Design and analysis of a SnS₂/WS₂/V₂O₅ double-heterojunction toward high-performance photovoltaics

Abstract

Tungsten disulfide (WS₂) transition metal dichalcogenide (TMDC) absorber-based solar cells comprising tin disulfide (SnS₂) buffer and vanadium (V) oxide V₂O₅ back surface field (BSF) layers have been designed and analyzed using a SCAPS-1D simulator in this study. The initial experimentation on back metal contact (BMC) and front metal contact (FMC) optimization involved the use of different materials to obtain the least resistive junction at the semiconductor–metal (M–S) interface, where the best potential was found. Following an extensive investigation nickel (Ni) and aluminum (Al) is determined to be the optimal material for the back and front contact, respectively. Subsequently, the impact of major parameters which affecting the photovoltaic (PV) performance, such as absorber layer thickness, doping concentration, bulk defect density, interface defect density, operating temperature, and surface recombination velocity, were studied systematically. An improved photoconversion efficiency (PCE) of over 32% (around 9% higher) was obtained with the open-circuit voltage (V_OC) of 1.1 V, short-circuit current (J_SC) of 37.2 mA cm⁻², and fill factor (FF) of 84% with the Al/FTO/SnS₂/WS₂/V₂O₅/Ni heterostructure, compared to 23.4%, 0.89 V, 31.2 mA cm⁻² and 81% for the pristine cell (without V₂O₅ BSF). These outcomes obtained from comprehensive studies reveal the huge potential of the SnS₂/WS₂/V₂O₅ double-heterostructure to be applied as a PV cell and pave a resourceful pathway for the experimental fabrication of WS₂-TMDC absorber-based high-performance photonic devices.

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1. Introduction

Energy demand is increasing quickly with the increase in population and rapid development of technology around the world; thereby, the predicted energy consumption is estimated to be 30 terawatts in 2050.^1–3 Scientists and researchers are highly concerned with the reduction of existing fossil fuel resources, their annual rate of price increment, and their dangerous impact on global warming, leading to the depletion of the ozone layer and air pollution.^4–8 Renewable energy derived from natural sources is abundant and sustainable, with the least impact on the environment. Solar energy, especially using photovoltaic technology, has the greatest potential, with the direct conversion of solar energy into electricity with a clean, economical, and sustainable pathway playing a key role in mitigating the global energy demand.^9–12 Transition-metal dichalcogenides (TMDCs) used in thin-film solar cells are one of the most attractive absorber materials for use in photovoltaic devices.^13–18 To date, there are several absorber semiconductors, including perovskites such as FeSi₂, Sb₂S₃, Sb₂Se₃, MoS₂, MoSe₂, WS₂, SnS_x, Cu₂MnSnS₄, CsSnCl₃, Cs₂BiAgI₆, CsPbBr₃, Sr₃NCl₃, and Sr₃AsI₃, that show considerable potential to be used for fabricating high-performance solar cells.^4–8 The choice of a favorable semiconductor as solar absorber material is a crucial issue to achieve the highest benefits, including inexpensive active materials, vacuum-free deposition, suitability for mass production, and markedly efficient power conversion.^13,19–21 However, the challenges in fabricating highly efficient solar cells based on these materials include the use of rare-earth materials, scarcity, and material toxicity.^{19,20,22–24} Additionally, the development of heterostructured devices with band alignment fulfilling the criteria of an ideal required band structure to absorb the major portion of incident visible photons and thereby generation of electron–hole pairs, their dissociation, and collection efficiency at the external circuit, is still a challenge for high-performance photovoltaics beyond the Shockley–Queisser (SQ) limit.^13,19–21

The WS₂ layered material has emerged as a promising one to be used as an absorber material, with an optical absorption coefficient exceeding 10⁵ cm⁻¹, a tunable gap of 1.3–2.1 eV for bulk and monolayer film, and the high carrier mobility of 486 cm² V⁻¹ s⁻¹ (at 5 K), while being much less hazardous and relatively abundant.²⁵ Therefore, the choice of WS₂ as a solar cell absorber may benefit the manufacture of inexpensive thin-film solar cells.^26–31 The n-type electron transport layer (ETL), i.e., the buffer layer, has a vital impact on the cell's open-circuit voltage and short-circuit current density. The band alignment at the absorber/buffer interface i.e., band offset which is specially (CBO) determines the band bending at the interface is a spike that inhibits recombination. SnS₂ is known as a low-cost, n-type layered semiconductor with nontoxic nature, having the tunable bandgap of 1.82–2.88 eV and electron mobility of 50 cm² V⁻¹ s⁻¹.²¹ Several numerical and experimental studies report the use of WS₂ as a solar cell photoactive material, with multiple heterostructures demonstrated.^29–36 The PCE of 5.0% with 625 nm exposure and a power density of 2.5 mW cm⁻² for the Au(10 nm)/WS₂(20 nm)/Ag(200 nm) structure,³⁷ and 0.31% with V_OC of 0.551 V, J_SC of 1.01 mA cm⁻², and FF of 47.6% for the ITO/WS₂ structure, have been obtained.^32,33 However, the improved PCE of 17.73% for Al:ZnO/WSSe/WS₂/TCO,³⁰ 25.71% in AZO/ZnO/WS₂/Mo,³⁵ 28.86% for n-TiO₂/p-WS₂/p-Cu₂O,³⁸ and 29.74% for ZnO/CdS/WS₂/Au³⁴ heterostructures are predicted by developing double heterojunction structures. These theoretical outcomes reveal that the limited V_OC obtained in WS₂-based photovoltaics are observed mostly owing to the Fermi level pinning, which could be solved by adding an interlayer (back surface field [BSF] layer) between WS₂ and the back metal contact, establishing a suitable band alignment with the least carrier recombination.^31,39–41 Also, the smooth transfer of holes from the absorber to the back metallic contact is facilitated by the low valence band offset with a highly doped p-type BSF.⁴² Thus, the PCE could be enhanced effectively by reducing the valence band offset at the BSF/absorber interface, consequently developing a high activation energy that is almost equal to the absorber's band gap.^43,44 Moreover, some inorganic metal oxides (MOs) such as vanadium pentoxide (V₂O₅), nickel oxide (NiO), and tungsten trioxide (WO₃) have been incorporated as a BSF layer in numerous structures to enhance both cell performance and stability.^45,46 Among them, p-type V₂O₅ (2.2 eV) showed promising features as BSF material, with moderate band gap, strong absorption coefficient, significant conductivity, and huge versatility, with abundance on Earth.¹¹ With this perspective, the double junction heterostructure of SnS₂/WS₂/V₂O₅ with WS₂ absorber, SnS₂ buffer and V₂O₅ BSF layer shows a favorable band alignment required for spontaneous carrier transportation with insignificant offsets (CBO at the SnS₂/WS₂ interface of ∼0.19 eV and VBO at WS₂/V₂O₅ of ∼0.26 eV), revealing its notable potential with nontoxic, low-cost and green characteristics for developing high-performance photovoltaics.

