To achieve efficient carrier separation in photoelectric, photovoltaic devices, it is crucial to explore suitable materials and structures. One promising solution is the use of van der Waals heterostructure (vdw HTS) with a type-II band alignment. These HTSs consist of two-different monolayers (MLs) stacked on top of each other without forming chemical bonds in between. The concept of vdw HTS was first introduced by Geim et al. 2013, and it has gained significant attention in the field of materials research (Meng et al., 2021). The weak vdw forces between these monolayers allow them to retain their original monolayer's optoelectronic characteristics, while the interlayer interaction brings out the novel characteristics. Many researchers have successfully constructed various 2D vdw HTS such as PtS2/ZrS2 (Parmar et al., 2023), GaTe/ZnI2, PtSe2/GaSe (Parmar et al., 2023), and many others. The discovery of graphene (Novoselov et al., 2005) has brought new insight into 2D materials. After that, many 2D materials are studied extensively because of their novel properties and potential applications (Khengar et al., 2022, Modi et al., 2023). Of all these various 2D materials, halides-dihalides have gained research interest due to their interesting properties, potential applications, and tunable bandgap (Hu et al., 2020). Recently, SnBr2 has been theoretically investigated, having an indirect bandgap of 3.02 eV (Naik et al., 2023). While CuI has a direct bandgap having an application in photovoltaic devices (Amrani et al., 2006). Due to these excellent properties of SnBr2 and CuI MLs, we choose to construct SnBr2/CuI vdw HTS. Recently, 2D materials are explored for many novel applications such as Gas sensors (Peng et al., 2021), storage devices (Mortazavi et al., 2016), Thermophotovoltaic (TPV) devices (Ismail et al., 2024), photocatalysis (Shakil et al., 2023), etc. Among which, TPV is gaining more research interest due its various potential use. TPV devices are energy conversion systems that directly transform thermal radiation into electricity which is the same fundamental principle as photovoltaic cells. Such TPV devices not only use sunlight just like solar cell but it harnesses the thermal radiation emitted from high-temperature sources such as combustion systems, concentrated solar absorbers, nuclear reactors, or even industrial waste heat. A typical TPV system consists of a thermal emitter, which radiates a broad spectrum of photons when heated to elevated temperatures, and a photovoltaic cell that absorbs these photons to generate electron-hole pairs and produce electrical power (Agarwal and Prajapati 2018). Spectral control elements such as filters or photonic structures are used in TPV which reflects sub-bandgap photons back to the emitter and hence efficiency improved.

The efficiency of TPV devices strongly depends on the choice of semiconductor material for the photovoltaic cell, as the bandgap must be carefully matched with the spectral distribution of the thermal emitter (Ahmed et al., 2025). Low bandgap semiconductors in the range of 0.2-0.7 eV, including materials such as InGaAsP (Tuley and Nicholas 2010), GeTe (Abir et al., 2023), have been widely studied because of their ability to absorb near-infrared thermal radiation effectively. In addition, achieving direct bandgaps and favorable carrier dynamics through Type-II band alignment in heterostructures has been shown to improve photon absorption and charge extraction, both of which are essential for efficient device performance (Almayyali et al., 2023). In this context, recent advances in two-dimensional (2D) materials and van der Waals heterostructures have opened new possibilities for TPV device design, owing to their tunable band structures, strong light-matter interactions, and potential to achieve Type-II band alignment with direct bandgaps. These features make them promising candidates for next-generation TPV applications.

Currently, no systematic research has been found on the SnBr₂/CuI vdW Heterostructure for the implications in Thermophotovoltaic Devices. So, this research consists of the DFT investigation on SnBr2/CuI vdw HTS thermophotovoltaic Devices. 

COMPUTATIONAL DETAILS

In this research, all the simulations have been performed within the DFT framework (Orio et al., 2009) by Quantum Espresso software. The exchange-correlation functional of the electron is described by utilizing the Generalized Gradient Approximation (GGA) given by the Perdew-Burke-Eruzerhof (PBE) (Perdew et al., 1996). To obtain a more accurate bandgap, a hybrid functional HSE06 given by Heyd-Scuseria-Ernzerhof (Heyd et al., 2003) is utilized. The DFT-D3 (Grimme et al., 2011) method given by Grimme has been utilized to represent the vdw interactions. A thick vacuum of 25 Å is given in a perpendicular direction in order to prevent interactions between two layers. The energy and force threshold is set to 10-8 Ry and 10-4 Ry/Bohr, respectively, to obtain structural optimization and system convergence. The Monkhorst-Pack k-point grid is fixed at 6×6×1 and plane wave energy cut-off is fixed at 550 Ry. The optical properties are obtained by using the Norm-conserving pseudopotential and frequency-dependent complex dielectric function ε (ꞷ).

