the total thermal conductivity consisting of the lattice part (κ L ) and the electronic part (κ e ). [6] Some of these parameters (S, ρ, and κ e ), however, are mutually interdependent and strongly coupled by way of the carrier concentration, making it difficult to attain high TE performance (zT) without making some compromise. Fortunately, in the past decade, several general strategies have been developed to achieve high zT in various materials. [7,8] Lowering the lattice thermal conductivity (κ L ), which is relatively independent of other parameters, is a comparatively easy method. Other well-documented methods for enhancing phonon scattering include nanostructuring, [9][10][11] dislocations, [12,13] point defects, [14][15][16][17] and strong lattice anharmonicity. [18][19][20] Additional strategies to boost the power factor (S 2 ρ −1 ) include carrier concentration optimization, [21,22] band convergence, [23,24] and creating band resonance levels. [25] Typically, practical TE materials are polycrystalline structures with grain boundaries that help to maintain a low lattice thermal conductivity but inevitably also degrade the charge carrier mobility. Recently, bulk SnSe [26][27][28][29][30] and In 4 Se 3[31] single crystals have attracted increasing amounts of attention. The layered structure and strong lattice anharmonicity of these materials cause ultralow lattice thermal conductivity, independent of grain boundary phonon scattering. Therefore, such single crystalline compounds are expected to yield superior performance as both high mobility and low lattice thermal conductivity can be maintained simultaneously. For instance, the ultrahigh zT of Lead-free tin sulfide (SnS), with an analogous structure to SnSe, has attracted increasing attention because of its theoretically predicted high thermoelectric performance. In practice, however, polycrystalline SnS performs rather poorly as a result of its low power factor. In this work, bulk sodium (Na)-doped SnS single crystals are synthesized using a modified Bridgman method and a detailed transport evaluation is conducted. The highest zT value of ≈1.1 is reached at 870 K in a 2 at% Na-doped SnS single crystal along the b-axis direction, in which high power factors (2.0 mW m −1 K −2 at room temperature) are realized. These high power factors are attributed to the high mobility associated with the single crystalline nature of the samples as well as to the enhanced carrier concentration achieved through Na doping. An effective single parabolic band model coupled with first-principles calculations is used to provide theoretical insight into the electronic transport properties. This work demonstrates that SnS-based single crystals composed of earth-abundant, low-cost, and nontoxic chemical elements can exhibit high thermoelectric performance and thus hold potential for application in the area of waste heat recovery.