In this work, transition-metal dichalcogenide WS₂-absorber-based high-performance thin film solar cells (TFSCs) with SnS₂ and V₂O₅ as buffer and back surface field (BSF) layers, respectively, have been designed and analyzed using the SCAPS-1D simulator. Firstly, the influence of various possible metals as back contact was investigated to obtain the least resistive junction at the semiconductor–metal (M–S) interface. After that, the impacts of major affecting parameters such as absorber layer thickness, doping density, defect density, interface defect density, operating temperature, and its back surface recombination velocity with the buffer and BSF layers were investigated. A comprehensive simulation study reveals the huge potential of WS₂, SnS₂, and V₂O₅ as an absorber, buffer, and back surface field (BSF) layer, respectively, for the fabrication of high-performance economical and green thin film solar cells (TFSCs).

2. Device modeling and simulation parameters

The realization of high-efficiency solar cells can be obtained by numerical modeling and a systematic investigation of the role of each influential parameter of the designed cells. The one-dimensional electrical solar cell simulation software SCAPS-1D developed at the Department of Electronic and Information Systems at the University of Ghent in Belgium has shown potential to design and analyze TFSCs. Optoelectrical simulations of PV device structures consisting of up to seven semiconductor layers can be carried out using the SCAPS-1D software. The optoelectronic properties of PV cell structures can be predicted and analyzed by applying fundamental equations, as well as electrostatic potential and continuity equations, under steady-state conditions.⁴⁷

Fig. 1(a)–(d) shows the schematic diagram and corresponding energy band alignment with and without V₂O₅ BSF of the proposed thin-film solar architectures. In the adjusted heterostructure with V₂O₅ BSF of Al/FTO/SnS₂/WS₂/V₂O₅/Ni, p⁺–p–n–n⁺ junctions are formed with a p-type 1.0 μm WS₂ absorber layer sandwiched between the highly doped p⁺-type 0.1 μm V₂O₅ BSF and n-type 0.05 μm SnS₂ buffer layer. The FTO (0.05 μm) layer is used as a transparent conductive oxide (TCO) layer along with the back metal contact. In the energy band diagram simulation for the architecture of Al/FTO/SnS₂/WS₂/V₂O₅/Ni, the energy levels (both VB and CB) of the p-type V₂O₅ BSF layer and the n-type SnS₂ buffer layer are notably higher than those of the WS₂ absorber layer. The difference in the energy levels of the conduction band (CBO) between the BSF and the absorber is determined to be 0.91 eV, which facilitates the easiest path for transporting the photogenerated holes (PGHs), and conversely, it blocks photogenerated electrons (PGEs) passing into the back contact Ni (5.01 eV). The difference in the energy levels of the conduction band (CBO) between the buffer and the absorber is determined to be 0.95 eV. This energy difference accelerates the transport of photogenerated electrons (PGEs) and inversely repels the way of photogenerated holes (PGHs) into the front contact.

Fig. 1 (a) and (b) Schematic diagram and (c) and (d) corresponding energy band alignment with and without V₂O₅ BSF of the proposed heterostructures.

In the SCAPS-1D simulator, the fundamental equations of one-dimensional semiconductors, Poisson's equation, hole and electron continuity equations, as well as the drift and diffusion drift eqn (1)–(5)^48–51 have been calculated to determine the PV parameters. The Poisson and continuity equations provide a set of coupled differential equations (Ψ, n, p) or (Ψ, E_Fn, E_Fp) with correct boundary conditions at interfaces and contacts. In this simulator, the intrinsic properties of each layer, such as layer thickness (nm); band gap E_g (eV); electron affinity χ (eV); dielectric permittivity ε_r; states density of the conduction band N_C (cm⁻³); density of the valence band N_V (cm⁻³); electron mobility μ_n (cm² V⁻¹ s⁻¹); hole mobility μ_p (cm² V⁻¹ s⁻¹); donor density N_D (cm⁻³); acceptor density N_A (cm⁻³); recombination coefficients (cm³ s⁻¹) of radiative recombination, Shockley–Read–Hall recombination (SRH) and Augur recombination; and the absorption coefficient, are required to be provided from experimental outcomes as well as reported literature. The simulation was performed under AM 1.5G standard spectrum.

