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First-principles many-body study for electronic, optical, and excitonic properties of RbTlCl3 perovskite for solar cells
Authors:
Siddharth,
Vinod Kumar Solet,
Sudhir K. Pandey
Abstract:
We present a detailed many-body ab initio study of the valence-skipper RbTlCl$_{3}$ perovskite compound for photovoltaic (PV) applications. The electronic and optical properties, both with and without spin-orbit coupling, have been calculated using density functional theory (DFT) and many-body excited-state calculations. The band gap, which is indirect in nature, is found to be 0.95 eV and 0.89 eV…
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We present a detailed many-body ab initio study of the valence-skipper RbTlCl$_{3}$ perovskite compound for photovoltaic (PV) applications. The electronic and optical properties, both with and without spin-orbit coupling, have been calculated using density functional theory (DFT) and many-body excited-state calculations. The band gap, which is indirect in nature, is found to be 0.95 eV and 0.89 eV from PBE and PBEsol, respectively. The optical properties have been computed using four different approximations: independent particle approximation (IPA), IPA with scissor correction (IQPA), random phase approximation for local-field effects (LFEs), and the Bethe-Salpeter equation (BSE). The estimated highest value of the imaginary part of the dielectric function using IQPA is 7 at 2 eV, which slightly decreases to 5.7 due to LFEs. Within BSE, the peak value is obtained to be maximum at 1.6 eV with a magnitude of 10.8, which indicates the strong excitonic effect below the optical gap. Large number of bright and dark bound excitons are found, where the binding energies of four main bound bright excitons are found in the range of 299-350 meV. The exciton amplitude in both reciprocal and real space is analyzed. The main bound bright exciton is localized in the reciprocal space, while this exhibits a delocalized nature in real space. The BSE predicts a highest absorption coefficient of 3.6 $\times$ $10^{6}$ cm$^{-1}$ at 1.7 eV, while a minimum reflectivity in the active region of the solar energy spectrum is obtained to be around 2.7\%. Finally, the solar efficiency has been estimated using the spectroscopic limited maximum efficiency approach and obtained highest value is 15.5% at a thickness of 0.5 $μ$m. These findings reveal a significant excitonic effect in the absorption spectra of RbTlCl$_{3}$ and highlight its potential as a promising material for single-junction thin-film solar cells.
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Submitted 22 August, 2025;
originally announced August 2025.
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Significant first-principles electron-phonon coupling effects in the LiZnAs and ScAgC half-Heusler thermoelectrics
Authors:
Vinod Kumar Solet,
Sudhir K. Pandey
Abstract:
The half-Heusler (hH) compounds are currently considered promising thermoelectric (TE) materials due to their favorable thermopower and electrical conductivity. Accurate estimates of these properties are therefore highly desirable and require a detailed understanding of the microscopic mechanisms that govern transport. To enable such estimations, we carry out comprehensive first-principles computa…
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The half-Heusler (hH) compounds are currently considered promising thermoelectric (TE) materials due to their favorable thermopower and electrical conductivity. Accurate estimates of these properties are therefore highly desirable and require a detailed understanding of the microscopic mechanisms that govern transport. To enable such estimations, we carry out comprehensive first-principles computations of one of the primary factors limiting carrier transport, namely the electron-phonon ($e-ph$) interaction, in LiZnAs and ScAgC. Our study first investigates their electron and phonon dispersions and then examines the temperature-induced renormalization of the electronic states. We then solve the Boltzmann transport equation (BTE) under multiple relaxation-time approximations (RTAs) to evaluate the carrier transport properties. Phonon-limited electron and hole mobilities are comparatively assessed using the linearized self-energy and momentum RTAs (SERTA and MRTA), and the exact or iterative BTE (IBTE) solutions within $e-ph$ coupling. Electrical transport coefficients for TE performance are also comparatively analyzed under the constant RTA (CRTA), SERTA, and MRTA schemes. The lattice thermal conductivity, determined from phonon-phonon interaction, is further reduced through nanostructuring techniques. The bulk LiZnAs (ScAgC) compound achieves the highest figure of merit ($zT$) of 1.05 (0.78) at 900 K with an electron doping concentration of 10$^{18}$ (10$^{19}$) cm$^{-3}$ under the MRTA scheme. This value significantly increases to 1.53 (1.0) for a 20 nm nanostructured sample. The remarkably high $zT$ achieved through inherently present phonon-induced electron scattering effects, combined with grain-boundary engineering, opens a promising path for discovering highly efficient and accurate next-generation hH TEs.
