First-principles study of photovoltaic and thermoelectric properties of AgBiSCl2

Technical Summary: First-Principles Study of Photovoltaic and Thermoelectric Properties of AgBiSCl2

Problem Statement
Mixed-anion compounds, such as sulfur halides, offer a rich platform for tuning electronic and thermal properties through the diversity of anion types. While AgBiSCl2 has been synthesized and its basic crystal structure and optical properties characterized experimentally, a comprehensive theoretical understanding of the correlation between its crystal structure, chemical bonding, and its coupled photovoltaic and thermoelectric transport properties remains lacking. This gap limits the material's potential application in high-performance energy conversion devices. This study aims to address this by systematically investigating AgBiSCl2 using first-principles calculations to evaluate its viability as a dual-function material for photovoltaics and thermoelectrics.

Methodology
The research employs a multi-scale computational framework based on Density Functional Theory (DFT) and Boltzmann transport theory:

• Electronic Structure & Optics: Calculations were performed using the Vienna ab initio Simulation Package (VASP) with the HSE06 hybrid functional to ensure accurate bandgap prediction. Spin-orbit coupling (SOC) was included due to the presence of heavy elements (Bi, Ag). Optical properties were derived from the complex dielectric function ($\epsilon_1$ and $\epsilon_2$).
• Electrical Transport: The AMSET software package was utilized to calculate electrical transport properties (Seebeck coefficient, electrical conductivity, power factor) by accounting for scattering mechanisms including acoustic deformation potential (ADP), ionized impurity (IMP), and polar optical phonon (POP) scattering.
• Thermal Transport: Lattice thermal conductivity ($\kappa_L$) was calculated using the ALAMODE software package. This involved computing interatomic force constants (IFCs) up to the sixth order using compressive sensing lattice dynamics (CSLD). The study adopted a two-channel transport model, separating heat transport into particle-like ($\kappa_p$) and wave-like/coherent ($\kappa_c$) contributions, governed by the Wigner and Ioffe-Regel limits. Anharmonic lattice dynamics were analyzed via self-consistent phonon (SCPH) theory.

Key Contributions and Results

1. Electronic and Optical Properties:

   • AgBiSCl2 is identified as a direct bandgap semiconductor with a gap of 1.72 eV (HSE06 + SOC). The valence band maximum is dominated by Ag 4d, Cl 3p, and S 3p orbitals, while the conduction band minimum is derived primarily from Bi 6p orbitals.
   • The material exhibits a high static dielectric constant ($\epsilon_1(0) = 5.60$) and strong optical absorption in the ultraviolet region, with coefficients exceeding $1 \times 10^6$ cm$^{-1}$.
   • Theoretical modeling predicts a Spectral-Limited Maximum Efficiency (SLME) of 28.06% for a 3 $\mu$m-thick absorber layer, outperforming several known chalcogenide perovskites.

2. Chemical Bonding and Lattice Dynamics:

   • Analysis of the Electron Localization Function (ELF) and Crystal Orbital Hamilton Population (COHP) reveals that Ag–S and Ag–Cl bonds are relatively weak and possess antibonding character near the Fermi level. In contrast, Bi–S bonds are stronger and directional.
   • This weak bonding leads to local structural softness, particularly around Ag atoms. The Ag atoms exhibit "rattling" vibrations within a flat potential well, significantly enhancing lattice anharmonicity.
   • Phonon dispersion analysis shows mode hardening with increasing temperature and the presence of flat optical branches (30–70 cm$^{-1}$) that facilitate extensive phonon scattering. Avoided crossings between acoustic and optical modes further reduce group velocities.

3. Thermal Transport:

   • The material exhibits extremely low lattice thermal conductivity. At 300 K, the particle-like component ($\kappa_p$) is 0.246 W·m$^{-1}$·K$^{-1}$ and the coherent component ($\kappa_c$) is 0.132 W·m$^{-1}$·K$^{-1}$.
   • The low thermal conductivity is attributed to the Ag-driven rattling vibrations, which reduce phonon lifetimes and group velocities, and the significant contribution of wave-like coherent transport in the low-frequency regime.

4. Thermoelectric Performance:

   • The material demonstrates anisotropic thermoelectric properties. At 700 K, the p-type material achieves a higher Seebeck coefficient, while the n-type shows higher electrical conductivity.
   • The maximum thermoelectric figure of merit (ZT) reaches 0.77 for p-type (along the c-axis) and 0.69 for n-type (along the a-axis) at 700 K.

Significance and Claims
The paper posits that AgBiSCl2 is a promising multifunctional material capable of serving in both photovoltaic and thermoelectric applications. Its significance lies in the unique interplay between its electronic structure (suitable for solar absorption) and its specific lattice dynamics (Ag rattling leading to ultra-low thermal conductivity).

The authors conclude that while the material shows excellent theoretical potential for photoelectric conversion and moderate thermoelectric performance, its ZT value has not yet exceeded 1, which limits immediate practical application. Consequently, the study suggests that future optimization through strain engineering, band engineering, and doping is necessary to further enhance the thermoelectric figure of merit. The work provides a theoretical foundation for designing similar mixed-anion systems for energy conversion devices.