A comprehensive first-principles study exploring the structural, electronic, optical, transport, and photovoltaic properties of Rb2LiGa (Br/I)6 utilizing DFT and SCAPS-1D simulations

doi:10.21203/rs.3.rs-4849239/v1

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A comprehensive first-principles study exploring the structural, electronic, optical, transport, and photovoltaic properties of Rb₂LiGa (Br/I)₆ utilizing DFT and SCAPS-1D simulations

https://doi.org/10.21203/rs.3.rs-4849239/v1

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Lead-free double perovskite halides are attracting substantial attention in optoelectronics due to their outstanding electronic, optical, and transport properties. These materials are not only stable and easy to synthesize but also present a wide range of potential applications. In this study, we explore the intriguing properties of Rb₂LiGa(Br/I)₆, emphasizing their structural, electronic, optical, transport, and photovoltaic characteristics. Our simulation results show that Rb₂LiGaBr₆ and Rb₂LiGaI₆ possess band gaps of 1.19 eV and 1.13 eV, respectively, highlighting their potential for various applications. Both compounds show remarkable optical performance, with high absorption coefficients and optical conductivity, along with low reflectivity across the UV-visible spectrum, making them excellent candidates for solar cell technologies. Furthermore, Rb₂LiGa(Br/I)₆ exhibit exceptional thermoelectric performance, with high figure-of-merit (ZT) values ranging from 200K to 800K, emphasizing their potential as efficient thermoelectric materials. Therefore, this study will provide guidelines for constructing efficient solar cells based on double perovskites.

Motivated by the higher absorption coefficient and optical conductivity of Rb₂LiGa(Br/I)₆, we have modeled an Au/Cu₂O/Rb₂LiGa(Br/I)₆/TiO₂/FTO solar cell. We have observed that Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO has higher efficiency than Au/Cu₂O/Rb₂LiGaBr₆/TiO₂/FTO. Additionally, the efficiency of Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO is higher than that reported previously. Thus, this study provides guidelines for constructing efficient solar cells based on double perovskites.

Physical sciences/Materials science/Materials for devices/Electronic devices

Physical sciences/Physics/Condensed matter physics

absorption coefficient

Seebeck coefficient

photovoltaic

thermoelectric

band gap.

With rising global energy demands and dwindling natural resources, the urgency for affordable, sustainable, and eco-friendly energy solutions has intensified. Solar cells and thermoelectric generators are leading the charge in this quest, converting solar energy and wasted heat into electricity. To fully realize the potential of these technologies, it is crucial to understand and optimize the structural, electronic, optical, and transport properties of materials. Materials with direct band gaps ranging from 0.9 eV to 1.6 eV are especially valuable for optoelectronic applications, while those with lower band gaps are better suited for transport-related uses. Traditional halide perovskites, represented by the chemical formula ABX₃, have attracted significant attention for their wide range of applications, such as catalysis, fuel cells, electrochemical sensing, photovoltaics, thermoelectric, and x-ray detection. These materials, with a simple cubic structure (space group Fm3m (225)), possess impressive properties such as a high dielectric constant, tunable band gap, and excellent carrier mobility, resulting in significantly boosted power conversion efficiencies from 3.8% in 2009 to 26.1% in 2024. However, the presence of toxic lead and instability caused by organic compounds in traditional perovskites impede their large-scale commercialization. This challenge has led to an active search for alternatives that possess similar optoelectronic properties while being stable, non-toxic, and cost-effective. In this context, lead-free double perovskite halides, characterized by the chemical formula A₂BB’X₆, have garnered significant interest from the research community. These materials are stable in the cubic phase (space group Fm3m (225)) and provide flexible optoelectronic properties due to the diverse selection of B and B’ elements.

Recent studies have achieved notable progress. Slavney et al. synthesized Cs₂AgBiBr₆, noting its long carrier lifetime and visible-range band gap; however, its indirect band gap limits its absorption capabilities. Conversely, Volonakis et al. pointed out the direct band gap characteristics of Cs₂AgInCl₆, although its wide band gap limits its effectiveness for renewable energy applications. In addition, Rb₂ASbX₆ (A = Tl, Cu, and X = Cl, I) has been explored for energy storage and optoelectronic applications [35]. However, their indirect band gap makes them inapt for such applications. Tang et al. investigated A₂CuSbX₆ (A = Cs, Rb, and X = Cl, Br, I) for photovoltaic applications, but their indirect band gap makes them unsuitable for this purpose [36]. Moreover, numerous other lead-free halide double perovskite materials have been explored [37–38]. However, none have shown promise for photovoltaic or thermoelectric applications. This is because they are either toxic or unstable, or they lack desirable optoelectronic properties. Despite these advancements, numerous lead-free double perovskites still face challenges with indirect band gaps or overly large band gap values, highlighting the necessity for new materials with optimal properties.

