Synergistically Optimizing the Thermoelectric Performance of n‑Type SnS through an Integrated Systematic Approach

Abstract

Tin­(II) sulfide (SnS) is a promising p-type semiconductor known for its high thermoelectric performance and eco-friendly properties, offering a viable alternative to group IV–VI compounds. However, achieving n-type conductivity in SnS has been challenging due to the propensity for native Sn vacancies. This study addresses this challenge by synthesizing polycrystalline n-type SnS_1−δ (δ = 0.05 and 0.075) samples through solid-state reaction. By introducing sulfur vacancies to counteract Sn vacancies, followed by aliovalent (Cl^–) and isoelectronic (Se^2–) substitutions, we significantly enhance the thermoelectric performance of n-type SnS. Chlorine doping further improves electrical conductivity, with SnS_0.455Se_0.45Cl_0.02 showing superior performance. Additionally, incorporating 0.03 mol % SnCl₂ in SnS_0.475Se_0.45 compensates for intrinsic Sn vacancies, optimizing the power factor and lowering lattice thermal conductivity. Consequently, we realized a figure of merit ZT (ZT _max) of ≈0.7 at 823 K and an average ZT (ZT _ave) of ≈0.2 from 308 to 823 K, the highest reported values for n-type SnS. This work advances the optimization of n-type SnS and lays the groundwork for developing SnS-based thermoelectric devices.

1. Introduction

Thermoelectric technology exemplifies an advanced paradigm in sustainable power generation, wherein charge carriers serve as working fluids to facilitate the direct transduction of thermal energy into electrical output, thereby offering a potential solution to energy and environmental challenges. The optimal materials for thermoelectric generators are characterized by a high dimensionless figure of merit ZT, expressed as ZT = (S ²σT/K), wherein S denotes the Seebeck coefficient, σ represents the electrical conductivity, K _e signifies the electronic thermal conductivity, K _l corresponds to the lattice thermal conductivity, and T designates the absolute temperature in the system. Since the efficiency of thermoelectric materials depends on the interplay of electrical and thermal transport carriers in both p- and n-type materials, the thermal carriers, particularly phonons, often impose a primary constraint on conversion efficiency.

Recent decades have witnessed remarkable advances in thermoelectric materials synthesis, with ZT values reaching or exceeding 2 through innovative design and advanced fabrication techniques. Although several high-performance Group IV–VI compounds such as lead Chalcogenide, − GeTe, , and SnTe − have been widely studied, their large scale application is limited by the toxicity of Pb and the scarcity and high cost of Ge and Te. In this context, SnSe has attracted significant attention from thermoelectric community, as it exhibits high ZT value in both p and n-type forms owing to its strong lattice anharmonicity, − multiband electronic transport, − and unique three-dimensional charge and two-dimensional phonon transport. − As a sulfur analogue of SnSe, SnS shares a similar crystal and electronic structure while offering additional advantages of low cost, high earth abundance, and better environmental compatibility, making it an attractive candidate for large scale thermoelectric application. In particular, p-type SnS has recently demonstrated outstanding thermoelectric performance, achieving a ZT of approximately 1.6 at 873 K and an average ZT of 1.25 over the temperature range of 300–873 K in SnS_0.91Se_0.09 alloys. These exceptional results highlight the strong potential of SnS as a sustainable thermoelectric material system.

Despite extensive efforts to enhance the electrical transport performance of p-type SnS, effective n-type conversion remains challenging due to the inherently low formation energy of Sn vacancies. − Under sulfur-rich conditions, Sn vacancies exhibit lower formation energies compared to sulfur vacancies, acting as shallow acceptors. − Conversely, under sulfur-poor conditions, sulfur vacancies, although having lower formation energies, fail to compensate for hole carriers generated by Sn vacancies due to their states being closer to the valence band maximum. − To enhance n-type conductivity, strategic modification of the defect chemical environment within SnS is required. This includes doping with transition metals and halogens (Cl and Br), as well as optimizing the sintering temperature. However, despite these efforts, halogen-doped SnS systems such as SnS_0.96Cl_0.04 and SnS_0.98Br_0.02 still exhibit thermally activated transport and limited carrier mobility, resulting in suboptimal ZT _max values of only ∼0.2 at 823 K. , Although n-type Bi₂Te₃ remains the benchmark thermoelectric material at near-room temperature, its performance rapidly degrades at elevated temperatures and it relies on tellurium, a scarce and expensive element. Consequently, Bi₂Te₃ cannot serve as a sustainable or temperature-matched counterpart for high-temperature p-type SnS. For optimal thermoelectric device performance and long-term stability, fabricating both p-type and n-type legs from the same material system is highly advantageous, as it minimizes thermal expansion mismatch, interfacial degradation, and chemical incompatibility. In this context, developing high-performance n-type SnS is not merely desirable but essential for realizing fully SnS-based, earth-abundant, and environmentally benign thermoelectric modules operating in the midto-high temperature range. Although SnS and SnSe share the same crystal and electronic framework, SnSe exhibits much stronger lattice anharmonicity, ,− more favorable band convergence, − and weaker self-compensation, resulting in superior thermoelectric performance. Se alloying enables SnS to progressively inherit these SnSe like transport characteristics while retaining the sulfur-rich host lattice. ,