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Submitted 4 April, 2026; v1 submitted 16 June, 2025;
originally announced June 2025.
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Many-body \textit{ab initio} study of quasiparticles, optical excitations, and excitonic properties in LiZnAs and ScAgC for photovoltaic applications
Authors:
Vinod Kumar Solet,
Sudhir K. Pandey
Abstract:
Using first-principles density-functional theory and many-body excited-state calculations, we study the quasiparticle band structure, optical and excitonic properties of two half-Heusler (HH) compounds, namely LiZnAs and ScAgC, for photovoltaic (PV) applications. Our results reveal a direct bandgap semiconducting behavior in LiZnAs (ScAgC) with a value of 1.5 (1.0) eV under an accurate G$_0$W$_0$…
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Using first-principles density-functional theory and many-body excited-state calculations, we study the quasiparticle band structure, optical and excitonic properties of two half-Heusler (HH) compounds, namely LiZnAs and ScAgC, for photovoltaic (PV) applications. Our results reveal a direct bandgap semiconducting behavior in LiZnAs (ScAgC) with a value of 1.5 (1.0) eV under an accurate G$_0$W$_0$ calculation. The highest value of the imaginary part of dielectric function is found as 52 (87), 77 (87), 88 (91) using the independent-quasiparticle approximation, local field effects in random-phase approximation, and electron-hole interaction in the Bethe-Salpeter equation, respectively. Both materials demonstrate a high refractive index, high absorption coefficients (1.2-1.6 $\times 10^6 cm^{-1}$), and low reflectivity (< 40%) in active region of the solar spectrum. The triply degenerate bright excitons (exciton A) at the main absorption peak and a considerable number of bright excitonic states in the visible region, are observed; however, the excitons oscillator strength are comparatively weaker in ScAgC than in LiZnAs. We further discuss the exciton character contributing to intense optical interband transitions and reveal that direct band gap is associated to the loosely bound exciton A state with binding energy of 45 (56) meV in LiZnAs (ScAgC). Exciton A is found to be highly localized (delocalized) in momentum (real) space, indicating the presence of Mott-Wannier type excitons at bandgap. Finally, we assess the solar efficiencies using the spectroscopic limited maximum efficiency (SLME) model and find SLME values of 32% for LiZnAs and 31% for ScAgC at a 0.4 $μ$m thin-film thickness. These findings highlight the significant role of excitons in solar energy absorption process and also suggest that both are highly suitable candidates for single-junction thin-film solar cells.
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Submitted 4 April, 2026; v1 submitted 1 April, 2025;
originally announced April 2025.