In this simulator, the absorption coefficient of each photoactive layer/material is required to be provided separately to simulate any homo/heterostructures from experimental results/literature, as summarized in Tables 1 and 2. The absorption coefficients of SnS₂, WS₂, and V₂O₅ were taken from reported literature.^{11,13,51–54} So, this modeling and the performed simulation is reliable. However, a noticeable discrepancy between simulation and experiment may be observed when the suggested device is fabricated experimentally owing to fabrication limitations, environmental impacts and measurement errors.

where ψ is the electrostatic potential; ε⁰ε_r is the vacuum and semiconductor permittivity; n and p are free carrier concentrations; and N⁺_d and N_A are ionized donor and acceptor densities. The defect charge density is denoted by ρ_def; G is the generation rate, and j_n and j_p are the electron–hole current densities; q is the elementary charge; U_n and U_p are respectively electron and hole recombination rate; μ_n and μ_p are electron and hole mobility, respectively.

Table 1 Simulation parameters of the proposed (Al/FTO/SnS₂/WS₂/V₂O₅/Ni) solar cells

Parameter (unit)	n^+-Type TCO (FTO)^57	n-Type buffer (SnS2)^52,53	p-Type absorber (WS2)^13,54	p^+-Type BSF (V2O5)^11,51
Thickness (μm)	0.05	0.05	1.00	0.10
Bandgap (eV)	3.6	2.24	1.29	2.2
Electron affinity (eV)	4.50	4.24	4.05	3.40
Dielectric permittivity (εr)	10.0	10.0	13.6	8.00
CB effective density of states (cm^−3)	2.0 × 10^18	2.0 × 10^18	2.2 × 10^18	9.2 × 10^19
VB effective density of states (cm^−3)	1.8 × 10^19	1.8 × 10^19	1.8 × 10^19	5 × 10^20
Electron thermal velocity (cm s^−1)	2 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7
Hole thermal velocity (cm s^−1)	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7
Electron mobility (cm^2 V^−1 s^−1)	100	50	200	150
Hole mobility (cm^2 V^−1 s^−1)	20	25	50	100
Shallow uniform donor density ND (cm^−3)	1 × 10^18	1 × 10^17	—	—
Shallow uniform acceptor density NA (cm^−3)	—	—	1 × 10^18	1 × 10^19
Bulk defect density (cm^−3)	—	1 × 10^15	1 × 10^14	1 × 10^15

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Table 2 Interface defect parameters of the proposed (Al/FTO/SnS₂/WS₂/V₂O₅/Ni) solar cells

Parameters (unit)	V2O5/WS2	WS2/SnS2
Defect type	Neutral	Neutral
Capture cross-section of electrons (cm^2)	1 × 10^−19	1 × 10^−19
Capture cross-section of holes (cm^2)	1 × 10^−19	1 × 10^−19
Reference for defect energy level Et	Above the highest Ev	Above the highest Ev
Energy with respect to a reference (eV)	0.6	0.6
Total density (cm^−2)	1 × 10^11	1 × 10^10

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The radiative recombination, Shockley–Read–Hall recombination (SRH) and Augur recombination were considered with recombination coefficients of 2.0 × 10⁻⁹ (cm³ s⁻¹), 1.0 × 10¹⁴ (cm³ s⁻¹) and 1.0 × 10⁻²⁹ (cm³ s⁻¹), respectively, in this study.

Recombination used for introducing the radiative and Auger (band-to-band) recombination in SCAPS-1D can be expressed as shown in eqn (6) and (7).⁵⁵

Here, c^A_n and c^A_p can be set from literature; i.e., for Si, K_Si = 1.8 × 10⁻¹⁵ cm⁻³ s⁻¹; GaAs, K_GaAs = 7.2 × 10⁻¹⁰ cm⁻³ s⁻¹; CIGS K = 0; and the Auger constants are: c^A_n ≈ c^A_p ≈ 3.0 × 10⁻³¹ cm⁻⁶ s⁻¹.⁵⁶

Table 1 lists the simulation parameters for the active layers used for modeling and designing the proposed devices. Herein, the thermal velocity of electrons and holes in each semiconductor at room temperature is fixed at approximately 10⁷ cm s⁻¹ throughout the study. The chosen value for surface recombination velocity of electrons and holes at the metallic contacts located at the front and back of the structure is 10⁷ cm s⁻¹. The chosen value for electron and hole capture cross-sections is 10⁻¹⁵ cm². The work function referring to aluminum (Al) and nickel (Ni) was used as an optimized front and back metallic contact.¹³