In this study, we focus on Rb₂LiGa(Br/I)₆, exploring their structural, electronic, optical, and transport properties. In this study, we focus on Rb₂LiGa(Br/I)₆, delving into their structural, electronic, optical, and transport properties. Our analysis shows Poisson's ratios of 0.27 and 0.28 for Rb₂LiGaBr₆ and Rb₂LiGaI₆, respectively, which indicate their ductility. Phonon spectra indicate the absence of imaginary frequencies, confirming stability against lattice distortions. With band gaps of 1.19 eV for Rb₂LiGaBr₆ and 1.13 eV for Rb₂LiGaI₆, these materials display remarkable optical properties, including high absorption coefficients, robust optical conductivity, and low reflectivity in the UV-visible range, positioning them as excellent candidates for solar cell applications. Our modeling of an Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO solar cell predicts a maximum efficiency of 19.06%, highlighting their potential in high-efficiency photovoltaic applications. This study paves the way for developing lead-free halide double perovskite-based solar cells, offering a promising pathway toward more sustainable energy solutions.

The structural, optoelectronic, and transport properties of Rb₂LiGa(Br/I)₆ are investigated using the Wien2k scientific package [12]. The structural parameters, including lattice parameters, bond lengths, bond angles, and stability, are calculated using the GGA-PBE exchange correlation. We have considered 1000 K-points for optimizing the crystal structure of Rb₂LiGa(Br/I)₆. The convergence criteria for total energy and force during electronic and ionic relaxation are set to 10^− 6 eV and 10^− 2 eV/Å, respectively. To calculate electronic, optical, and transport properties, we employed the mBJ potential, which provides results closely aligned with experimental values [13]. 20,000 K-points are used to calculate the optical properties. The transport properties are analyzed using the BoltzTraP package [14]. The electrical conductivity coefficient can be expressed as

$$\:{\sigma\:}_{\alpha\:\beta\:}\left(\epsilon\:\right)=\frac{1}{N}\sum\:_{i,k}\:{\sigma\:}_{\alpha\:\beta\:}(i,k)\frac{\delta\:\left(\epsilon\:-{\epsilon\:}_{i,k}\right)}{\delta\:\left(\epsilon\:\right)}$$

$$\:{\sigma\:}_{\alpha\:\beta\:}(i,k)={e}^{2}{\tau\:}_{i,k}{v}_{\alpha\:}(i,\overrightarrow{k}){v}_{\beta\:}(i,\overrightarrow{k})$$

where N, v_β, and $\:\tau\:$ are the number of k-points, group velocity, and relaxation time, respectively.

The Seebeck coefficient and electrical conductivity as functions of the chemical potential can be expressed as

$$\:{S}_{\alpha\:\beta\:}(T,\mu\:)=\frac{1}{eT{\Omega\:}{\sigma\:}_{\alpha\:\beta\:}(T,\mu\:)}\int\:\:{\sigma\:}_{\alpha\:\beta\:}\left(\epsilon\:\right)(\epsilon\:-\mu\:)\left[-\frac{\partial\:{f}_{0}(T,\epsilon\:,\mu\:)}{\partial\:\epsilon\:}\right]d\epsilon\:$$

$$\:{\sigma\:}_{\alpha\:\beta\:}(\alpha\:,\mu\:)=\frac{1}{{\Omega\:}}\int\:\:{\sigma\:}_{\alpha\:\beta\:}\left(\epsilon\:\right)\left[-\frac{\partial\:{f}_{0}(T,\epsilon\:,\mu\:)}{\partial\:\epsilon\:}\right]d\epsilon\:$$

The above equations use the BoltzTraP code to compute their numerical values.