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Band gap renormalization, carrier mobility, and transport in Mg$_{2}$Si and Ca$_{2}$Si: \textit{Ab initio} scattering and Boltzmann transport equation study
Authors:
Vinod Kumar Solet,
Sudhir K. Pandey
Abstract:
We perform first-principles electron-phonon interaction (EPI) calculations based on many-body perturbation theory to study the temperature-dependent band-gap and charge-carrier transport properties for Mg$_{2}$Si and Ca$_{2}$Si using the Boltzmann transport equation (BTE) under different relaxation-time approximations (RTAs). For a PBE band gap of 0.21 (0.56) eV in Mg$_{2}$Si (Ca$_{2}$Si), a zero-…
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We perform first-principles electron-phonon interaction (EPI) calculations based on many-body perturbation theory to study the temperature-dependent band-gap and charge-carrier transport properties for Mg$_{2}$Si and Ca$_{2}$Si using the Boltzmann transport equation (BTE) under different relaxation-time approximations (RTAs). For a PBE band gap of 0.21 (0.56) eV in Mg$_{2}$Si (Ca$_{2}$Si), a zero-point renormalization correction of 29-33 (37-51) meV is obtained using various approaches, while the gap at 300 K is 0.15-0.154 (0.46-0.5) eV. The electron mobility ($μ_{e}$), with a detailed convergence study at 300 K, is evaluated using linearized (self-energy and momentum RTA, or SERTA and MRTA) and iterative BTE (IBTE) solutions. At 300 K, the $μ_{e}$ values are 351 (100), 573 (197), and 524 (163) cm$^{2}V^{-1}s^{-1}$ from SERTA, MRTA, and IBTE, respectively, for Mg$_{2}$Si (Ca$_{2}$Si). SERTA (MRTA) provides results in better agreement with IBTE at higher (lower) temperatures, while SERTA-derived $μ_{e}$ closely matches experimental $μ_{e}$ values for Mg$_{2}$Si. Thermoelectric (TE) transport coefficients significantly influenced by the choice of RTA, with SERTA and MRTA yielding improved agreement with experimental results compared to constant RTA (CRTA) for Mg$_{2}$Si over an electron concentration range of $10^{17}$ to $10^{20}$ cm$^{-3}$. The lattice thermal conductivity ($κ_{ph}$) at 300 K due to phonon-phonon interactions is estimated to be 22.7 (7.2) W m$^{-1}K^{-1}$ for Mg$_{2}$Si (Ca$_{2}$Si). The highest calculated figure of merit (zT) under CRTA is 0.35 (0.38), which decreases to 0.08 (0.085) when EPI is included using MRTA. This study clearly identifies the critical role of EPI in accurate transport predictions of TE silicides. Finally, we explore strategies to enhance zT by reducing $κ_{ph}$ through nanostructuring and mass-difference scattering.
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Submitted 4 April, 2026; v1 submitted 27 January, 2025;
originally announced January 2025.
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Understanding the transport behaviour of PbSe: A combined experimental and computational study
Authors:
Isha Sihmar,
Abhishek Pandey,
Vinod Kumar Solet,
Neeru Chaudhary,
Navdeep Goyal,
Sudhir K. Pandey
Abstract:
Lead chalcogenides are the promising thermoelectric (TE) materials having narrow band gap. The present work investigates the TE behaviour of PbSe in the temperature range 300-500 K. The transport properties of the sample have been studied using the Abinit and BoltzTrap code. The experimentally observed value of \textit{S} at 300 and 500 K is found to be $\sim$ 198 and 266 $μ$V K$^{-1}$, respective…
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Lead chalcogenides are the promising thermoelectric (TE) materials having narrow band gap. The present work investigates the TE behaviour of PbSe in the temperature range 300-500 K. The transport properties of the sample have been studied using the Abinit and BoltzTrap code. The experimentally observed value of \textit{S} at 300 and 500 K is found to be $\sim$ 198 and 266 $μ$V K$^{-1}$, respectively. The rate of increase in \emph{S} from 300 to 460 (460 to 500) K is found to be $\sim$ 0.4 (0.09). The temperature dependent electrical conductivity \textit{($σ$)} shows the increasing trend, with values of $\sim $ 0.35 $\times $ 10$^{3}$ and $\sim$ 0.58 $\times$ 10$^{3}$ $Ω$$^{-1}$ m$^{-1}$ at 300 and 500 K, respectively. Further, the value of thermal conductivity \textit{($κ$)} at 300 (500) K is found to be 0.74 (1.07) W m$^{-1}$ K$^{-1}$. The value of \textit{$κ$} is found to be increasing upto 460 K and then starts decreasing. The dispersion plot indicates that PbSe is a direct band gap semiconductor with band gap value of 0.16 (0.27) eV considering spin-orbit coupling (without SOC). The partial density of states (PDOS) plot shows that Pb 6p and Se 4p states have a major contribution in the transport properties. The observed and calculated values of \textit{S} gives a good match for SOC case. The calculated \textit{$σ$} and electronic part of thermal conductivity (\textit{$κ{_e}$}) gives good match with the experimental data. The maximum power factor (PF) value of $\sim$ 4.3 $\times$ 10$^{-5}$ W/mK$^{2}$ is observed at 500 K. This work helps in understanding the TE behaviour of PbSe through a novel and insightful alliance of experimental measurements and DFT approach.