3. Results and discussion

3.1 Influence of thickness and carrier concentration of WS₂ absorber layer on PV performance

Fig. 2(a) displays how the performance parameters of the solar cell are influenced by changes in thickness within the range of 0.25–3.0 μm, keeping unchanged the rest of the parameters as shown in Tables 1 and 2, under AM 1.5G illumination at a constant working temperature of 300 K. The V_OC is significantly changed at a thin absorber thickness of ≤1.0 μm. A markedly improved V_OC (from ∼0.84 to ∼1.0 V) was seen in the solar cell structure with V₂O₅ BSF at the absorber thickness of 0.08–1.0 μm, which is consistent with previous reports.^40,58 The efficiency of the cell increased from 16.03% to 27.06% and 25.88% to 32.09% for the heterostructure without and with BSF layer, respectively, with an increase in absorber thickness from 0.25–3.0 μm. The J_SC increased almost linearly when absorber thickness was varied from 0.25–0.75 μm, and thereafter, it reached a saturated value of 35.6 (with BSF) and 33.7 mA cm⁻² (without BSF) with further increase in absorber thickness beyond 0.75 μm. The enhancement in J_SC values is a consequence of the increased absorption of incident photons at higher wavelengths in the WS₂ absorber with the V₂O₅ BSF layer. Logically, an absorber with a higher thickness having BSF absorbs more photons, resulting in a greater number of electron–hole pairs (EHPs) generated.^59,60 A smaller increase in FF was found at a lower thickness of <0.75 μm, which is almost constant over the absorber thickness of up to 3.0 μm. The tendency of reduction in V_OC refers to the gradual increase of recombination of photogenerated carriers. This process may come to an end when the absorber thickness reaches >3.0 μm. The PCE gradually reduces (from 26 to 24%) at higher acceptor concentrations, above 10¹⁶ cm⁻³, when no BSF layer is added, while the PCE increases sublinearly after insertion of the V₂O₅ BSF between the WS₂ absorber and Ni back contact.

Fig. 2 Effect of absorber layer parameters: (a) thickness and (b) acceptor density with and without V₂O₅ BSF layer of the proposed heterostructure (Al/FTO/SnS₂/WS₂/Ni).

Though the performance parameters are insignificantly changed with an increase in acceptor concentration from 10¹⁰ to 10¹⁶ cm⁻³, surprisingly, a marked increase in PCE from 26.3% to 36.5% with FF from 77% to 81% and V_OC from 0.77 V to 0.81 V was observed with increasing acceptor concentration from 10¹⁶ to 10²⁰ cm⁻³ (Fig. 2(b)). This significant improvement was obtained due to the enhancement of carrier (hole) density, resulting in the formation of a strong built-in field, which causes the improved V_OC as well as FF.

Fig. 3 shows the J–V characteristics and corresponding quantum efficiency (QE) response to varying absorber thicknesses from 0.25 to 3.0 μm for structures with and without the BSF layer, respectively. In Fig. 3(a), the photocurrent J_SC increases exponentially from 27 to 36 mA cm⁻² with an increase of absorber thickness in the entire region of 0.25 to 0.9 μm. The corresponding QE response demonstrates the improvement of photoabsorption by expanding the area coverage (indicated by arrow) with increasing absorber thickness, which causes the resulting enhancement in J_SC (Fig. 3(b)). A similar characteristic was also observed in the structure without the BSF layer, having a current of 33 mA cm⁻² and relatively smaller area of coverage in the QE response, as shown in Fig. 3(c) and (d). This is because a thicker absorber layer leads to an increase in cell resistance and diffusion length, which causes severe unwelcomed recombination of photogenerated carriers.^59,61 Thereby, the optimal absorber thickness of 1.0 μm was used for further investigation based on the adjusted photovoltaic parameter values for both configurations, resulting in the highest η of 32.02% and 23.39% achieved for the structure with and without the BSF layer, respectively.

Fig. 3 Effects of absorber layer thickness on the J–V and Q–E of solar cells constructed (a) and (b) with BSF V₂O₅ and (c) and (d) without BSF V₂O₅, respectively.

There is a significant improvement in V_OC with the increase in WS₂ acceptor density from 10¹⁰ to 10²⁰ cm⁻³. In the case with BSF, there is a shift in the V_OC value from 0.918 to 1.152 V, whereas without BSF, the V_OC value changes from 0.84 to 1.01 V, and the conversion efficiency increases from 25.82 to 36.07% due to the variation in acceptor density from 10¹⁰ to 10²⁰ cm⁻³. Without the BSF layer, the J_SC changes from 35.01 to 29.37 V, the FF from 84.73 to 88.22%, and consequently, the conversion efficiency changes from 24.83 to 26.08% with the acceptor density variation in the range of 10¹⁰–10²⁰ cm⁻³. In both cases, FF and efficiency did not show appreciable changes with acceptor density up to 10¹⁵ at the WS₂ absorber layer. To attain the best possible photovoltaic performance, this study has chosen the layer thickness of 1.0 μm and acceptor density of 10¹⁸ cm⁻³ in the WS₂ absorber layer for further investigations.