In the present work, we used the Solar Cell Capacitance Simulator-One Dimension (SCAPS-1D) version 3.3.10 due to its benefits, including its user-friendly interface, capability to model both dark and bright environments, and ability to create multi-layer heterostructure systems. SCAPS-1D was developed by Marc Burgelman and colleagues at the University of Ghent, Belgium. SCAPS-1D can simulate various properties of a solar cell, including power conversion efficiency (PCE), fill factor (FF), energy band, short-circuit current density (JscJ_{sc}Jsc), quantum efficiency (QE), and current-voltage (J-V) characteristics. The SCAPS-1D software primarily operates by solving the Poisson and continuity equations as follows:

Poisson equation: $\:-\frac{\partial\:}{\partial\:x}\left(-\epsilon\:\left(x\right)\frac{\partial\:V}{\partial\:x}\right)$

$$\:=q\left[p\left(x\right)-n\left(x\right)+{N}_{D}^{+}\left(x\right)-{N}_{A}^{-}\left(x\right)+{p}_{t}\left(x\right)-{n}_{t}\left(x\right)\right]$$

Continuity equation for the hole $\::\frac{\partial\:p}{\partial\:t}=\frac{1}{q}\frac{\partial\:{J}_{p}}{\partial\:x}+{G}_{P}-{R}_{p}$ (6)

Continuity equation for electron: $\:\frac{\partial\:n}{\partial\:t}=\frac{1}{q}\frac{\partial\:{J}_{n}}{\partial\:x}+{G}_{n}-{R}_{n}$ (7)

Where q, $\:\epsilon\:$, and V represent charge, dielectric permittivity and potential respectively. On the other hand, p(x), $\:n\left(x\right),\:\:{N}_{D}^{+}\left(x\right)$ and $\:\:{N}_{A}^{-}\left(x\right)\text{a}\text{r}\text{e}$free holes, free electrons concentration, ionized donor concentration, and ionized acceptor concentration respectively. Also, $\:{p}_{t}\left(x\right)$ is the hole trap density, $\:{n}_{t}\left(x\right)$ denote the trap density of electron, $\:{J}_{p}$ represents the current density of hole, $\:{J}_{n}$ is the current density of electron, $\:{G}_{p}$ is the holes generation rate, $\:{G}_{n}$ is the electrons generation rate$\:,\:\:{R}_{n}$ represents the recombination rate of an electron, $\:{R}_{p}$ is the recombination rate of the hole.

3.1. Structural properties, mechanical and thermodynamic properties

Initially we optimized the volume of Rb₂LiGa(Br/I)₆. Figure 1 (a, b) shows the volume optimization curve of Rb₂LiGa(Br/I)₆. The lattice parameters obtained are 11.13 Å for Rb₂LiGaBr₆ and 11.17 Å for Rb₂LiGaI₆. Additionally, the relaxed Rb₂LiGa(Br/I)₆ systems exhibit a cubic crystal phase with an Fm3m (No. 225) space group. The optimized crystal structure of Rb₂LiGa(Br/I)₆ is depicted in Fig. 2 (a, b), showing that each Rb atom is positioned at the centre of the octahedral cavity, sharing twelve Br atoms. Conversely, each Li and Ga atom is positioned at the centre of their respective octahedra, coordinated with six (Br/I) atoms [15]. Detailed structural analysis reveals that the Li(Br/I)₆ and Ga(Br/I)₆ octahedra are alternately arranged along the [001], [100], and [010] directions. The volume optimization curve of Rb₂LiGa(Br/I)_6, shown in Fig. 2 (a, b), is used to calculate the lattice parameters of these systems. We have calculated the tolerance factor (τ) as follows:

The τ values for Rb₂LiGa(Br/I)₆ are listed in Table 1, demonstrating their stability in the cubic phase [16]. Additionally, the charge density of Rb₂LiGa(Br/I)₆ along the (001) plane is illustrated in Fig. 2 (c, d).

Table 1

The estimated values of the tolerance factor (τ), lattice parameter (a), bond lengths (r), and bond angles of Rb₂LiGa(Br/I)₆.
Systems	a (Ȧ)	τ	r (Rb -Br/I) (Ȧ)	r (Li-Br/I) (Ȧ)	r (Ga-Br/I) (Ȧ)	Bond angles (⁰)
Rb₂LiGaBr₆	11.13	0.95	3.63	2.57	2.87	α = β = γ = 90⁰
Rb₂LiGaI₆	11.17	0.94	3.67	2.59	2.91	α = β = γ = 90⁰