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Submitted 8 August, 2024; v1 submitted 7 August, 2024;
originally announced August 2024.
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Mg$_{2}$Si and Ca$_{2}$Si semiconductors for photovoltaic applications: Calculations based on density-functional theory and the Bethe-Salpeter equation
Authors:
Vinod Kumar Solet,
Sudhir K. Pandey
Abstract:
We conduct a comprehensive assessment of the electronic and optical properties, as well as photovoltaic (PV) performance parameters for Mg$_{2}$Si and Ca$_{2}$Si using density-functional theory and Bethe-Salpeter equation (BSE) based methods. The band-gap for Mg$_{2}$Si (Ca$_{2}$Si) is found to be in the range of 0.25-0.6 (0.57-0.96) eV when PBE, PBEsol and mBJ functionals are used. In the indepen…
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We conduct a comprehensive assessment of the electronic and optical properties, as well as photovoltaic (PV) performance parameters for Mg$_{2}$Si and Ca$_{2}$Si using density-functional theory and Bethe-Salpeter equation (BSE) based methods. The band-gap for Mg$_{2}$Si (Ca$_{2}$Si) is found to be in the range of 0.25-0.6 (0.57-0.96) eV when PBE, PBEsol and mBJ functionals are used. In the independent-particle approximation (IPA), the real and imaginary parts of dielectric function show maximum values of 50 (16.3) at 2.6 (1.0) eV and 61 (16.2) at $\sim$3.24 (3.4) eV, respectively. Within BSE, these respective values change to 59 (17) at 2.5 (0.86) eV and 65 (16.6) at 2.68 (3.1) eV. The excitonic effect is found to be crucial in understanding the experimental optical spectra of Mg$_{2}$Si. However, this effect is relatively weaker in Ca$_{2}$Si. Present study highlights the importance of different levels of theoretical approximations for obtaining the optical spectroscopy data of silicides with a high level of accuracy. Finally, we have evaluated PV efficiency by using spectroscopic limited maximum efficiency (SLME) calculation. On the top of radiative recombination, we have also incorporated non-radiative carrier recombination at a defect trap state via Shockley-Read-Hall (SRH) mechanism to evaluate the efficiency. Among the studied defects, the interstitial Mg (Si) is identified as the most stable in Mg$_{2}$Si (Ca$_{2}$Si) and this provides SRH lifetime of 2 $μs$ (11.3 $ms$). The estimated maximum SLME using BSE absorption spectrum is 1.3 (31.2)\%, which decreases to 1.2 (28.5)\% due to SRH recombination. The present study suggests that Ca$_{2}$Si (Mg$_{2}$Si) is a potential candidate for single-junction (bottom cell in multi-junction) thin-film PV devices.
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Submitted 1 March, 2025; v1 submitted 10 January, 2024;
originally announced January 2024.