3.2 Influence of thickness, carrier concentration, and defect density variation of SnS₂ buffer layer on PV performance

Fig. 4 illustrates the variation of photovoltaic parameters of the proposed thin film heterojunctions with varying layer thickness, carrier concentration, and defect density of the SnS₂ buffer. In Fig. 4(a), PV parameters were found almost unchanged with the variation of SnS₂ buffer layer thickness from 0.03 to 0.5 μm in both structures with and without the BSF. This may be due to the thin layer thickness with higher carrier density (∼10¹⁷ cm⁻³) of the SnS₂ buffer layer. Considering an effective transmission of incident photons from SnS₂ to WS₂ absorber, a thin thickness of 0.05 μm was chosen as the optimal thickness of the SnS₂ buffer layer. Further, an insignificant change in PV parameters was observed when the carrier concentration in SnS₂ increased up to 10²⁰ cm⁻³ in both structures, as depicted in Fig. 4(b). In the structure with BSF, the V_OC, J_SC, FF, and efficiency decreases from 1.035 to 1.030 V, 35.04 to 35.03 mA cm⁻², 88.39 to 87.92%, and 32.05 to 31.72%, respectively, while these were changed from 0.891 to 0.890 V, 30.268 to 30.960 mA cm⁻², 86.44 to 85.99%, and 23.32 to 23.74%, respectively, for the heterostructure without BSF.

Fig. 4 Effect of varying (a) thickness, (b) donor concentration and (c) bulk defect density of the SnS₂ buffer layer in both structures with and without BSF layer.

The photogenerated electrons would have to travel farther to get to the front contact as the SnS₂ buffer layer's thickness increases, which would enhance the potential for carrier recombination. Therefore, the optimum values for thickness and doping concentration were found at 0.05 μm and 10¹⁷ cm⁻³, respectively, considering photon transmission and material utilization of the efficient buffer layer.

Fig. 4(c) shows the variation of PV parameters at different n-type defect densities of the SnS₂ buffer layer from 10¹⁰ to 10¹⁸ cm⁻³, keeping unchanged the rest of the parameters. PV parameters drastically change as the bulk defect density increases from 10¹⁶ cm⁻³ for both scenarios with and without the BSF structure. An increase in bulk defect density in the SnS₂ layer leads to an augmentation in the rate of Shockley-Read-Hall (SRH) recombination, causing a severe deterioration in the overall performance of cells.⁶² So, the optimum value of defect density should be lower than 10¹⁵ cm⁻³ to obtain the best performance from the proposed cells.⁶² Thus, a thickness of 0.05 μm, doping concentration of 10¹⁷ cm⁻³, with a defect density of 10¹⁵ cm⁻³ in SnS₂, are optimal for use as a potential buffer in the proposed device structures.⁶³

3.3 Influence of thickness and bulk defect density variation in the WS₂ absorber layer on PV performance

Fig. 5 illustrates the variation of PV parameters corresponding to relative bulk defect density and layer thickness of the WS₂ absorber from 10¹⁰ to 10¹⁸ cm⁻³ and 0.25 to 3.0 μm, respectively. A marked reduction in solar cell parameters was found when the defect density of WS₂ exceeds 10¹⁴ cm⁻³ with a layer thickness of ≥0.5 μm. The J_SC, V_OC, FF, and PCE of cells with BSF (Al/FTO/SnS₂/WS₂/V₂O₅/Ni) decreased from 35.71 to 26.05 mA cm⁻², 1.2 to 0.6 V, 88.31 to 68.54%, and 35.55 to 10.78%, respectively, when the bulk defect density and absorber layer thickness were varied from 10¹⁰ to 10¹⁸ cm⁻³. In Fig. 5(a), the highest V_OC of 1.0983 V is achieved when the BSF layer thickness is ∼1.0 μm and defect density is ≤10¹⁴ cm⁻³; however, it sharply dropped to 0.604 V at a defect density higher than this critical value (∼10¹⁴ cm⁻³). As shown in Fig. 5(b), the highest J_SC value of 35.71 mA cm⁻² was achieved at a defect density of ≤10¹⁶ cm⁻³ with layer thickness larger than 1.0 μm. The FF reached the maximum value of 88.31% when defect density was less than or equal to 10¹⁴ cm⁻³ and layer thickness was higher than 1.1 μm, so far, and it dropped noticeably to 68.54% corresponding to a defect density beyond 10¹⁴ cm⁻³, as shown in Fig. 5(c). As a result, the highest conversion efficiency, exceeding 30%, was obtained at a layer thickness of ∼1.0 μm and defect density of ∼10¹⁴ cm⁻³ (Fig. 5(d)). A high carrier recombination rate occurs across the cell as a result of the introduction of several defect states in the absorber layer, which in turn lowers cell performance.^13,64,65 Thus, the highest PCE exceeding 32% with V_OC of 1.035 V, a J_SC of 35.03 mA cm⁻², and an FF of 88.31% were obtained at a layer thickness and bulk defect density of ≥1.0 μm and ≤10¹⁴ cm⁻³ of the WS₂ absorber, respectively.

Fig. 5 Effects of WS₂ absorber thickness and bulk defect density variations on the photovoltaic parameters: (a) V_OC, (b) J_SC, (c) FF, and (d) η.