To examine how the materials behave under external forces, we evaluated the mechanical properties of the Rb₂LiGa(Br/I)₆ double perovskites (DP). Initially, we determined the three main elastic constants: C₄₄, C₁₂, and C₁₁ as detailed in Table 2. We found that the elastic constants for Rb₂LiGaBr₆ are greater than those for Rb₂LiGaI₆. Moreover, the elastic constants for both Rb₂LiGa(Br/I)₆ met the essential criteria: (a) C₁₁ > B > C₁₂, (b) C₁₁ + 2C₁₂ > 0, (c) C₁₁ and C₄₄ > 0, and (d) C₁₁ – C₁₂ > 0 [32]. Using these elastic constants in equations (9) through (14), we were unable to calculate the other mechanical parameters for Rb₂LiGa(Br/I)₆. The final numerical values are presented in Table 2.

$$\:B=\frac{{C}_{11}+2{C}_{12}}{3}$$

$$\:E=\frac{9BG}{3B+G}$$

$$\:{G}_{V}=\frac{{C}_{11}-{C}_{12}+3{C}_{44}}{5}$$

$$\:{G}_{R}=\frac{5{C}_{44}({C}_{11}-{C}_{12})}{4{C}_{44}+3({C}_{11}-{C}_{12})}$$

$$\:G=\frac{{G}_{V}+{G}_{R}}{2}$$

$$\:A=\frac{2{C}_{44}}{{C}_{11}-{C}_{12}}$$

Table 2 shows that Rb₂LiGaI₆ exhibits lower values for the bulk modulus (B), shear modulus (G), and Young's modulus (E) compared to Rb₂LiGaBr₆, indicating that Rb₂LiGaI₆ is more compressible under applied stress. It is noted that ductile materials typically have Pugh's ratio (B/G) and Poisson's ratio (ν) greater than 1.71 and 0.23, respectively [33]. The numerical values for both the B/G and Poisson's ratio (ν) for Rb₂LiGa(Br/I)₆ are greater than the cutoff which suggest that these materials display ductile characteristics. Moreover, the anisotropic factor (A) for both materials is not precisely 1, signifying their anisotropic nature [34].

Table 2

Mechanical parameter values obtained for Rb₂LiGa(Br/I)₆.
Materials	C₄₄ (GPa)	C₁₂ (GPa)	C₁₁ (GPa)	E (GPa)	B (GPa)	G (GPa)	A	B/G	V
Rb₂LiGaBr₆	24.33	7.050	67.14	64.82	29.15	27.27	0.76	1.87	0.27
Rb₂LiGaI₆	17.13	6.41	55.76	49.98	22.26	23.31	0.68	1.74	0.26

To evaluate thermal stability, we computed the root mean square deviation (RMSD) over time (in femtoseconds) using AIMD simulations. Ideally, the RMSD should remain at zero, but due to statistical uncertainties, some nonzero RMSD values are observed. Figure 3(a) and 3(b) illustrate these fluctuations in RMSD for both Rb₂LiGaBr₆ and Rb₂LiGaI₆. Despite these fluctuations, the minimal RMSD variations over time indicate that both systems are thermodynamically stable.

3.2 Electronic properties

Investigating electronic energy band dispersion is essential for assessing a material's suitability for photovoltaic and thermoelectric technologies. Figure 4(a) and 4(b) display the energy band dispersion spectra for Rb₂LiGa(Br/I)₆. Both systems exhibit similar band structures, likely due to the comparable electronic configurations of the elements in Rb₂LiGa(Br/I)₆. The valence band maxima and conduction band minima occur at the same symmetry point (┌). The fundamental energy band gap in these materials is direct, with estimated values of 1.19 eV for Rb₂LiGaBr₆ and 1.13 eV for Rb₂LiGaI₆. These suitable direct band gap values suggest that Rb₂LiGa(Br/I)₆ could be promising candidates for solar cell applications.

To gain a detailed understanding of the electronic properties of Rb₂LiGa(Br/I)₆, we have characterized their total and partial density of states, as shown in Fig. 5 (a-d). The valence state is primarily composed of Br-p and Ga-s orbitals in both systems. However, the conduction band minimum of Rb₂LiGaBr₆ is shaped by Br-p and Ga-s orbitals, while the conduction band of Rb₂LiGaI₆ is mainly influenced by I-p orbitals.

