Stability Criteria and Optoelectronic Properties of Mg3ZBr3 (Z = As, Sb, Bi) Perovskites for Evaluating the Performance in PIN Photo Diode

TL;DR

This work investigates lead-free Mg$_3$ZBr$_3$ (Z = As,Sb,Bi) halide perovskites as stable optoelectronic materials using first-principles methods. Employing DFT with PBE and HSE06, phonon analyses, and optical/elastic characterizations, the study reveals dynamic stability for Mg$_3$AsBr$_3$ and Mg$_3$SbBr$_3$ in the cubic phase and a tendency toward instability for Mg$_3$BiBr$_3$, with indirect band gaps spanning $E_g^{\mathrm{HSE}}\approx$ 1.5–2.1 eV and strong anharmonic lattice dynamics (large Grüneisen parameters). The materials exhibit moderate mechanical stiffness, sizable dielectric responses, and pronounced light–matter coupling, while device-level drift–diffusion simulations demonstrate tunable near-IR to visible spectral responsivity in Mg$_3$ZBr$_3$ PIN photodiodes. Together, these results position Mg$_3$ZBr$_3$ as promising non-toxic, stable candidates for lead-free photodiodes and photovoltaic applications, providing a framework for future experimental validation of thermal transport and finite-temperature phase behavior. The work highlights how pnictogen substitution modulates lattice dynamics, band structure, and optoelectronic performance in a simple perovskite motif, enabling targeted design of stable, low-toxicity optoelectronic devices.

Abstract

The toxicity and stability issues of lead-based perovskites motivate the search for non-toxic, durable alternatives. This work examines lead-free $\mathrm{Mg_3ZBr_3}$ ($Z=\mathrm{As,Sb,Bi}$) halide perovskites as optoelectronic materials, with emphasis on $\mathrm{Mg_3AsBr_3}$ and $\mathrm{Mg_3SbBr_3}$. First-principles calculations establish cubic $Pm\bar{3}m$ frameworks stabilized by strong Mg--Br linkages, and indirect band gaps of $2.0645\,\mathrm{eV}$ for $\mathrm{Mg_3AsBr_3}$ and $1.6533\,\mathrm{eV}$ for $\mathrm{Mg_3SbBr_3}$ obtained using hybrid functionals. Optical spectra show a rapid rise in absorption above the gap and an increasing static dielectric response along $\mathrm{As}\rightarrow\mathrm{Sb}\rightarrow\mathrm{Bi}$, indicating strengthened light--matter coupling. Phonon dispersions lack imaginary branches, confirming dynamical stability, and exhibit large mode anharmonicity (Grüneisen signatures) consistent with soft-lattice heat transport. Moving down the pnictogen series expands the lattice and lowers the Goldschmidt tolerance factor, while enhanced pnictogen--Br $p$-orbital hybridization and stereochemically active $n\mathrm{s}^{2}$ lone pairs (Sb, Bi) narrow the band gap and increase the optical dielectric response. Elastic analyses confirm Born stability and moderate stiffness, with Hill-averaged bulk moduli decreasing from approximately $44\,\mathrm{GPa}$ ($\mathrm{Mg_3AsBr_3}$) to $35\,\mathrm{GPa}$ ($\mathrm{Mg_3BiBr_3}$). Drift--diffusion $p$--$i$--$n$ simulations qualitatively track band-edge-limited spectra, aligning with the computed gaps. Together, these results position these materials as promising lead-free candidates for stable thin-film photodiode and photovoltaic applications.

Figures (15)

• Figure 1: Crystal structures of (a) Mg$_{3}$AsBr$_{3}$ with As atoms (green) surrounded by Br atoms (pink) with Mg atoms (blue) at the corners, (b) Mg$_{3}$SbBr$_{3}$ with Bi atom (yellow) at the center of the octahedron and (c) Mg$_{3}$BiBr$_{3}$ with Sb atom (yellow) at the center of the octahedron.
• Figure 2: Equation-of-state (EOS) analysis for the Mg$_3$AsBr$_3$, Mg$_3$SbBr$_3$, and Mg$_3$BiBr$_3$ perovskites. Here (a),(c) and (e) correspond to the total energy per atom versus volume, respectively. The red star indicates the energy minimum. Here (b),(d) and (e) subplot plots display the corresponding pressure–volume curves; the red star indicates the zero-pressure crossing that coincides with $V_0$. Dashed grey lines denote the zero-pressure reference.
• Figure 3: Phonon dispersion relations of cubic Mg$_3Z$Br$_3$ (Z = As, Sb, Bi) along high-symmetry directions and the frequency range that each branch spans along the computed path (a) - (b) for Mg$_3$AsBr$_3$; (c) - (d) Mg$_3$SbBr$_3$; (e) - (f) Mg$_3$BiBr$_3$.
• Figure 4: Phonon DOS and PDOS of Mg$_3Z$Br$3$ (a) Z = As, (c) Z = Sb, (e) Z = Bi. And mode Grüneisen parameter $\gamma{\mathbf{q}\nu}$ vs.mode frequency for (b) Z = As, (d) X = Sb, (f) Z = Bi. In the Grüneisen plots, each point corresponds to a phonon mode (color-coded by frequency for clarity). The average Grüneisen parameter (dashed line) and the heat-capacity-weighted average at 300K (dotted line) are indicated.
• Figure 5: Thermodynamic functions of Mg$_3Z$Br$_3$ (Z = As, Sb, Bi) obtained from phonon calculations; (a) molar Helmholtz free energy $F$, (b) entropy $S$, and (c) constant-volume heat capacity $C_V$, as a function of temperature.