3.4 Influence of layer thickness and interface defect density variation of the WS₂ absorber on PV parameters

The quality of the interface has a significant impact on the cell's performance and thereby plays a vital role in the interaction between light and electricity. Fig. 6(a)–(d) displays the changes in PV parameters for various interface defect densities of 10¹⁰–10²⁰ cm⁻² for the V₂O₅/WS₂ interface at a thickness of 0.25–3.0 μm of the WS₂ absorber layer. The PV parameters were insignificantly affected at the interface defect density of ≤10¹⁰ cm⁻², while they were markedly decreased with an increase in interface defect densities over 10¹⁴ cm⁻² for both of the V₂O₅/WS₂ interfaces. The J_SC, V_OC, FF, and PCE decreased from 35.64 to 20.04 mA cm⁻², 1.065 to 0.845 V, 88.31% to 84.49%, and 32.1% to 14.3% for the structure with the BSF layer, while it drastically decreased from 35.641 to 1.971 mA cm⁻², 1.065 to 0.455 V, 88.31% to 60.38%, and 32.09% to 0.65%, respectively, for the structure without BSF layer, which requires proper steps during practical cell fabrication to obtain the predicted optimized cell performance by tackling interface defects. Thus, the highest conversion efficiency of over 30% was achieved when the interface defect density was adjusted to ≤10¹⁶ cm⁻² at a layer thickness of ∼1.0 μm of WS₂ absorber.

Fig. 6 Effects of absorber thickness and interface (V₂O₅/WS₂) defect density variations on the photovoltaic parameters: (a) V_OC, (b) J_SC, (c) FF, and (d) η.

Fig. 7(a)–(d) depicts the change of PV with varying WS₂/SnS₂ interface defect density from 10¹⁰–10²⁰ cm⁻² and WS₂ absorber layer thickness of 0.25–3.0 μm. The highest V_OC of 1.098 V was obtained for all WS₂ thicknesses when the defect density was below 10¹¹ cm⁻²; however, it severely dropped from 1.098 to 0.464 V for the defect density beyond 10¹² cm⁻² (Fig. 7(a)). In Fig. 7(b), the maximum value for J_SC of 35.642 mA cm⁻² was achieved at a defect density of ≤10¹⁴ cm⁻² and an absorber thickness of ≥0.75 μm. In the meantime, the FF decreased gradually from 88.31% to 72.41% at each thickness value of the WS₂ absorber in the range of 0.25–3.0 μm and interface defect density of ≤10¹⁰ cm⁻², as shown in Fig. 7(c). As a result, the highest conversion efficiency exceeding 30% was achieved at the defect density of ≤10¹¹ cm⁻² and absorber thickness of ≥0.75 μm, as demonstrated in Fig. 7(d). These detailed simulation outcomes reveal that the defects formed at the WS₂/SnS₂ interface have a severe impact on solar cell performance compared to defects at the WS₂/WS₂/V₂O₅ interface, which is consistent with reported studies.^40,44

Fig. 7 Effects of absorber thickness and interface (WS₂/SnS₂) defect density variations on the photovoltaic performance parameters: (a) V_OC, (b) J_SC, (c) FF, and (d) η.

3.5 Influence of the absorber layer's carrier concentration on the G–R profile

Fig. 8(a)–(d) demonstrates the impact on the carrier (electron and hole) concentration and total generation–recombination (G–R) profiles in configurations with and without the BSF layer at a constant acceptor concentration N_A of 10¹⁸ cm⁻² and defect density N_t of 10¹⁴ cm⁻². Regardless of whether a solar cell has a back surface field (BSF) layer or not, the carrier concentration of the absorber layer shows a noticeable effect on G–R by the incident photon. In the cell structure of the BSF layer, a higher concentration of carriers in the WS₂ absorber increased G–R recombination closer to the front surface of the cell. This happens due to an increase in available carriers at the higher carrier concentration, which leads to a rise in the rate of recombination. With a structure incorporating BSF, the impact of the carrier concentration in the absorber layer on the G–R profile is mitigated because of the formation of an aligned energy band with the least band offset among the absorber, BSF and metal contact. Because the BSF layer is designed to reduce recombination at the back surface of the cell, it can also affect the G–R profile near the front surface, which is consistent with previous reports.⁶⁶ Total (electron and hole) generation at the absorber thickness of ∼1.0 μm was found to be the maximum, and on the contrary, resulted in the least total recombination loss.

Fig. 8 Impact on (a) hole and (b) electron carrier concentration, (c) total generation, and (d) recombination of the absorber layer thicknesses.

3.6 Capacitance–voltage (C–V) characteristics of the proposed heterojunctions

Fig. 9 illustrates the capacitance–voltage characteristics of the proposed heterojunction solar cells over a frequency range of 0.5 kHz to 1 MHz. Conventionally, p–n junctions are linked to the development of depletion and diffusion of charges. At reverse bias voltage, the depletion capacitance surpasses the diffusion capacitance, whereas the diffusion capacitance predominates at forward bias. These C–V characteristics under a range of frequencies, absorber concentrations, and thicknesses reveal the versatility of charge modulation in the proposed WS₂-absorber-based solar cells. For a specific frequency, the capacitance increases sublinearly with the increase in polarization potential, which indicates a negligible acuity of the absorber traps at all frequencies, as shown in Fig. 9(a). Under reverse bias conditions, the existing traps are unable to mitigate the effective charge, leading to a decrease in capacitance, which is consistent with previous studies.^67,68 With increasing carrier concentration, the capacitance increases markedly, while it is absolutely amplified with increasing polarization potential. Surprisingly, the capacitance increased almost linearly with absorber thickness up to 0.8 μm.

Fig. 9 (a) Impact on C–V in solar cells with varying frequency; (b) 1/C²–V Mott–Schottky curve; C–V curves with (c) varying absorber doping and (d) varying absorber thickness.

