Low carrier concentration leads to high in-plane thermoelectric performance in n-type SnS crystals

ARTICLES SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 16 June 2021 | https://doi.org/10.1007/s40843-021-1684-0 Low carrier concentration leads to high in-plane thermoelectric performance in n-type SnS crystals 1 1,2 1 2 Wenke He , Tao Hong , Dongyang Wang , Xiang Gao and Li-Dong Zhao ABSTRACT As a simple binary compound, p-type SnS shows great competitiveness in thermoelectrics due to the certain appealing carrier and phonon transport behaviors, coupled with its cost-effectiveness, earth-abundance and environmental compatibility. To promote the application of low-cost thermoelectric devices, we synthesized n-type SnS crystals through bromine doping. Herein, we report a high in-plane −1 −2 power factor of ~28 µW cm K , and attribute it to an outstanding in-plane carrier mobility in the crystal form and the large Seebeck coefficient benefitting from the low carrier concentration. The calculations of elastic properties show that the low lattice thermal conductivity in SnS is closely related to its strong anharmonicity. Combining the excellent electrical transport properties with low thermal conductivity, a final ZT of ~0.4 is attained at 300 K, projecting a conversion efficiency of ~5% at 873 K along the in-plane direction. Keywords: tin sulfide, n-type, layered structure, carrier concentration, thermoelectric transports INTRODUCTION To cope with the issues of worldwide fossil fuel shortage and the serious environmental pollution caused by excessive consumption of this resource, many new energy technologies have emerged and developed over the past decades [1–3]. Among them, thermoelectric conversion is deemed to be a thriving energy utilization technology as it can generate electrical power through harvesting exhausted heat and realize solid-state cooling by applying electric current. The thermoelectric conversion efficiency for a given material is determined by a dimensionless 2 figure of merit, defined as ZT = S σT/κ, where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature in Kelvin and total thermal conductivity (sum of the contributions from lattice thermal conductivity κlat and electronic thermal conductivity κele), 1* respectively [4–6]. Under great explorations and efforts, increasing progress in advanced synthesis techniques [7–10], modern theories [11–13] on thermoelectric transports and strategies for improving thermoelectric performance have been made in previous decades. In addition, it is particularly important to explore and research new thermoelectric materials to meet the market demands of low cost, high effectiveness, component nontoxicity and environmental compatibility [14]. Tin sulfide (SnS) is one of the most representative thermoelectric candidates in recent years [15–17]. Unlike traditional extensively studied thermoelectrics (such as Bi2Te3 [8,18], PbTe [7,19,20], and GeTe [21,22]), SnS is made of low-cost or no-toxic elements, and crystallized in layered structure with lower symmetry and possesses large bandgap. This kind of material has the following characteristics. First, the carrier mobility can be achieved orders of magnitude improvement by utilizing its layered structure to grow crystals compared to the polycrystalline phase [16,23]. Although wide-bandgap semiconductors have intrinsically low carrier concentration, high carrier mobilities in crystals can compensate the deterioration in electrical conductivity (σ = neµ). Further, through effective doping, the electrical conductivities in the form of crystals are superior to the polycrystalline SnS, as well as the multi-band transport effects can be activated to enhance the Seebeck coefficient owing to its complex electronic band structure caused by the asymmetric crystal structure [15,16]. Consequently, the excellent electrical transport properties (power factor, 2 PF = S σ) in SnS crystals can be comparable to traditional high-performance thermoelectrics, and even beyond them at low temperatures especially [17]. Not only the electrical performance improvement benefits from these features, but also the thermal conductivity is related to them. Low symmetric structure means the complexity of 1 School of Materials Science and Engineering, Beihang University, Beijing 100191, China Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094, China * Corresponding author (email: ) 2 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021 1 ARTICLES SCIENCE CHINA Materials chemical bondings (e.g., anharmonic bondings) in materials, which bring about strong anharmonicity, thus leading to a low lattice thermal conductivity κlat [24,25]. −1 −1 The κlat can reach ~0.4 W m K in the high-temperature region in polycrystalline SnS [26,27], approaching the theoretical limit. Moreover, large bandgap can avoid the adverse effect on thermal conductivity of bipolar diffusion from intrinsic excitation at high temperatures, which usually occurs in semiconductors with narrow bandgaps. Thus, combining the electrical and thermal transports, these characteristics in SnS crystals can broaden the temperature range of high ZTs rather than confine the maximum ZTs within a narrow range [14,17]. Wide-ranged high ZTs (namely, high average ZT) are conducive to the overall thermoelectric conversion efficiency. Therefore, low-cost SnS becomes a competitive candidate for thermoelectric device applications. Based on the above characteristics, increasing progress on the thermoelectric performance improvement has been made in p-type SnS recently. As is well known, a thermoelectric device consists of p-type and n-type legs. Therefore, it is of great importance to develop n-type SnS and match its p-type counterpart to promote the applications of this low-cost thermoelectric device with potential high economic benefits. To date, several attempts on how to realize n-type SnS have been conducted, including aliovalent (Pb) or isovalent (Sb, Bi) ion doping in Sn sites to fabricate thin films [28–30], or halogen anions (Cl, Br) substitution in S sites to prepare bulk polycrystalline SnS [31,32]. The carrier concentration and carrier mobility, however, are too low to achieve good electrical conductivity for thermoelectrics. By the methods of crystal growth, the carrier mobility can achieve a huge improvement in SnS. Iguchi et al. [33] successfully fabricated n-type SnS single crystals using a self-flux 2 −1 −1 method, the carrier mobility reaches 252 cm V s with 17 −3 a carrier concentration of 3 × 10 cm . Subsequently, Kawanishi et al. [34] reported a growth method of n-type SnS single crystals through halogen-doping from Snbased flux, with the carrier mobilities up to 155 and 2 −1 −1 154 cm V s as well as carrier concentrations up to 4.5 17 17 −3 × 10 and 7.6 × 10 cm for SnS crystals with Cl and Br doping, respectively. Although these crystal-growth methods have improved the electrical cond (...truncated)