Modular Nanostructures Facilitate Low Thermal Conductivity and Ultra‐High Thermoelectric Performance in n‐Type SnSe

Abstract

Single crystals of SnSe have gained considerable attention in thermoelectrics due to their unprecedented thermoelectric performance. However, polycrystalline SnSe is more favorable for practical applications due to its facile chemical synthesis procedure, processability, and scalability. Though the thermoelectric figure of merit (zT) of p‐type bulk SnSe polycrystals has reached >2.5, zT of n‐type counterpart is still lower and lies around ≈1.5. Herein, record high zT of 2.0 in n‐type polycrystalline SnSe0.92 + x mol% MoCl5 (x = 0–3) samples is reported, when measured parallel to the spark plasma sintering pressing direction due to the simultaneous optimization of n‐type carrier concentration and enhanced phonon scattering by incorporating modular nano‐heterostructures in SnSe matrix. Modular nanostructures of layered intergrowth [(SnSe)1.05]m(MoSe2)n like compounds embedded in SnSe matrix scatters the phonons significantly leading to an ultra‐low lattice thermal conductivity (κlat) of ≈0.26 W m−1 K−1 at 798 K in SnSe0.92 + 3 mol% MoCl5. The 2D layered modular intergrowth compound resembles the nano‐heterostructure and their periodicity of 1.2–2.6 nm in the SnSe matrix matches the phonon mean free path of SnSe, thereby blocking the heat carrying phonons, which result in low κlat and ultra‐high thermoelectric performance in n‐type SnSe.

Improved thermoelectric properties in n-type polycrystalline SnSe0.95 by PbCl2 doping

“…The increased k at a higher temperature may result from the excited bipolar effect, which was also found in a previous report. 43,44 At a certain temperature, k of PbCl 2 -doped SnSe 0.95 firstly decreases with an increasing content of PbCl 2 , reaches a minimum value with 6% PbCl 2 doping, and then slightly increases. For instance, the total thermal conductivity at room temperature decreases from 1.2 W m À1 K À1 for the undoped SnSe 0.95 to 0.85 W m À1 K À1 for SnSe 0.95 + 6% PbCl 2 .…”

“…The IV-VI compound SnSe is composed of the non-toxic and abundant elements Sn and Se, and it possesses a layered orthorhombic crystal structure. 18 The high anharmonicity resulting from the weak chemical bonds of Sn and Se promotes phononphonon scattering, which makes SnSe an intrinsically low thermal conductivity material. [19][20][21] Although the thermoelectric property of a tin selenide single crystal is excellent, its harsh preparation conditions and weak mechanical performance of the single crystal severely limit its practical applications.…”

“…However, the thermoelectric properties of polycrystalline SnSe are really inferior to those of the single crystal, because of the lower electrical conductivity and higher thermal conductivity. 18,24 Many efforts have been made to improve the electrical conductivity of polycrystalline SnSe. For example, K, Na, Cu, and Zn doping are adopted for the p-type polycrystalline SnSe [25][26][27][28] while Cl-, Br-, I-based dopants are used for the n-type one.…”

Recent Advances in Ultrahigh Thermoelectric Performance Material SnSe

“…[60] The band gap modulation via PbTe alloying and the simultaneous formation of Sn nanophase at crystal boundaries in Sn 0.85 Pb 0.15 Se 0.85 Te 0.15 B r0.03 results in a ZT max of ~1.7 at 793 K and a ZT ave of ~0.58 from 300 to 793 K. [61] SnSe doped with 3mol% MoCl 2 exhibits a ZT max of ~2 at 798 K parallel to the SPS pressing direction. [62] All these studies on SnSe TE materials (single-and polycrystalline forms) raise very important questions: 1) is ultralow κ lat intrinsic property of SnSe? 2) why do polycrystalline SnSe samples exhibit higher κ lat than single crystal samples?…”

Study on the Thermoelectric Properties of n-Type Polycrystalline SnSe by CeCl3 Doping

“…Interestingly, different cations have quite different effects, which indicates that the co-doping of different cations and halogens can flexibly adjust the thermoelectric properties of SnSe. For example, the co-doping of Pb and Br , and the doping of halides such as BiCl 3 and MoCl 5 , can increase the electron concentration to 10 19 cm –3 , while the doping of LaCl 3 leads to an increase of hole concentration. Recently, we investigated the effect of NdCl 3 doping on the thermoelectric properties of n-type polycrystalline SnSe .…”