3.3 Optical properties

Studying optical properties is crucial for assessing a material's effectiveness in solar cells. Therefore, the optical characteristics of Rb₂LiGa(Br/I)₆ are analyzed across the 0–8 eV energy range. The optical absorption, conductivity, and reflectivity of a material can be determined from its frequency-dependent complex dielectric constant ε, which describes how the material responds to external electromagnetic radiation. The ɛ can be written as follows

ε(ω) = ε₁ (ω) + iε₂(ω) (9)

where ε₁ (ω) and ε₂(ω) are the real and imaginary parts of the dielectric constant, respectively. The imaginary part of the dielectric constant represents the electronic transition from the valence band to the conduction band, which can be expressed as [19]:

$$\:{\epsilon\:}_{2}\left(\omega\:\right)\:=\:\frac{8}{2\pi\:{\omega\:}^{2}\:}*{\sum\:P}_{nn{\prime\:}}^{2}\:\frac{{ds}_{k}}{{\nabla\:\omega\:}_{nn{\prime\:}}}$$

Here, P_nn′(k) represents the dipole matrix corresponding to the transition from the valence state n to the conduction state n′. On the other hand, S_k is the energy surface with a constant value. The frequency ω_nn′ arises from the energy difference between E_n′(k) and E_n(k).

The real part of the dielectric constant, ε₁(ω), can be derived using the Kramers-Kronig relation [20]

$$\:\:\:\:\:\:\:\:{\epsilon\:}_{1}\left(\omega\:\right)\:=\:\frac{2}{\pi\:\:}*\underset{0}{\overset{\alpha\:}{\int\:}}\omega\:{\prime\:}\:{\epsilon\:}_{2\:\:}\left(\omega\:\right)\:\frac{d\omega\:{\prime\:}}{{(\omega\:}^{2}\:-{\omega\:{\prime\:}}^{2})}$$

The real dielectric constant is key to comprehending a system's defect tolerance. Figure 6(a) illustrates the energy dependence of ϵ₁(ω). Notably, the static dielectric constants ϵ₁(0) for Rb₂LiGaBr₆ and Rb₂LiGaI₆ are 4.73 and 5.91, respectively. These values are consistent with Penn’s model, validating the accuracy of our calculations [21]. The relatively high $\:{\epsilon\:}_{1}\left(0\right)\:$values suggest that these materials exhibit good defect tolerance.

Furthermore, we determined the imaginary dielectric constant spectra as a function of energy, as depicted in Fig. 6(b). The absorption edges for Rb₂LiGaBr₆ and Rb₂LiGaI₆ are 1.24 eV and 1.16 eV, respectively. These absorption edges result from electronic transitions from Br/I-p to Ga-s orbitals. Our detailed analysis shows a sharp rise in $\:{\epsilon\:}_{2}$ beyond the optical band gap, reaching a peak at a specific photon energy. After this peak, $\:{\epsilon\:}_{2}$ decreases, a typical behavior of semiconducting materials.

We also analyzed the absorption spectra as a function of photon energy. Both materials show significant absorption coefficients throughout the UV-Visible range, extending beyond the optical band gap [25]. The spectra display multiple absorption peaks associated with various electronic transitions. Photon absorption creates electron-hole pairs, thereby increasing optical conductivity. Figure 6(c) illustrates how optical conductivity changes with photon energy, showing a pattern similar to that of absorption behaviour [22]. This correlation arises because absorbed photons generate charge carriers, leading to increased conductivity.

Lastly, we assessed the reflectivity as a function of incoming energy, depicted in Fig. 6(d). Both materials exhibit low reflectivity throughout the entire energy range up to 5 eV. Overall, the high absorption coefficients, strong conductivity, and low reflectivity indicate that Rb₂LiGa(Br/I)₆ could be excellent candidates for photovoltaic applications.

3.4 Thermoelectric properties

Currently, most energy needs are met by non-renewable and environmentally harmful sources. As a result, there is an urgent need to explore renewable and eco-friendly energy alternatives. In this context, thermoelectric technology is promising as it converts waste heat into electricity. The effectiveness of a thermoelectric material is evaluated using the thermoelectric figure of merit ZT = S²σT / (k_e + k_l), where S represents the Seebeck coefficient, σ is the electrical conductivity, T is the temperature, k_e is the electronic part of the thermal conductivity, and k_l is the lattice part of the thermal conductivity. A high ZT value, approaching unity, is characteristic of a good thermoelectric material. This can be achieved when the material has a high Seebeck coefficient (S) and electrical conductivity (σ), while maintaining low thermal conductivity (k).

