi2 bottom cell for the short-circuit condition under illumination

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3.2.2 The thickness of the absorber layers

Figure 6 represents a contour graph of TSC efficiency dependence on the thickness of both the top and bottom absorber layers. To investigate device performance, the thickness of the top absorber was increased from 0.5 up to 1.6 μm and that of the bottom absorber was increased from 1 to 4 μm while the other parameters remained constant. As obvious from Fig. 6, there is an inconsiderable effect on η as the thickness of the rear absorber grows from 2 to 4 μm and that of the front absorber raises from 0.8 to 1.6 μm. As the bottom absorber thickness falls below 0.8 μm, the efficiency gradually decreases from 32.4 to 30.4%. Furthermore, there is no change in η as the bottom absorber thickness changes from 2 to 4 μm, while the thickness of the bottom absorber layer remains constant below 0.8 μm. The best performance can be determined when the thicknesses of the top and bottom cells are chosen to be 1.1 and 3 μm, respectively.

Fig. 6
figure 6

Contour graph of TSC efficiency dependency on the thickness of the top and bottom absorber layers

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3.2.3 Current matching point

In this subsection, the absorber thickness (d) of the top cell has been altered from 0.85 to 1.25 μm, while the bottom BaSi2 was fixed at 3 μm. Figure 7a shows the Jsc of the top and bottom sub-cells versus the thickness of the Ba(C0.78Si0.22)2 absorber layer in the top cell, in which significant dependency on d is shown. As d becomes thicker, the Jsc of the top cell rises, and the Jsc of the bottom cell, in turn, reduces. The reason is that the thicker the top cell is, the more photons are absorbed implying lesser transferred light to the bottom cell. A current matching point occurs at Jsc = 16.47 mA/cm2 and is satisfied at d = 1.05 μm for the absorbing film of the top cell. As displayed in Fig. 7b, η of the tandem cell reaches a maximum value at the current matching point. In this condition, the performance of the Ba(C0.78Si0.22)2/BaSi2 tandem cell has been simulated. The J–V characteristics of tandem, top, and bottom cells are drawn in Fig. 7c. The maximum value of Jsc of tandem, top, and bottom cells is equal to 16.47 mA/cm2 with Voc = 2.24 V and η = 32.83% for the tandem cell. The value of Voc = 2.24 V is equal to the sum of those of the top cell (1.37 V) and bottom cell (0.87 V), indicating the efficient operation of the recombination junction. Moreover, the EQE of both cells at the current matching condition is demonstrated in Fig. 7d. The EQE of the bottom BaSi2 sub-cell surpasses 95% at a wavelength around 800 nm.

Fig. 7
figure 7

Variation of cell parameters versus d of top cell absorber layer from 0.85 to 1.25 μm a Jsc of top and bottom sub-cells and b η of the bottom, top and tandem cells. Device characteristics at the current matching point c J–V of the bottom, top, and tandem cells and d EQE spectra of bottom and top cells

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Figure 8a illustrates the structure of the Ba(C0.78Si0.22)2/BaSi2 tandem cell produced by the Atlas simulator at the current matching point. As the bottom BaSi2 cell is exceedingly thicker than the top sub-cell, the extended view of the red dashed area in Fig. 8a is displayed in Fig. 8b.

Fig. 8
figure 8

a TSC architecture created by Atlas. b Expanded picture of the red dashed area in a. c Electrostatic potential distribution and d photogeneration rate (G) inside the tandem cell with a magnified view

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Figure 8c depicts the electrostatic potential established through the TSC. The built-in potential of the tunnel junction indicates a greater potential in this specific region. Meanwhile, the electrostatic potential decreases as one proceeds in two directions away from the tunnel junction. Additionally, Fig. 8d displays the photogeneration rate inside the different layers. The graph reveals that the top Ba(C0.78Si0.22)2 cell has a greater photogeneration rate than the bottom BaSi2 cell. Photogenerated carriers recombine in the tunnel junction area. A low photogeneration rate has been observed as a result of this recombination.