In this study, we successfully synthesized n-type SnS_1−δ polycrystalline materials with enhanced anisotropic properties through a three-step process. First, we achieved a controlled transition to n-type conductivity by precisely adjusting the sulfur vacancy concentration (δ). Second, we employed dual substitution (aliovalent and isoelectronic alloying) on SnS_0.925. Finally, we added metal chloride (SnCl₂) for dual vacancy compensation. We systematically synthesized and characterized a series of sulfur-deficient compositions (SnS_1−δ, where δ = 0.05–0.075) using solid-state melting (≈10^–6 Torr) and spark plasma sintering (SPS). The SnS_0.925 (δ = 0.075) sample exhibited optimal performance, achieving a ZT of ≈0.03 at 770 K (Figure S1). Subsequently, the SnS_0.905Cl_0.02 sample was selected as a matrix for further research, with chlorine doping (0.02) acting as an additional free electron source by substituting sulfur atoms. The effect of Cl doping was further enhanced in SnS_0.455Se_0.45Cl_0.02 due to the higher Se content, which lowers the band gap and facilitates Cl-induced carrier conduction (Figure b). Finally, we maintained a fixed Se content for SnS_0.925 + x mol % SnCl₂ samples to compensate for Sn vacancies and provide Cl for charge compensation. These modifications reduced lattice thermal conductivity (Figure c) and maintained electron transport properties. The cumulative effect resulted in a high thermoelectric figure of merit of approximately 0.7 at 823 K for n-type SnS_0.925 + 0.03 mol % SnCl₂.

1.

(a) XRD patterns of SnS_1−δ (X = 0.05, 0.075) samples, thermoelectric transport performance of n-type polycrystal SnS_1−δ samples (X = 0.05, 0.075). (b) Electrical resistivity. (c) Seebeck coefficient. (d) Power factor. (e) Lattice and electronic thermal conductivity. (f) Figure of merit (ZT) value.

2. Results and Discussion

2.1. Intrinsic n-Type SnS Enabled by Sulfur Vacancies

Figure summarizes the structural and thermoelectric transport properties of sulfur-deficient SnS_1−δ (δ = 0.05 and 0.075). The XRD patterns (Figure a) confirm that both compositions crystallize in the orthorhombic SnS phase. To better visualize weak reflections and assess phase purity in the presence of preferred orientation, the corresponding log-scale XRD patterns and Rietveld refinement results are provided in Figures S1 and S2b. The temperature-dependent electrical resistivity (Figure b) decreases continuously with increasing temperature, consistent with semiconducting transport. Compared with SnS_0.95, SnS_0.925 exhibits substantially lower resistivity across the measured temperature range, reflecting a higher electron concentration (−6.77 × 10¹⁵ cm^–3) introduced by sulfur vacancies acting as donor defects. ,, The Seebeck coefficient (Figure c) are negative for both samples, confirming n-type conduction. The magnitude of S is moderately reduced for SnS_0.925, consistent with its increased carrier density, as shown in Table S2. The combination of electrical resistivity and Seebeck coefficient led to a significant enhancement in the power factor for SnS_0.925 (Figure d), which reaches a maximum near 700 K and exceeds that of SnS_0.95 over the entire temperature range. As shown in Figure e, the lattice thermal conductivity decreases with increasing temperature for both samples due to intensifies phonon–phonon scattering. As a result, SnS_0.925 achieves maximum ZT of 0.03 achieved at 770 K (Figure f), demonstrating that sulfur deficiency induces donor defects and provides an effective pathway to achieve and stabilize n-type conduction. Having established n-type SnS through S vacancy engineering, further enhancement of thermoelectric performance requires increasing the electron concentration, which is realized here by Cl and Se codoping in SnS_0.925–x Cl_0.02Se _x .