The relationship between S and T is shown in Fig. 7(a). It is evident from Fig. 7(a) that S decreases as the temperature increases. This decline is attributed to the increase in carrier density with rising temperature. Moreover, the positive Seebeck coefficient (S) confirms that both systems are p-type. Additionally, both compounds exhibit high values of S across the entire temperature range, which is advantageous for potential thermoelectric applications. Figure 7(b) illustrates how electrical conductivity changes with temperature. It shows a linear increase in conductivity with rising temperature, which is attributed to the increase in carrier concentration as the temperature rises. Significantly, the high carrier conductivity in both Rb₂LiGa(Br/I)₆ indicates their potential as effective thermoelectric materials. Figure 7(c) illustrates the variation in thermal conductivity for Rb₂LiGaBr₆. The thermal conductivity of these compounds increases linearly with temperature, in accordance with Wiedemann-Franz law. Notably, both systems exhibit exceptionally low thermal conductivity. Finally, Fig. 7(d) shows the variation of ZT with temperature. It is observed that ZT decreases gradually with rising temperature. Despite this, ZT remains high throughout the entire temperature range, indicating that Rb₂LiGa(Br/I)₆ could be promising materials for thermoelectric applications.

3.5 Modelling of heterojunction solar cell

Motivated by higher absorption coefficient and optical conductivity of Rb₂LiGa(BrI)_{6 ,} we have Modelled Au/Cu₂O/Rb₂LiGa(Br/I)₆/TiO₂/FTO solar cell. Figure 8 exhibited the p-i-n structure Rb₂LiGa(Br/I)₆ based solar cell. In this structure, Au works as a back contact, Cu₂O as a hole transparent layer, Rb₂LiGa(BrI)₆ is the absorber layer, TiO₂ is the hole blocking layer and FTO is the front contact. The input parameters used for simulation is summarized in Table 3. Here, $\:\chi\:$ represents the electron affinity, E_g is the band gap, $\:{\epsilon\:}_{\text{r}}$ represents the dielectric permittivity, $\:{\mu\:}_{\text{e}}$ is the electron mobility, $\:{\mu\:}_{\text{p}}$ is the hole mobility, $\:{\text{N}}_{\text{C}}\:\:\text{r}\text{e}\text{p}\text{r}\text{e}\text{s}\text{n}\text{e}\text{t}\text{s}\:\text{t}\text{h}\text{e}\:$conduction band density of states, $\:{\text{N}}_{\text{V}}$represents the valence band density of states, $\:{\text{N}}_{\text{D}}$ is the donor concentration, and $\:{\text{N}}_{\text{T}}$ is the defect concentration. All simulation were performed at 300 K with illumination $\:\:of\:1000{\text{W}\text{m}}^{-2}$ and at AM 1.5G light spectrum. It is worthy to be mentioned here that Rb₂LiGaI₆/TiO₂ and Rb₂LiGaI₆ /Cu₂O interface layers are used to perfect a more accurate perovskite solar cell. We have considered that both interfaces have neutral defect densities of $\:{10}^{14}{\text{\:}\text{c}\text{m}}^{-2}$.The back contact Au has a work function value of $\:5.1\text{e}\text{V}\:\text{i}\text{s}\:\text{c}\text{o}\text{n}\text{s}\text{i}\text{d}\text{e}\text{r}\text{e}\text{d}.\:$

Figure 9 presented the J-V spectrum of Au/Cu₂O/Rb₂LiGa(BrI)₆/TiO₂/FTO solar cell. The results are summarised Table 4. We can see that Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO is having higher efficiency than Au/Cu₂O/Rb₂LiGaBr₆/TiO₂/FTO. Also, the obtained efficiency of Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO is higher than the previous report [39]. Thus, this study will provide the guidelines for construction of double perovskites based efficient solar cell.

