First-principles study of anisotropic thermoelectric transport properties of IV-VI semiconductor compounds SnSe and SnS (1505.02601v3)

Abstract: We conduct comprehensive investigations of both thermal and electrical transport properties of SnSe and SnS using first-principles calculations combined with the Boltzmann transport theory. Due to the distinct layered lattice structure, SnSe and SnS exhibit similarly anisotropic thermal and electrical behaviors. The cross-plane lattice thermal conductivity $\kappa_{L}$ is 40-60% lower than the in-plane values. Extremely low $\kappa_{L}$ is found for both materials because of high anharmonicity. It is suggested that nanostructuring would be difficult to further decrease $\kappa_{L}$ because of the short mean free paths of dominant phonon modes (1-30 nm at 300 K) while alloying would be efficient in reducing $\kappa_{L}$ considering that the relative $\kappa_{L}$ contribution ($\sim$ 65%) of optical phonons is remarkably large. On the electrical side, the anisotropic electrical conductivities are mainly due to the different effective masses of holes and electrons along the $a$, $b$ and $c$ axes. This leads to the highest optimal $ZT$ values along the $b$ axis and lowest ones along the $a$ axis in both $p$-type materials. However, the $n$-type ones exhibit the highest $ZT$s along the $a$ axis due to the enhancement of power factor when the chemical potential gradually approaches the secondary band valley that causes significant increase in electron mobility and density of states. SnSe exhibits larger optimal $ZT$s compared with SnS in both $p$-type and $n$-type materials. For both materials, the peak $ZT$s of $n$-type materials are much higher than those of $p$-type ones along the same direction. The predicted highest $ZT$ values at 750 K are 1.0 in SnSe and 0.6 in SnS along the $b$ axis for the $p$-type doping while those for the $n$-type doping reach 2.7 in SnSe and 1.5 in SnS along the $a$ axis, rendering them among the best bulk thermoelectric materials for large-scale applications.

• The paper demonstrates pronounced anisotropy in thermal conductivity, with cross-plane values 40–60% lower than in-plane measurements.
• The paper shows that first-principles calculations predict optimal ZT values of 1.0 (p-type) and 2.7 (n-type) for SnSe, and 0.6 (p-type) and 1.5 (n-type) for SnS.
• The paper highlights that tuning electronic band structures via doping and nanostructuring can enhance thermoelectric efficiency while minimizing lattice thermal conductivity.

Anisotropic Thermoelectric Properties of SnSe and SnS: A First-Principles Study

The paper provides a detailed examination of the thermoelectric properties of tin selenide (SnSe) and tin sulfide (SnS) utilizing first-principles calculations and Boltzmann transport theory. The paper focuses on understanding the anisotropic thermoelectric behavior, essential for enhancing the efficiency of these materials in large-scale applications.

Key Findings and Numerical Outcomes

1. Thermal Conductivity: The thermal properties of both SnSe and SnS show significant anisotropy due to their layered lattice structure. The cross-plane lattice thermal conductivity is found to be 40-60% lower than the in-plane values. The average lattice thermal conductivity (κL) of SnS is ~8% higher than SnSe over a temperature range of 300 to 750 K, due predominantly to larger phonon group velocities and longer relaxation times in SnS.
2. Electrical Conductivity: Anisotropic behavior is also observed in electrical conductivity, attributed to variations in the effective masses of holes and electrons along different crystallographic axes. This anisotropy results in higher optimal ZT values along the b axis and lower values along the a axis for p-type materials. Contrarily, n-type materials exhibit the highest ZTs along the a axis due to enhanced electron mobility and density of states as the chemical potential approaches a secondary conduction band valley.
3. Optimal ZT Values: The maximum ZT at 750 K for p-type SnSe is predicted at 1.0 along the b axis, while for n-type, it reaches a peak of 2.7 along the a axis. Correspondingly, SnS displays a peak ZT of 0.6 for p-type along the b axis, increasing to 1.5 for n-type along the a axis. These results position SnSe and SnS among the most efficient bulk thermoelectric materials, especially when doped for n-type conductance.

Theoretical and Practical Implications

This research underscores the substantial potential for enhancing the thermoelectric performance of SnSe and SnS, especially in n-type applications. The insights into anisotropic phonon and electron transport properties highlight avenues for materials engineering, particularly through manipulating band structures to optimize thermoelectric efficiency.

The investigation notes that despite the inherent material challenges in reducing lattice thermal conductivity, tuning the electronic properties via doping remains a promising approach. This understanding supports the pursuit of improved thermoelectric applications leveraging the “phonon-glass electron-crystal” principles, emphasizing selective band valley contributions for superior electronic performance while maintaining low thermal conductance.

Future Directions

The findings suggest several pathways for future research. Exploring alloying strategies and advanced nanostructuring techniques could further decrease lattice thermal conductivity, despite its limited impact due to the short mean free paths of phonons. Additionally, refining synthesis techniques for high-quality n-type crystals will play a crucial role in realizing the theoretical thermoelectric potential predicted for SnSe and SnS.

The paper provides a comprehensive theoretical framework foundational to advancing the application of SnSe and SnS in thermoelectric devices, with its predictions aligning closely with experimental observations. Continued empirical validation and material optimization are expected to bridge the gap between theoretical potential and practical application, cementing these materials' roles in sustainable energy solutions.