2.2. Carrier Density Optimization in n-Type SnS_0.925

2.2.1. Structural Analysis of SnS_0.925–x Cl_0.02Se _x

Figure a presents room-temperature X-ray diffraction (XRD) patterns of SnS_0.905–x Se _x Cl_0.02 (x = 0.25, 0.35, 0.45) samples. The observed diffraction peaks align precisely with standard JCPDS card data, confirming the formation of an orthorhombic layered structure with Pnma space group symmetry. With increasing selenium content, the prominent (400) diffraction peak systematically shifts toward lower 2θ values (Figure b), indicating lattice expansion, as shown in Figure c. The sample without Se (SnS_0.905Cl_0.02) exhibits phase-pure characteristics with no detectable impurities. In contrast, selenium-containing samples (x = 0.25–0.45) display an additional diffraction peak at 2θ = 32.01°, denoted by an asterisk in Figure a. This peak, matching the diffraction pattern of elemental Sn, suggests secondary phase formation. The predominant intensity of the (400) peak reveals the preferential growth orientation of the samples. Rietveld refinement analysis of the SnS sample’s XRD pattern is illustrated in Figure S5.

2.

(a) XRD patterns of SnS_1–x Se _x Cl_0.02 samples, asterisk symbols indicate the Sn impurities, which are prominent in x = 0.35 composition (b) closed view of (400) peak of SnS_1–x Se _x Cl_0.02 samples, and (c) calculated lattice parameters for SnS_1–x Se _x Cl_0.02 samples (x = 0.25, 0.35, 0.45).

2.2.2. Electric Transport Properties

Figure a depicts the temperature-dependent electrical resistivity measurements of SnS_0.905–x Se _x Cl_0.02 (x = 0.25, 0.35, 0.45) samples conducted along the parallel to the pressing direction. The temperature-dependent behavior demonstrates characteristic semiconducting transport properties across all compositions. The systematic incorporation of selenium results in a marked reduction in electrical resistivity compared to the selenium-free SnS_0.905Cl_0.02 sample. Among the studied compositions, SnS_0.455Se_0.45Cl_0.02 exhibits lower resistivity. Specifically, at 308 K, SnS_0.455Se_0.45Cl_0.02 achieves a minimal resistivity of 0.0049 Ω·m, representing an order of magnitude improvement over SnS_0.905Cl_0.02 (0.034 Ω·m). This enhanced electrical performance can be attributed to two synergistic effects: (i) the band gap reduction induced by selenium substitution and (ii) the improved activation of chlorine dopants. The combined effect manifests as a significant increase in carrier concentration from −2.66 × 10¹⁸ cm^–3 in SnS_0.905Cl_0.02 to −1.48 × 10¹⁹ cm^–3 in SnS_0.455Se_0.45Cl_0.02 at room temperature, demonstrating successful carrier density optimization through compositional engineering. The activation energies derived from temperature-dependent resistivity measurements using the Arrhenius equation: $\rho (T)={\rho }_0.{\mathrm{e}}^{(E_{\mathrm{a}}/K_{\mathrm{B}}T)}$ , were found to be significantly lower than half of the transport band gaps (E _a ≪ E _g/2), estimated via the Goldsmid–Sharp relation: E _g = 2e|S _max|T _max, as shown in Table S2. This observation confirms extrinsic conduction behavior. In SnS₀.₉₀₅Cl₀.₀₂, where specific values of E _a and E _g are observed, charge transport is attributed to thermally activated electrons originating from shallow donor levels, likely introduced by Cl substituting sulfur. For Se-substituted samples, the activation energy systematically decreases from 0.102 eV in SnS_0.655Se_0.25Cl_0.02 to 0.069 eV in SnS_0.455Se_0.45Cl_0.02. This reduction in activation energy, coupled with lower electrical resistivity, indicates an increase in carrier concentration. This trend suggests that Se incorporation modifies the defect structures, potentially increasing donor-like defects such as Se on S, thereby generating more free electrons and promoting n-type conduction at lower temperatures. Overall, transport in all samples is predominantly governed by thermally activated extrinsic electrons from shallow donor levels, as opposed to intrinsic excitation across the band gap.