A. Structural Properties

The monolayers SnBr2 and CuI have similar hexagonal symmetry and almost the same lattice parameter, so it feasible to construct heterostructure easily. The obtained lattice constant (a) of SnBr2/CuI vdw HTS, SnBr2 ML, and CuI ML are 4.32 Å, 4.37 Å, and 4.33 Å, respectively. The lattice mismatch between their constitute monolayer for SnBr2/CuI vdw HTS is given by equation δ=aCuI-aSnBr2aCuI (Parmar et al., 2023), which is 1.14% for this vdw HTS. The optimized structure of constructed SnBr2/CuI vdw HTS is illustrated in Fig. 1(a).

Fig. 1. (a) Optimized structure and (b) AIMD calculation of SnBr2/CuI vdw HTS.

To check the energetic stability of this constructed vdw HTS, we have to find its adhesion energy Eae that is given by equation (Parmar et al., 2022), Eae=Evdw HTS-ESnBr2-ECuI. Where Evdw HTS is the total energy of a HTS, and ESnBr2 and ECuI represent the energy of their respective monolayers. The obtained negative value of adhesion energy -1.01 eV for the SnBr2/CuI vdw HTS shows that the material is energetically stable. However, the AIMD simulation is also performed to ensure the thermal stability of SnBr2/CuI vdw HTS for 5 ps with a time interval of 1fs. The obtained results as shown in Fig. 1(b), illustrates that there is minimal deviation in total energy and negligible structural fluctuations throughout the entire simulation. Hence, the constructed SnBr2/CuI vdw HTS is energetically and thermally stable material. 

It can also be added that the extremely low lattice mismatch (1.14%) implies negligible interfacial strain, favorable for the retention of the intrinsic quality of the constituting layers while allowing for tight interlayer coupling (Guo et al., 2017). The geometrically optimized structure implies that the van der Waals interaction maintains the interlayer separation without structural distortions, confirming structural soundness. Such low values for the mismatch also favour easier experimental realization, due to the lack of strain-related defects. Furthermore, the stability also agreed by the room-temperature AIMD implies the potential for the heterostructure to survive real-device operation conditions. Collectively, these results imply that the SnBr₂/CuI vdW HTS possesses the dual characteristic of robustness and compatibility, preconditions for the realization of practical optoelectronic as well as photovoltaic device fabrication.

B. Electronic Properties

Since SnBr2 ML, CuI ML, and SnBr2/CuI vdw HTS have hexagonal symmetry, the band diagram is obtained along highly symmetric direction ‘Γ-M-K-Γ’ with HSE06 functional as shown in Fig. 2(a,b,c). The SnBr2 ML has an indirect bandgap of 3.02 eV, while CuI ML has a direct bandgap of 2.50 eV. Interestingly, SnBr2/CuI vdw HTS exhibits a direct bandgap of 1.21 eV, which is lesser than each monolayer of constructive components. The CBM resembles SnBr2 while VBM mainly originates from CuI. Hence, in the presence of radiation, the SnBr2/CuI vdw HTS exhibits efficient electron transfer, resulting in the effective separation of photoexcited electron-hole pairs. These findings also highlight the enhanced optical properties. The Fig. 2(d) displays the Projected Density of State (PDOS). 

It is seen that Cu-d state is more dominant in valance band while Br-p and Sn-p states have significant effect in conduction band. Additionally, the overlap of Cu-d states with Sn-p/Br-p states around the Fermi level indicates interlayer hybridization, which enhances interfacial coupling and supports efficient charge transfer. The band curvature in Fig. 2 also suggests relatively light effective masses for electrons in the SnBr2 conduction band and holes in the CuI valence band. Lower effective masses are typically associated with higher carrier mobilities, which is a desirable feature for fast charge transport in photovoltaic applications. The materials work function Φ is characteristics as the energy required to eliminate an electron from its surface is obtained by employing the equation, =Evac-Ef , where Ef and Evac denote the fermi and vacuum level. The obtained Φ is 8.71 eV for vdw HTS.

As shown in Fig. 3(a), the CuI ML exhibits a higher work function compared to the SnBr2 ML, which indicates that upon contact electrons will transfer from CuI to SnBr2 surface until their fermi level matches. Furthermore, the charge density difference ∆ρ is obtained (Parmar et al., 2023) from ∆ρ=∆ρvdw HTS-∆ρSnBr2-∆ρCuI. The yellow clouds below SnBr2 ML indicate charge accumulation, while green spheres above CuI ML suggest charge depletion, implying substantial charge transfer from CuI ML to SnBr2 ML, as depicted in Fig. 3(b). The redistribution of charges forms an intrinsic inbuilt electric field at the junction, which in turn drives the photogenerated carriers away and hinders the loss due to recombination. The generated dipole layer at the interface at the end adjusts the electrostatic potential at the junction, favoring enhanced type-II band alignment and easier carrier separation. Such effects demonstrate the working principle of a normal p–n junction but in the pure 2D vdW regime (Chapin et al., 1954). 

Fig. 2. Band structure of (a) SnBr2 ML (b) CuI ML (c) SnBr2/CuI vdw HTS and (d) PDOS of SnBr2/CuI vdw HTS.

Besides the photovoltaic uses, the offset alignment can be utilized for photocatalytic reaction, where spatially separated carrier pairs are utilized to drive reduction and oxidation processes on different layers (Dalsaniya et al., 2021). 

