First-principles study of photovoltaic and thermoelectric properties of AgBiSCl2

This work systematically investigates the potential of the hybrid anion semiconductor AgBiSCl2 for photovoltaic and thermoelectric applications, aiming to provide theoretical guidance for high-performance energy conversion devices. Structural analysis reveals favorable ductility and a relatively low Debye temperature. Analysis of interatomic interactions indicates that Ag-S and Ag-Cl bonds are relatively weak, resulting in local structural softness and enhanced lattice anharmonicity. These weak bonds facilitate phonon scattering and give rise to low-frequency localized rattling vibrations primarily associated with Ag atoms, contributing to reduced lattice thermal conductivity. In contrast, Bi-S bonds exhibit stronger, directional interactions, which help stabilize the overall structure. The coexistence of weak bonding and strong lattice coupling enables favorable modulation of thermal transport properties.Optically, AgBiSCl2 possesses a high static dielectric constant and exhibits strong absorption in the ultraviolet region. In terms of thermal transport, phonon spectrum exhibit mode hardening with temperature increasing. The localized Ag vibrations intensify the anharmonicity, reducing phonon lifetimes and group velocities.For electronic transport, the p-type material maintains a higher Seebeck coefficient than the n-type, while the latter shows greater electrical conductivity. At 700 K, the figure of merit reaches 0.77 for p-type and 0.69 for n-type AgBiSCl2, indicating promising high-temperature thermoelectric performance.In summary, AgBiSCl2 exhibits excellent potential for dual photovoltaic and thermoelectric applications. Its unique bonding features and lattice response mechanisms offer valuable insights into designing multifunctional energy conversion materials.

💡 Research Summary

This paper presents a comprehensive first‑principles investigation of the hybrid‑anion semiconductor AgBiSCl₂, focusing on its suitability for both photovoltaic (PV) and thermoelectric (TE) applications. Using density‑functional theory with the HSE06 hybrid functional and spin‑orbit coupling, the authors determine that AgBiSCl₂ is a direct‑gap semiconductor with a band gap of ~1.72 eV. The conduction band is dominated by Bi 6p states, while the valence band derives mainly from Ag 4d, Cl 3p and S 3p orbitals. Structural analysis shows an orthorhombic Cmcm lattice (a = 4.07 Å, b = 14.17 Å, c = 8.69 Å) with good ductility (B/G = 2.19, Poisson’s ratio = 0.30) and a low Debye temperature of 219 K, indicating intrinsic lattice softness.

A key finding is the coexistence of weak Ag–S/Ag–Cl bonds and stronger, more directional Bi–S bonds. The weak Ag bonds generate low‑frequency, localized “rattling” vibrations of Ag atoms, which strongly enhance anharmonicity, shorten phonon lifetimes, and reduce group velocities. Consequently, the lattice thermal conductivity κL is exceptionally low, calculated as 0.246 W m⁻¹ K⁻¹ at 300 K and 0.132 W m⁻¹ K⁻¹ at 700 K. Phonon spectra harden with temperature, further supporting reduced heat transport at elevated temperatures.

Electronic transport is evaluated with the AMSET package, incorporating acoustic‑deformation‑potential (ADP), ionized‑impurity (IMP), and polar‑optical‑phonon (POP) scattering via Matthiessen’s rule. The material can be doped p‑type or n‑type. The p‑type exhibits a higher Seebeck coefficient (≈ ‑250 µV K⁻¹) but lower electrical conductivity, whereas the n‑type shows greater conductivity with a reduced Seebeck value. At 700 K, the thermoelectric figure of merit reaches ZT ≈ 0.77 for p‑type and ZT ≈ 0.69 for n‑type, indicating promising high‑temperature TE performance.

Optically, AgBiSCl₂ possesses a high static dielectric constant (ε₁(0) = 5.60) and strong absorption in the ultraviolet region, with an absorption coefficient exceeding 1 × 10⁶ cm⁻¹. Simulated PV performance for a 3 µm thick absorber predicts a theoretical conversion efficiency of up to 28 %, highlighting its potential as a high‑efficiency UV‑absorbing photovoltaic material.

In summary, the paper demonstrates that AgBiSCl₂’s unique bonding landscape—weak Ag‑X bonds that generate rattling modes combined with robust Bi‑S interactions that maintain structural integrity—enables simultaneous suppression of lattice thermal conductivity and preservation of favorable electronic transport. This dual functionality makes AgBiSCl₂ a compelling candidate for integrated PV‑TE devices, and the authors suggest that experimental synthesis, doping optimization, and device engineering could translate these theoretical advantages into practical energy‑conversion technologies.