Fig. 9(b) displays the Mott–Schottky plot of the proposed Al/FTO/SnS₂/WS₂/V₂O₅/Ni heterostructured cell. The intersection of the 1/C² plot with the voltage axis gives rise to the flat-band potential of the proposed solar cell. The p-type WS₂ layer is mostly occupied by the space charge area, as indicated by the negative slope of the plot, suggesting holes are the dominant carriers. The WS₂ layer may have a high carrier density due to the photogenerated carriers caused by exposure to sunlight. A possible reason for the slight variation in 1/C² is the presence of deep states in the absorber layer, which are localized. The deep states do not have a considerable effect; therefore, the modulation of majority carriers is the primary cause of the observed impact.⁶⁷

Fig. 9(c) illustrates the capacitance–voltage (C–V) properties of a solar cell based on WS₂ carrier concentration. The capacitance changes with increasing doping concentration of the absorber layer. As the forward bias voltage increases, the capacitance values increase and function sublinearly as Mott–Schottky junctions. The Mott–Schottky plot under AM 1.5G sunlight illumination indicated a lower built-in potential, which could be attributed to the capacitance generated by photogenerated carriers within photoactive materials. An earlier study reported that the increase in doping density leads to an increase in charge accumulation at the interface, which in turn results in an improvement in capacitance.⁶⁹ In Fig. 9(d), the graph shows the relationship between capacitance and bias voltage at different absorber thicknesses at a constant frequency of 1 MHz. The change in capacitance with thickness variation shows an almost linear relation. However, after the voltage of 0.70 V, a significant rise in capacitance value is observed with the increase in voltage. Thus, the WS₂ absorber has the potential to design high-efficiency photovoltaics with significant C–V behavior.

3.7 Influence of operating temperature on the performance of the solar cell

Fig. 10 illustrates the impact of operating temperatures on photovoltaic parameters of the proposed WS₂-based solar cells for both configurations, with and without the BSF layer. The stability of the PV cell could be explored by the investigation of operating temperatures ranging from 275 to 475 K. A drastic reduction in PV parameters is observed for both WS₂-based heterojunctions with and without V₂O₅ BSF when the working temperature rises from 275 to 475 K. The PCE decreased from 33.19% to 23.27% and from 24.56% to 15.75% as the operating temperature increased from 275 to 475 K for heterostructures with and without BSF, respectively. An increase in operating temperature leads to the shrinking of the band gap of WS₂ and an increase in the reverse saturation current.⁷⁰ As a result, the values of V_OC, FF, and consequently the cell efficiency were affected noticeably at higher working temperatures. The declination in power conversion efficiency at higher operating temperatures observed in this study is consistent with previous reports.^40,71–74

Fig. 10 Effect of temperature on the PV parameters of the solar cell.

3.8 Performance of the device output

Fig. 11 displays the current density–voltage (J–V) characteristics of the WS₂-based TFSCs. From Fig. 11, it is evident that the Al/FTO/SnS₂/WS₂/V₂O₅/Ni heterojunction solar cell exhibits a notably superior estimated current density compared to the reference WS₂-based heterojunction solar cell, Al/FTO/SnS₂/WS₂/Ni. Without BSF, the WS₂-based solar cell achieves a V_OC of 0.89 V, J_SC of 31.2 mA cm⁻², FF of 81.0%, and power conversion efficiency of 23.4%. On the other hand, the configuration with BSF results in a V_OC of 1.1 V, J_SC of 37.2 mA cm⁻², FF of 84.0%, and efficiency of 32.02%. The solar cell with BSF configuration achieves improved electrical device outputs in terms of V_OC and J_SC (as observed from the J–V analysis). This enhancement leads to an overall increase in the power conversion efficiency of the solar cell. Therefore, the inclusion of V₂O₅ as a BSF layer is suggested to significantly enhance the performance of the proposed WS₂-based solar cell. In earlier experiments, CdS was used as buffer layer, which is toxic; here, SnS₂ was used instead of CdS, being nontoxic and readily available.^13,52Table 3 presents a comparative overview of the advantages brought about by the incorporation of V₂O₅ BSF in WS₂ solar cells, along with its positive impacts when compared to analogous materials. The TFSCs proposed in this study could be more commercially attractive than other structures mentioned in Table 3.

Fig. 11 J–V characteristics of the WS₂-based solar cells.