3.

Thermoelectric transport performance of n-type polycrystal SnS_1–x Se _x Cl_0.02 samples (x = 0.25, 0.35, 0.45). (a) Electrical resistivity. (b) Seebeck coefficient. (c) Density of states effective mass estimated from eq at 308 K. (d) Power factor. (e) Lattice thermal conductivity. (f) Figure of merit (ZT) value.

The temperature-dependent Seebeck coefficient of SnS_0.905–x Se _x Cl_0.02 (x = 0.25, 0.35, 0.45) samples, as shown in Figure b. The Seebeck coefficient (S) is inversely proportional to the carrier concentration. It follows Mott law $(S=\frac{{\pi }^2{K}_{\mathrm{B}}^2}{3q}{[\frac{1}{n}\frac{\partial (E)}{\partial E}+\frac{1}{\mu }\frac{\partial \mu (E)}{\partial E}]}_{E=E_{\mathrm{f}}})$ , and it is also influenced by the effective mass of the carriers and the density of states near the Fermi level. Substitution with isovalent (Se^2–) and aliovalent (Cl^–) elements could alter band structures, leading to modified electronic transport properties. When engineering bands to enhance thermoelectric performance, the density of state effective mass (m _d*) serves as a key indicator for band modification. m _d* calculations, derived from Hall effect measurements and experimental Seebeck coefficient (S) data. , It provides critical insights into how doping affects both S and electrical resistivity, ultimately influencing thermoelectric efficiency.

$$m_{\mathrm{d}}^{*}=0.857\left(\frac{300\mathrm{K}}{T}\right)m_{\mathrm{e}}{\left(\frac{n}{{10}^{20}{\mathrm{c}\mathrm{m}}^{-3}}\right)}^{2/3}[\frac{3{(\exp [\frac{|S|}{K_{\mathrm{B}}/e}-2]-0.17)}^{2/3}}{1+\exp [-5(\frac{|S|}{K_{\mathrm{B}}/e}-\frac{K_{\mathrm{B}}/e}{|S|})]}+\frac{\frac{|S|}{K_{\mathrm{B}}/e}}{1+\exp \left[\right(\frac{|S|}{K_{\mathrm{B}}/e}-\frac{K_{\mathrm{B}}/e}{|S|}\left)\right]}]$$ 1

where m _d* is the density of state effective mass, m _e is the rest mass of an electron, n is the carrier concentration, K _B is the Boltzmann constant, e is the charge of the electron, and S is the Seebeck coefficient. The density of state effective mass (m _d*) varies with Se­(x) content in SnS_0.905Cl_0.02, as illustrated in Figure c. At room temperature, SnS_0.655Se_0.25Cl_0.02 and SnS_0.555Se_0.35Cl_0.02 compositions demonstrate enhanced S values relative to SnS_0.905Cl_0.02. This enhancement correlates with the increased m _d* values, which rise from 0.231 m _e in SnS_0.905Cl_0.02 to 0.421 m _e in SnS_0.655Se_0.25Cl_0.02 and 0.919 m _e in SnS_0.555Se_0.35Cl_0.02. In contrast, the SnS_0.455Se_0.45Cl_0.02 composition exhibits a lower S than compositions with lower Se content. This reduction is attributed to its increased carrier density and lower density of state effective mass (m _d* = 0.651 m _e). However, these characteristics contribute to increased carrier density, resulting in an enhanced power factor of ≈198.89 μWm^–1 K^–2 at 820 K SnS_0.455Se_0.45Cl_0.02, as shown in Figure d.