3.2.4 The defect density of the absorber layers

Figure 9 represents contour graphs of TSC performance metrics dependence on the defect concentration of both the top and bottom absorber layers. The defect density ranges from 1012 to 1018 cm−3 while preserving the other parameters unchanged to explore the device performance. As clear from Fig. 9a, the Jsc significantly decreases as the defect density of the top and bottom absorber layers increases. For example, Jsc drops from 16 to 12 mA/cm2 as the defect density of the bottom absorber rises from 1015 to 1018 cm−3, and that of the top layer grows from 1016 to 1018 cm−3. Besides, there is no change in Jsc as the defect density of the top absorber layer is below 1016 cm−3 or that of the bottom absorber layer is below 1015 cm−3.

Fig. 9
figure 9

Contour graphs of TSC performance figure of merit dependency on the defect density of top and bottom absorber layers

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It has been reported in the literature that Voc is extremely affected by the defect concentration of the absorber layer (Soucase et al. 2016). In this context, it can be shown in Fig. 9b that Voc decreases from 2.10 to 1.55 V when the defect density of the absorber layers raises from 1015 to 1018 cm−3. For absorber layers defect density lower than 1015 cm−3, there is no appreciable effect on Voc. Figure 9c shows that the FF gradually declines from 86 to 73% when the defect density of the absorber layers raises from 1015 to 1018 cm−3. Again, for small values of defect density of absorber layers (below 1015 cm−3), there is no significant effect on FF. Theoretically, FF is solely a function of Voc, however, FF is not only related to Voc but in addition, it is strongly correlated to the recombination processes inside the depletion region.

Finally, as clarified in Fig. 9d, we noticed an expected changing profile in η due to its dependency on Jsc, Voc, and FF. There is an insignificant effect on η as the defect density of the absorber layers is below 1015 cm−3. η sharply decreases from 30 to 13% as the defect density of the absorber layers grows from 1015 to 1018 cm−3.

3.2.5 Work functions of the front and rear contacts

Work functions of the front and rear contacts are crucial factors that affect the device’s performance as they modulate the metal/semiconductor contact barrier height and therefore the transportation of charge carriers (Anwar et al. 2017). The front Schottky barrier at the metal/n+-Ba(C0.78Si0.22)2 interface is computed using the equation (Liao et al. 2020):

$$\phi_{{{\text{b}},{\text{front}}}} = \varphi_{{{\text{m}},{\text{front}}}} - \chi$$
(1)

where \(\varphi_{{{\text{m}},{\text{ front}}}}\) is the front contact work function, and \(\chi\) is the electronic affinity of Ba(C0.78Si0.22)2. while the rear Schottky barrier at the metal/p+-BaSi2 interface is calculated using the equation (Liao et al. 2020):

$$\phi_{{{\text{b}},{\text{rear}}}} = \chi + E_{g} - \varphi_{{{\text{m}},{\text{rear}}}}$$
(2)

where \(\chi\) and \(E_{g}\) are the electronic affinity and the bandgap energy of BaSi2, respectively, and \(\varphi_{{{\text{m}},{\text{ rear}}}}\) is the rear contact work function. The front contact work function is changed from 3 to 4.8 eV, while that of the rear contact materials is summarized in Table 4.

