Currently, double halide perovskites are being explored as viable replacements for fossil fuels in green energy sectors such as photovoltaic and thermoelectric applications. Using the full-potential linearized augmented-plane-wave (FP-LAPW) method within the WIEN2k implementation of density functional theory (DFT), this research provides an extensive characterization of A2AlTlI6 (A = K, Rb, and Cs). The study focuses on their structural, mechanical, optoelectronic, and thermoelectric properties. Structural optimization was performed using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) functional, while transport coefficients were calculated using the BoltzTraP code. The dynamic stability of these compounds is confirmed by the absence of negative frequencies in the phonon dispersion relations, while the calculated negative formation energies ensure their thermodynamic stability. Additionally, mechanical stability is verified by satisfying the Born–Huang criteria, and the analysis of the Pugh ratio confirms the ductile nature of these materials. The optimized lattice parameters are 12.21 Å, 12.26 Å, and 12.34 Å for K2AlTlI6, Rb2AlTlI6, and Cs2AlTlI6, respectively. Electronic structure analysis via the Tran–Blaha modified Becke‒Johnson potential (TB-mBJ) potential revealed indirect bandgaps (L-Γ) of 2.27 eV, 2.30 eV, and 2.28 eV for K2AlTlI6, Rb2AlTlI6, and Cs2AlTlI6, respectively, which positions them as ideal candidates for top-cell absorbers in tandem solar cell architectures. Furthermore, these materials exhibit superior optical properties, including high conductivity and strong absorption with low reflectivity (<35%), which positions them as promising candidates for photovoltaic applications. The figure of merit (ZT) at 100 K is 0.12, 0.23, and 0.25 for K2AlTlI6, Rb2AlTlI6, and Cs2AlTlI6, respectively. The Seebeck coefficient analysis reveals p-type conduction for K-based compounds and n-type behavior for Cs-based analogs. These findings suggest that the A2AlTlI6 series holds significant potential for optoelectronic devices, while further strategies, such as doping or vacancy engineering, are recommended to increase their thermoelectric efficiency. Graphical Abstract

First-Principles Investigation of the Structural Stability and Optoelectronic Properties of Thallium-Based Double Perovskites A₂AlTlI₆ (A = K, Rb, Cs)

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发表日期：

2026年3月14日

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Tanvir Ahmed,
Anirbun Paul Arko,
M. Shahidul Islam,
Md.Nahid Hasan Shefat,
Dipannita Basak,
M. Abdur Razzaq

摘要

Currently, double halide perovskites are being explored as viable replacements for fossil fuels in green energy sectors such as photovoltaic and thermoelectric applications. Using the full-potential linearized augmented-plane-wave (FP-LAPW) method within the WIEN2k implementation of density functional theory (DFT), this research provides an extensive characterization of A₂AlTlI₆ (A = K, Rb, and Cs). The study focuses on their structural, mechanical, optoelectronic, and thermoelectric properties. Structural optimization was performed using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) functional, while transport coefficients were calculated using the BoltzTraP code. The dynamic stability of these compounds is confirmed by the absence of negative frequencies in the phonon dispersion relations, while the calculated negative formation energies ensure their thermodynamic stability. Additionally, mechanical stability is verified by satisfying the Born–Huang criteria, and the analysis of the Pugh ratio confirms the ductile nature of these materials. The optimized lattice parameters are 12.21 Å, 12.26 Å, and 12.34 Å for K₂AlTlI₆, Rb₂AlTlI₆, and Cs₂AlTlI₆, respectively. Electronic structure analysis via the Tran–Blaha modified Becke‒Johnson potential (TB-mBJ) potential revealed indirect bandgaps (L-Γ) of 2.27 eV, 2.30 eV, and 2.28 eV for K₂AlTlI₆, Rb₂AlTlI₆, and Cs₂AlTlI₆, respectively, which positions them as ideal candidates for top-cell absorbers in tandem solar cell architectures. Furthermore, these materials exhibit superior optical properties, including high conductivity and strong absorption with low reflectivity (<35%), which positions them as promising candidates for photovoltaic applications. The figure of merit (ZT) at 100 K is 0.12, 0.23, and 0.25 for K₂AlTlI₆, Rb₂AlTlI₆, and Cs₂AlTlI₆, respectively. The Seebeck coefficient analysis reveals p-type conduction for K-based compounds and n-type behavior for Cs-based analogs. These findings suggest that the A₂AlTlI₆ series holds significant potential for optoelectronic devices, while further strategies, such as doping or vacancy engineering, are recommended to increase their thermoelectric efficiency.

Graphical Abstract

First-Principles Investigation of the Structural Stability and Optoelectronic Properties of Thallium-Based Double Perovskites A2AlTlI6 (A = K, Rb, Cs)

摘要

First-Principles Investigation of the Structural Stability and Optoelectronic Properties of Thallium-Based Double Perovskites A₂AlTlI₆ (A = K, Rb, Cs)