Figure e illustrates the temperature-dependent thermal conductivity of SnS_0.905–x Cl _x (x = 0.25–0.45) samples measured parallel to the pressing direction. The lattice thermal conductivity (κ_l) consistently decreases with increasing temperature across all Se­(x)-substituted compositions. Notably, the SnS_0.555Se_0.35Cl_0.02 composition exhibits an optimal lattice thermal conductivity of 0.30 Wm^–1 K^–1 at 822 K, surpassing the performance of the higher Se-substituted SnS_0.455Se_0.45Cl_0.02 composition (κ_l ≈ 0.35 Wm^–1 K^–1 at 822 K). This superior thermal behavior in SnS_0.555Se_0.35Cl_0.02 can be attributed to its optimized ratio of Sn–S (S ∼ 100 pm) to Sn–Se (Se ∼ 120 pm) bonds, which induces lattice anharmonicity distortion and strain, thereby enhancing phonon scattering efficiency. Moreover, the presence of SnCl₂ and Sn precipitates further enhances phonon scattering, leading to a reduction in lattice thermal conductivity. The electronic contribution to thermal conductivity (κ_ele) was evaluated using the Wiedemann–Franz relationship κ_ele = LσT, where L represents the Lorenz constant (detailed in Figure S6, Supporting Information). Analysis reveals exceptionally low κ_ele values, demonstrating its negligible contribution to the total thermal conductivity (κ_tot). This finding indicates that κ_tot is primarily dominated by lattice thermal conductivity (κ_lat), a characteristic that suggests the carrier concentration (n) remains suboptimal. The current carrier concentration state presents substantial opportunities for enhancement through further materials engineering, doping, and vacancy compensation strategies. As illustrated in Figure f, a significant enhancement in thermoelectric figure of merit (ZT) was achieved, increasing from ≈0.22 in SnS_0.905Cl_0.02 at 773 K to ≈0.45 in SnS_0.455Se_0.45Cl_0.02 at 823 K. This substantial improvement can be attributed to the synergistic effect of enhanced power factor (PF) and moderate total thermal conductivity (κ_tot).

2.3. SnS_0.475Se_0.45 Vacancy Compensation

The thermoelectric performance of n-type polycrystalline SnS_0.455Se_0.45Cl_0.02 was optimized by strategically incorporating SnCl₂, which effectively compensates for intrinsic Sn vacancies. Under S-poor conditions, despite S exhibiting lower formation energy compared to Sn vacancies, it proves ineffective in compensating for hole carriers generated by Sn vacancies. , This behavior is attributed to the energetic positioning of both Sn vacancy states and Sn _i transition levels, which lie in closer proximity to the valence band maximum relative to the conduction band minimum. − Consequently, residual Sn vacancies play a crucial role in determining the electronic properties of n-type SnS polycrystals, even under S-poor synthesis conditions. The formation energy of Sn vacancies increased under Sn-rich conditions compared to Sn-poor conditions. This higher formation energy indicates the reduced probability of Sn vacancy formation following Sn addition. These results demonstrate that incorporating excess Sn compensates for intrinsic Sn vacancies, thereby reducing the overall Sn vacancy concentration. − The XRD patterns of SnS_0.475Se_0.45 samples with varying SnCl₂ content (x = 0.01, 0.02, 0.03 mol %) are presented in Figure a. All samples exhibit primary diffraction peaks consistent with the SnS phase, alongside additional Sn-related diffraction peaks. With increasing SnCl₂ concentration, the diffraction peaks initially shift to lower angles (at 0.01 and 0.02 mol % SnCl₂) and subsequently to higher angles (at 0.03 mol % SnCl₂), as shown in Figure b. This systematic peak shift can be attributed to the initial substitution of Sn²⁺ ions at lattice sites. At higher concentrations, excess Sn preferentially forms Sn–Se bonds, driven by their significant electronegativity difference. The addition of SnCl₂ partially results in the formation of SnCl₂ along with Sn impurity phases, which impacts thermal transport. Rietveld refinement analysis of the SnS sample’s XRD pattern is illustrated in Figure S7. The role of SnCl₂ addition should be understood in terms of defect-equilibrium modification rather than idealized full lattice substitution. , SnCl₂ does not fully dissolve into the SnS lattice as a simple substitutional defect complex. Instead, its addition increases the Sn chemical potential while introducing Cl^– donors. The elevated Sn chemical potential raises the formation energy of Sn vacancies (V_Sn), thereby suppressing their formation, while Cl^– substitution on S sites provides electronic doping. , Excess Sn or SnCl₂ beyond the solubility limit forms isolated secondary phases, as confirmed by SEM–EDS mapping (Figures S8 and S9), rather than contributing to substitutional defects. This defect-equilibrium shift underlies the simultaneous suppression of Sn vacancies and control of electrical conductivity.

4.

(a) XRD patterns of SnS_0.475Se_0.45 + x mol % SnCl₂ samples, (b) closed view of (400) peak of SnS_0.475Se_0.45 + x mol % SnCl₂ samples, and (c) calculated lattice parameters for SnS_0.475Se_0.45 + x mol % SnCl₂ samples (x = 0.01, 0.02, 0.03).

