Exploring Anharmonicity-induced high thermoelectric performance in α-In2X3 (X=S, Se) monolayers

Abstract

The growing interest in thermoelectric materials stems from their capability to convert thermal energy into electrical power, addressing critical challenges in sustainable energy generation and utilization. While maximizing the Seebeck coefficient is crucial, this can be achieved not solely by focusing on electrical conductivity, but rather through the reduction of lattice thermal conductivity (${\kappa }_l$), which is influenced by material anharmonicity. This ab initio study, based on density functional theory, investigates the thermoelectric properties of $\alpha$-In${}_2$S${}_3$ and $\alpha$-In${}_2$Se${}_3$ monolayers, through a detailed analysis of phonon modes, revealing that these materials exhibit low group velocities in acoustic (ZA, TA, LA) and optical modes, short phonon lifetimes, and high Grüneisen parameters, suggesting strong anharmonicity, especially at low frequencies. At room temperature, significantly low ${\kappa }_l$ values of 1.90 W m⁻¹ K⁻¹ and 0.56 W m⁻¹ K⁻¹ are predicted for $\alpha$-In${}_2$S${}_3$ and $\alpha$-In${}_2$Se${}_3$, respectively, highlighting their potential for thermoelectric applications. Low-frequency optical modes play a dominant role in the total lattice thermal conductivity, contributing 41.54% and 36.78% in $\alpha$-In${}_2$S${}_3$ and $\alpha$-In${}_2$Se${}_3$, respectively. This results in dimensionless figures of merit ($ZT$) reaching 1.12 and 2.30 at 900 K for $\alpha$-In${}_2$S${}_3$ and $\alpha$-In${}_2$Se${}_3$, respectively, under $n$-type doping, underscoring the viability of these materials for efficient $n$-type and $p$-type thermoelectric devices. These findings not only advance our understanding of anharmonicity-induced thermoelectric performance in 2D materials but also pave the way for the design of more efficient energy conversion devices, potentially contributing to future energy policies focused on sustainable and clean energy technologies.