The band alignment is displayed in Fig. 3(c). Under solar light, the SnBr2/CuI vdw HTS exhibits photogenerated electron transfer from CuI ML to the SnBr2 conduction band and photoexcited holes move to CuI valance band, achieving spatial electron-hole separation. This is attributed to the type-II band alignment, effectively reducing electron-hole recombination.

C. Optical Properties 

The optical response is calculated from the dielectric function (that is the function of frequency) ɛ(ω) given as =1+i2. Here, ɛ1(ω) and ɛ2(ω) corresponds to the real and imaginary portion, respectively. The real part in Fig. 4(a) shows that the static dielectric constant (SDC) of SnBr2 ML, CuI ML, and SnBr2/CuI vdw HTS is 2.54, 1.76, 2.91, respectively. The increment in SDC of ɛ1(ω) implies that lower exciton binding energy which leads to an effective hole and electrons separation. The presence of a negative value indicates the metallic characteristics at that particular frequency. The dielectric function’s imaginary portion is related to the interband transition, which is shown in Fig. 4(b) exhibits several distinct peaks. Further, the relation between absorption coefficient and energy is displayed in Fig. 4(c). and is obtained the equation (Mehta et al., 2023),

=2c12+2212-112.

The highest peak is observed at 6.74 eV, 5.54 eV, and 7.55 eV for SnBr2 ML, CuI ML, and SnBr2/CuI vdw HTS, respectively. It is seen from the figure that the absorption of SnBr2/CuI vdw HTS is enhanced from CuI ML. Also, SnBr2/CuI vdw HTS exhibits a starting noticeable absorption in visible region in comparison with SnBr2 ML.  So, by comparing the absorption curve of both MLs with SnBr2/CuI vdw HTS, it is seen that SnBr2/CuI vdw HTS has better absorption performance. So, SnBr2/CuI vdw HTS has great potential for efficient optoelectronic-nanoelectronics devices and photovoltaic application.

Fig. 4. (a) Real portion (b) Imaginary portion and (c) Absorption coefficient.

Besides the improved absorption, the red-shifted absorption edge for SnBr₂/CuI vdW HTS holds special significance, for it enables the material to exploit a larger fraction of the solar spectrum than either of the individual layers. The relatively larger dielectric constant also translates to lower exciton binding energy, implying that the excitons can be readily split into free carriers—a characteristic important for photovoltaic conversion. Lastly, the sharp absorption peaks in the UV regime point to possible suitability for UV photodetectors (Patel et al., 2021). The simultaneous occurrence of visible-range absorption and UV sensitivity also serves to reinforce the optoelectronic tunability of the heterostructure, making it suitable for various optoelectronic applications (Yang et al., 2019). Such spectral tunability is characteristic for vdW heterostructures and fortifies the candidacy for multifunctional devices for SnBr₂/CuI.

In this work, we have systematically investigated the structural and optoelectronic properties of the SnBr2/CuI van der Waals heterostructure (vdW HTS) using first-principles density functional theory calculations. Our results confirm that the heterostructure is both energetically and thermally stable, as evidenced by the negative adhesion energy and the minimal fluctuations observed in AIMD simulations. The compatibility of lattice constants between SnBr₂ and CuI enables the formation of a stable interface with minimal mismatch, further supporting its structural integrity.

From an electronic point of view, SnBr2/CuI vdW HTS has a direct bandgap in the region of about 1.17 eV, within the optimum value for photovoltaic absorbers. Note well that the type-II band alignment for efficient charge transfer over the interface takes place due to the spatial separation of electrons and holes in multiple layers. The band alignment strongly suppresses the recombination losses, thus increasing the probable efficiency in devices by virtue of this heterostructure. Charge density difference calculations and work functions are additional proof to confirm the taking place of the interfacial charge redistribution in support of effective carrier separation.

Optical characterizations reveal weak absorption in the visible regime for the pure SnBr2 monolayer, while the SnBr2/CuI heterostructure red-shifts the absorption edge to align the absorption within the visible regime. The above-mentioned features along with the relatively low value of the exciton binding energy indicated by the larger dielectric constant direct towards efficient separation and generation of the photoexcited carrier pairs in the heterostructure. Consequently, the synergistic combination of inherent structural stability, appropriate band alignment, and improved optical response makes the SnBr2/CuI vdW HTS an extremely viable prospect for next-generation photovoltaic and nano-optoelectronic devices. The SnBr2/CuI vdW heterostructure demonstrates type-II band alignment and a direct bandgap, both of which are highly favorable for efficient carrier separation and strong light absorption. These features suggest that the system could be a promising candidate for thermophotovoltaic applications, where optimized bandgaps and charge dynamics are essential for high conversion efficiency.

The SnBr2/CuI heterostructure from the present study has the appropriate stability, band alignment, and optical response for next-generation photovoltaic and TPV devices. Further simulations at the device level, experimental synthesis, and tuning by strain, doping, or external fields are suggested to further optimize the performance of SnBr2/CuI. Its strong interlayer charge separation and visible-range absorption also make it a good candidate for broader optoelectronic applications.