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Ab-initio study of phononic thermal conduction in ScAgC half-Heusler
Authors:
Vinod Kumar Solet,
Sudhir Kumar Pandey
Abstract:
We present a first-principles lattice calculations to comprehend the thermal expansion $α(T)$ and lattice thermal conductivity $κ_{ph}$ of ScAgC. The obtained positive frequencies of phonon dispersion shows the dynamical stability of ScAgC in FCC structure. The estimated $α(T)$ from quasi-harmonic approximation (QHA) at 300(1200) K is $\sim$4(4.6)$\times$$10^{-6}$ K$^{-1}$. The predicted value of…
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We present a first-principles lattice calculations to comprehend the thermal expansion $α(T)$ and lattice thermal conductivity $κ_{ph}$ of ScAgC. The obtained positive frequencies of phonon dispersion shows the dynamical stability of ScAgC in FCC structure. The estimated $α(T)$ from quasi-harmonic approximation (QHA) at 300(1200) K is $\sim$4(4.6)$\times$$10^{-6}$ K$^{-1}$. The predicted value of total $κ_{ph}$ from phonon-phonon interaction (PPI) at 300(1200) K is $\sim$7.4(1.8) Wm$^{-1}$K$^{-1}$. The highest group velocity for acoustic $\&$ optical branches (AB $\&$ OB) is $\sim$6.7 and $\sim$3.5 km/s, respectively. The predicted average phonon lifetime ($τ_λ$) for AB(OB) is $\sim$2.5(1.65) ps at 300 K, whereas it is $\sim$0.6(0.4) ps at 1200 K. The estimated highest heat capacity ($C_λ$) at 200 K for AB(OB) is $\sim$23.5 (19.5) meV/K. We fitted the equation $A_κ$T$^{-x_κ}$($A_τ$T$^{-x_τ}$) in the $κ_{ph}$($τ_λ$) curve to gain a thorough understanding of temperature-dependent $κ_{ph}$ trend. The $x_κ$ for total branches is calculated to be $\sim$1.02. The $x_τ$ value due to total AB(OB) is estimated to be $\sim$1.04(1.02), while it is $\sim$1.03 for total branches. The calculated $x_κ$ for total AB(OB) is $\sim$1.04(0.95), implying that AB contributes more to the total $κ_{ph}$. This research could be important for enhancing the properties of ScAgC regarding thermoelectric and photovoltaic applications.
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Submitted 1 March, 2025; v1 submitted 3 January, 2023;
originally announced January 2023.
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First-principles study of optoelectronic and thermoelectronic properties of the ScAgC half-Heusler compound
Authors:
Vinod Kumar Solet,
Shamim Sk,
Sudhir K. Pandey
Abstract:
Here, we present a theoretical study in the context of photovoltaic (PV) and thermoelectric (TE) applications of ScAgC. The electronic, optical, and thermoelectric properties have been investigated systematically using density functional theory (DFT) and semi-classical Boltzmann transport theory. DFT calculates a direct band gap of 0.47 eV, whereas the $G_{0}W_{0}$ method estimates a band gap of 1…
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Here, we present a theoretical study in the context of photovoltaic (PV) and thermoelectric (TE) applications of ScAgC. The electronic, optical, and thermoelectric properties have been investigated systematically using density functional theory (DFT) and semi-classical Boltzmann transport theory. DFT calculates a direct band gap of 0.47 eV, whereas the $G_{0}W_{0}$ method estimates a band gap of 1.01 eV. We used parabola fitting to estimate the effective mass values for bands B1 to B4 at $Γ$-point, which are -0.087 (-0.075), -0.17 (-0.27), -0.17 (-0.27), and 0.049 (0.058) along the $Γ$-$X$ ($Γ$-$L$) direction, respectively. Furthermore, the optoelectronic properties are calculated and analyzed over an energy range of 0 to 10 eV. The optical conductivity, refractive index, and dielectric function show strong optical transitions in the visible region. The lowest calculated reflectivity is 0.24 at 4.7 eV, and the highest calculated value of the absorption coefficient is $1.7\times10^{6}$ cm$^{-1}$ at $8.5$ eV. At 300 K, we expect a maximum solar efficiency (SLME) of 33\% at a thickness of $1~μ$m. The lattice part of the thermal conductivity shows a maximum value of 3.8 Wm$^{-1}$K$^{-1}$ at $1200$ K. At 1200 K, for electron doping of $3.9\times10^{21}$ cm$^{-3}$, the maximum value of $S^{2}σ/τ$ is $145 \times 10^{14}$ $μ$WK$^{-2}$cm$^{-1}$s$^{-1}$, while for hole doping of $1.5\times10^{21}$ cm$^{-3}$, it is $123 \times 10^{14}$ $μ$WK$^{-2}$cm$^{-1}$s$^{-1}$. The highest $ZT$ at $1200$ K is expected to be $0.53$, whereas the optimal efficiency is predicted to be $8.5\%$ for cold and hot temperatures of $300$ K and $1200$ K, respectively. The collected results suggest that the ScAgC compound is a potential candidate for renewable energy sources such as solar cell and TE applications
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Submitted 4 March, 2025; v1 submitted 12 July, 2022;
originally announced July 2022.