Table 3

Used parameters on SCAPS simulation
Properties	TiO₂ [26 − 3]	$\:{\mathbf{C}\mathbf{u}}_{2}\mathbf{O}$ [26, 30,31 ]	Rb₂LiGaBr₆	Rb₂LiGaI₆
thickness (nm)	30 nm	30 nm-	600nm	700nm
$\:{E}_{\text{g}}\left(\text{e}\text{V}\right)$	3.2	$\:2.17$	1.19	1.13
$\:\chi\:\left(\text{e}\text{V}\right)$	4	3.0	3.87	3.94
$\:{\epsilon\:}_{\text{r}}\left(\text{e}\text{V}\right)$	100	7.5	4.1	4.13
$\:{N}_{\text{v}}\left({\text{c}\text{m}}^{-3}\right)$	1x10²¹	$\:1.1\times\:{10}^{19}$	$\:2.21\times\:{10}^{18}$	$\:2.23\times\:{10}^{18}$
$\:{N}_{\text{c}}\left({\text{c}\text{m}}^{-3}\right)$	2x10²¹	$\:2\times\:{10}^{18}$	$\:5.82\times\:{10}^{18}$	$\:5.81\times\:{10}^{18}$
$\:{\mu\:}_{e}\left({\text{\:}\text{c}\text{m}}^{2}/\left(\text{V}\text{s}\right)\right)$	0.006	200	100	100
$\:{\mu\:}_{\text{p}}\left({\text{c}\text{m}}^{2}/\left(\text{V}\text{s}\right)\right)$	0.006	80	100	100
$\:{N}_{\text{D}}\left({\text{c}\text{m}}^{-3}\right)$	0	o	0	0
$\:{N}_{\text{A}}\left({\text{c}\text{m}}^{-3}\right)$	5.06$\:\times\:{10}^{19}$	$\:1\times\:{10}^{15}$	10X10¹⁰	10X10¹⁰
$\:{N}_{\text{t}}\left({\text{c}\text{m}}^{-3}\right)$	$\:10\times\:{10}^{15}$	$\:1\times\:{10}^{14}$	10X10¹⁰	10X10¹⁰

Table 4

The obtained solar cell parameters.
Solar cell	J_sc (mA/cm²)	V_oc (v)	FF	PCE (%)
Au/Cu₂O/Rb₂LiGaBr₆/TiO₂/FTO	32.1	1.06	78	23.1
Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO	32.2	1.19	79	25.6
Previour report
TCO/IDL1/Cs2AuBiCl6/IDL2/Cu2O	31.69	0.86	81.17	22.18

In conclusion, lead-free double perovskite halides are becoming prominent in optoelectronics because of their remarkable electronic, optical, and transport properties. These materials are stable, easy to synthesize, and offer a broad range of potential applications. In this study, we delve into the fascinating properties of Rb₂LiGa(Br/I)₆, focusing on its structural, electronic, optical, transport, and photovoltaic characteristics. Our results show that Rb₂LiGaBr₆ and Rb₂LiGaI₆ have band gaps of 1.19 eV and 1.13 eV, respectively, highlighting their potential for a range of applications. Both materials exhibit exceptional optical properties, including high absorption coefficients and optical conductivity, coupled with low reflectivity throughout the UV-visible spectrum, positioning them as strong candidates for solar cell technologies. Furthermore, Rb₂LiGa(Br/I)₆ exhibits remarkable thermoelectric properties, with high figure-of-merit (ZT) values between 200K and 800K, highlighting their potential as highly effective thermoelectric materials. Therefore, this study offers valuable insights for developing efficient double perovskite-based solar cells.

Encouraged by the superior absorption and optical conductivity of Rb₂LiGa(Br/I)₆, we modeled an Au/Cu₂O/Rb₂LiGa(Br/I)₆/TiO₂/FTO solar cell. Our findings reveal that the Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO configuration delivers higher efficiency compared to the Au/Cu₂O/Rb₂LiGaBr₆/TiO₂/FTO setup and exceeds previous efficiency reports. This study establishes a new benchmark for the development of high-performance double perovskite-based solar cells and paves the way for future advancements in this field.

Conflict of interest

The author does not have any further interest.

Acknowledgement

Dr. Mukaddar Sk acknowledge SRMIST for providing HPCC computational facility.

Data Availability

The data will be made available on reasonable request. The data will be available from Mukaddar Sk.

Author contribution

Dr. Mukaddar Sk has given the idea, performed the theoretical calculation and written the manuscript. Dr. M. T. Islamhas done the correction of original manuscript.

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No competing interests reported.

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A comprehensive first-principles study exploring the structural, electronic, optical, transport, and photovoltaic properties of Rb₂LiGa (Br/I)₆ utilizing DFT and SCAPS-1D simulations

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Abstract

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1. Introduction

2. Computational details

3. Results and Discussion

3.1. Structural properties, mechanical and thermodynamic properties

3.2 Electronic properties

3.3 Optical properties

3.4 Thermoelectric properties

3.5 Modelling of heterojunction solar cell

Conclusion

Declarations

References

Additional Declarations

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