Table 3 Summary of the reported experimental and theoretical work

Sl No	Type	Cell structure	Cell thickness (μm)	V OC (V)	J SC (mA cm^−2)	FF (%)	η (%)	Ref.
Note that * means this work, T = theoretical, and E = experimental.
1	E	ZnO/WS2		0.362	0.65	41.1	1.7	33
2	E	ITO/WS2		0.551	1.01	47.6	3.9	33
3	E	FTO/dye/WSG2		0.79	18.6	66	9.6	77
4	T	ZnO/SnS2/CZTS/MoS2	0.08/0.05/1.5/0.1	0.7178	26.9976	65.67	12.73	52
5	T	ZnO/ZnSe/WS2/Mg	0.03/0.045/2.5/0.05	0.9	25	85	20	78
6	T	ITO/WS2/CdS/Sb2Se3	0.025/0.05/0.06/0.4	0.706	40.52	72	20.6	79
7	T	SnO2: F(FTO)/WS2/Cu2Te	0.05/0.05/1.0	0.68	42.95	82.77	24.23	80
8	T	ZnO: Al/ZnO/WS2	0.2/0.05/2	0.90	32.70	86.5	25.71	35
9	T	Spiro-OMeTAD/CH3NH3PbI3/WS2	0.1/0.35/0.15	1.056	25.483	88.54	25.70	81
10	T	FTO/n-WS2/Sb2S3	0.05/0.03/1.2	1.23	26.6	89.60	26.60	10
11	T	FTO/n-WS2/Sb2Se3	0.05/0.03/1.2	0.85	38.40	86.56	28.20	10
12	T	n-ZnO/n-CdS/p-WS2	0.2/0.05/2	0.8872	39.34	84.93	29.64	82
13	T	FTO/CdS/WS2	0.05/0.05/1.0*	0.83	31.88	83.66	22.09	13
14	T	FTO/CdS/WS2/CuI	0.05/0.05/1.0*/0.1	0.98	35.19	87.08	29.87	13
15	T	FTO/SnS2/WS2	0.05/0.05/1.0	0.89	31.2	81	23.4	**
16	T	FTO/SnS2/WS2/V2O5	0.05/0.05/1.0/0.1	1.1	37.2	84	32.02	**

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In double heterostructures, with the insertion of a BSF layer like V₂O₅, the sub-bandgap photons may get absorbed significantly, specifically in the longer wavelength, which results in the improvement in photovoltaic performance. The sub-bandgap photons may be absorbed by the Urbach energy states, and these lower energy sub-bandgap photons participate in tail-state-assisted (TSA) two-step photon upconversion. These absorbed photons generate additional electron–hole pairs, resulting in a noticeable improvement in cell photocurrent. With the assistance of Urbach tail states, two sub-bandgap photons are absorbed in a row in this process. However, the degree of upconversion and the resulting enhancement in photocurrent depends on the Urbach energy, E₀, of the photoactive material. This TSA upconversion takes place in a photoactive material when it possesses adequate doping concentration, high absorption coefficient and favorable bandgap.^75,76 The higher Urbach energy significantly contributes to enhancing the quantum efficiency (QE), specifically in the longer wavelength.^48–51,76 The equation for Urbach energy is shown in eqn (8). Thus, the authors believe that enhanced PV performance can be obtained in the proposed double-junction solar cells with relevant modelling and the convincing simulation parameter data.

α = α₀ exp(E/E_u), (8)

where α is the absorption coefficient, α₀ is a constant, E is the photon energy, and E_u is the Urbach energy. The Urbach energy is calculated from inverse of ln α vs. E slope.

4. Conclusions

Through numerical modeling with a SCAPS-1D simulator, high-efficiency transition metal dichalcogenide thin-film heterojunction solar cells with SnS₂ buffer and V₂O₅ BSF layers have been explored. Potential V₂O₅ BSF and SnS₂ buffer layer materials with the required layer properties, such as thickness, carrier doping concentration, and defect densities, have been investigated and adjusted through a comprehensive study. The best and adjusted layer thicknesses for the SnS₂ buffer, WS₂ absorber, and V₂O₅ BSF layer were found to be 0.05, 1.0, and 0.10 μm, respectively, with a bulk defect density of 10¹⁴ cm⁻³, and the interface defect densities of 10¹¹ cm⁻² at WS₂/V₂O₅ and 10¹⁰ cm⁻² at WS₂/SnS₂ were determined. These results indicate that the addition of a V₂O₅ BSF to the reference SnS₂/WS₂ TFSCs offers an efficient dual-heterojunction solar cell showing the improved (by almost 9%) PCE of 32.02% with V_OC of 1.1 V, J_SC of 37.2 mA cm⁻² and FF of 84% for the Al/FTO/SnS₂/WS₂/V₂O₅/Ni heterostructure, compared to the PCE of 23.4% with V_OC of 0.89V, J_SC of 31.2 mA cm⁻² and FF of 81% in the reference cell. The proposed WS₂ cell has been shown to have better stability and performance than the reference WS₂ solar cell. The results show the enormous potential of the SnS₂/WS₂/V₂O₅ heterostructure for designing and manufacturing high-efficiency photovoltaic cells and open a clear path for the production of WS₂-based photovoltaic and photonic devices based on transition-metal dichalcogenides.

Author contributions

Md. Ferdous Rahman and Abdul Kuddus: conceptualization, methodology, software, validation, formal analysis, visualization, investigation, data curation, supervision, writing – original draft, review & editing. Jubair Al Mahmud: methodology, software, validation, formal analysis, visualization investigation, data curation, writing – original draft, review & editing. Md. Hasan Ali, A. T. M. Saiful Islam, Md Dulal Haque, Sheikh Rashel Al Ahmed, Muhammad Mushtaq, Abu Bakar Md. Ismail: validation, formal analysis, writing – original draft, review & editing.

Ethical approval

All authors declare that the manuscript does not have studies on human subjects, human data or tissue, or animals.

Conflicts of interest

The authors have no conflicts of interest.

Acknowledgements

The authors would like to offer their heartfelt appreciation to Dr Marc Burgelman and his colleagues at the EIS Department at the University of Gent in Belgium for providing the opportunity to use the SCAPS-1D software. The authors are also grateful to the Department of Electrical and Electronic Engineering, Begum Rokeya University, Rangpur 5400, Bangladesh, for the use of the Advanced Energy Materials and Solar Cell Research Laboratory.

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