Table 4 Inorganic rear contact materials showing their work function values and indicating the corresponding references from the literature (Anwar et al. 2017; Sawicka-Chudy et al. 2019; Salem et al. 2022; Derry et al. 2015; Singh et al. 2019)
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Figure 10a, b illustrate the energy band diagrams for various work function values of front and rear contacts at the short circuit and dark conditions. As apparent in Fig. 10a, increasing \(\varphi_{{{\text{m}},{\text{front}}}}\) results in increasing the front barrier height \(\phi_{{{\text{b}},{\text{front}}}}\), which leads to sufficient bending up of Ec close to metal/n+-Ba(C0.78Si0.22)2 interface irrespective of n+-Ba(C0.78Si0.22)2 thickness. This bending impedes the transfer of photo-excited electrons to the front electrode. On the other hand, decreasing \(\varphi_{{{\text{m}},{\text{front}}}}\) leads to a decrease of \(\phi_{{{\text{b}},{\text{front}}}}\), and thus, improves the device performance. Similarly, in Fig. 10b, the rear barrier height \(\phi_{{{\text{b}},{\text{ rear}}}}\) produces sufficient bending down of Ev adjoining to the p+-BaSi2/metal interface and so the series resistance increases at the contact interface which results in impeding the transport of photoexcited holes to the back electrode. In addition, \(\phi_{{{\text{b}},{\text{ rear}}}}\) decreases with increasing \(\varphi_{{{\text{m}},{\text{ rear}}}}\) and improves the device performance.

Fig. 10
figure 10

Energy band diagram at short-circuit and dark conditions of a the top 10 nm for different values of front contact work function, and b the bottom 10 nm for distinct values of rear contact work function

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Figure 11 exhibits contour graphs of the photovoltaic parameters of Ba(C0.78Si0.22)2/BaSi2 tandem solar cells under different values of the work function for both contacts. As apparent in Fig. 11, there is no effect on the performance parameters as \(\varphi_{{{\text{m}},{\text{ rear}}}}\) is larger than 4.5 eV while \(\varphi_{{{\text{m}},{\text{front}}}}\) remains constant. On the other hand, with increasing \(\varphi_{{{\text{m}},{\text{front}}}}\) from 3.8 to 4.8 eV, Jsc and FF values gradually decrease while Voc drastically drops from 2.18 to 1.18 V. Finally, as clarified in Fig. 11d, an expected trend in η is observed due to its dependency on Jsc, Voc, and FF. There is an insignificant effect on η when \(\varphi_{{{\text{m}},{\text{front}}}}\) is below 3.8 eV, and \(\varphi_{{{\text{m}},{\text{ rear}}}}\) remains constant, while it sharply decreases from 31 to 15% as \(\varphi_{{{\text{m}},{\text{front}}}}\) is increased above 3.8 eV. The optimum efficiency has been attained with a terbium front contact (\(\varphi_{{{\text{terbium}}}}\) = 3 eV) and a silver rear contact, resulting in the following PV parameters: Jsc = 16.48 mA/cm2, Voc = 2.27 V, FF = 91.45%, and η = 34.25%.

Fig. 11
figure 11

Contour graphs of tandem cell performance parameters dependence on front and rear contact work function

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Finally, we provide a comparison between our optimized TSC and other tandem candidates, as shown in Table 5. Some of the reported tandem cells are based on experimental investigations, while others are computed numerically, as illustrated in the table. Most tandem cells with lead-based perovskite top cells give higher efficiencies; however, the toxicity of such tandems is a serious issue that limits their use and commercialization. Moreover, our simulated tandem solar cell shows promising properties with a high η and Voc.

Table 5 A state-of-art comparison between performance parameters of Ba(C0.78Si0.22)2/BaSi2 tandem cell and some multi-junction solar cells stated in the literature
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4 Conclusion

In this work, all-BaSi2 2T monolithic tandem solar cell is presented. In the proposed design, the bottom cell comprises BaSi2 material with a 1.3 eV bandgap, while for the top cell, Ba(C0.78Si0.22)2 with a 1.8 eV bandgap is employed. After optimizing the doping and the thickness of both absorber layers, and at the designed matching point, the current density and η of the TSC are enhanced up to 16.47 mA/cm2 and 32.83%, respectively. Additionally, we have inspected the consequence of the absorbers defect density in the range (1012–1018 cm−3) and the impact of the work functions of the contacts on the performance parameters of the TSC. It was found that contacts having a work function ≥ 4.5 eV for rear contact and ≤ 3.8 eV for front contact were adequate for better performance. These results can offer possible routes leading in the direction of the advancement of low-cost, environmentally friendly, and efficient TSCs.