Figure a illustrates the temperature-dependent electrical resistivity measurements of SnS_0.475Se_0.45 + x mol % SnCl₂ (x = 0.01, 0.02, 0.03) samples along the parallel direction. All samples exhibit decreasing electrical resistivity with increasing temperature, confirming semiconductor behavior. The resistivity anomaly observed at 550 K in n-type SnS, while the actual phase transition occurs at 880 K, suggests the presence of other influencing factors. This anomaly could be attributed to enhanced defect states, such as increased activation of Sn or S vacancies, which temporarily trap charge carriers and increase resistivity. Additionally, increased carrier scattering due to impurity phases or subtle structural rearrangements might contribute to this behavior. Despite the rise in resistivity, the Seebeck coefficient remains unaffected, indicating stable carrier concentration and energy distribution. The incorporation of SnCl₂ systematically increases the electrical resistivity at 300 K from 0.003 Ω·m (x = 0.01) to 0.004 Ω·m (x = 0.02) and 0.005 Ω·m (x = 0.03). This trend correlates with a concurrent decrease in carrier concentration and carrier mobility from −6.61 × 10¹⁸ cm^–3 & 2.742 cm² v^–1 s^–1 (x = 0.01) to −5.41 × 10¹⁸ cm^–3 & 2.643 cm² v^–1 s^–1 (x = 0.02) and −3.79 × 10¹⁸ cm^–3 & 2.334 cm² v^–1 s^–1 (x = 0.03) at 308 K, as shown in Figure b. Moreover, across all SnS_0.475Se_0.45 + x mol % SnCl₂ (x = 0.01, 0.02, 0.03) samples were shown the activation energy was significantly lower than half the Goldsmid–Sharp band gap, confirming conduction via extrinsic donor levels. A shallow activation energy of 0.055 eV at 0.01 mol % SnCl₂ indicates efficient thermal ionization of dopants. However, increasing the doping level to 0.03 mol % SnCl₂ resulted in a rise in activation energy to 0.079 eV, suggesting either deeper donor states or potential compensation effects. These observations indicate an optimal dopant range of 0.01–0.02 mol % SnCl₂ for maximizing carrier activation without degrading electronic transport. Furthermore, the 0.03 mol % SnCl₂ sample exhibits higher electrical resistivity, likely due to dopant oversaturation. This oversaturation can lead to defect compensation, such as the formation of charge-neutral Cl–V_Sn complexes, and clustering of SnCl₂ atoms. Such phenomena reduce the effective donor concentration and increase carrier scattering, thereby limiting free electron generation and degrading conductivity despite the increased nominal doping.

5.

Thermoelectric transport performance of n-type polycrystal SnS_0.475Se_0.45 + x mol % SnCl₂ samples (x = 0.01, 0.02, 0.03). (a) Electrical resistivity. (b) Carrier concentration and mobility at 308 K. (c) Seebeck coefficient. (d) Power factor. (e) Lattice thermal conductivity. (f) ZT value.

Furthermore, the Seebeck coefficient (S) of n-type polycrystalline SnS_0.475Se_0.45 + x mol % SnCl₂ samples shows systematic enhancement with increasing SnCl₂ content, consistent with the observed carrier concentration trends (Figure b,c). The optimized interplay between electrical resistivity and the Seebeck coefficient results in an enhanced power factor of ≈217 μWm^–1 K^–2 at 823 K (Figure d). The temperature-dependent lattice thermal conductivity of SnS_0.475Se_0.45 + x mol % SnCl₂ samples was measured from 308 to 823 K, as shown in Figure e. The SnS_0.475Se_0.45 + 0.03SnCl₂ sample exhibited a low lattice thermal conductivity of 0.63 Wm^–1 K^–1 at 300 K, which decreased to 0.24 Wm^–1 K^–1 at 823 K, lower than previously reported values. ,, The reduced K _l in the SnS_0.475Se_0.45 + 0.03SnCl₂ sample is attributed to solid solution point defects and secondary phase precipitates, creating more scattering centers that effectively impede phonon transport. This leads to a significant reduction in K _tot (approximately 0.26 Wm^–1 K^–1 at 823 K), as shown in Figure S10. The combined effect of improved power factor (PF) and low lattice thermal conductivity results in an impressive ZT value of ≈0.7 at 823 K for the SnS_0.475Se_0.45 + 0.03SnCl₂ sample, as shown in Figure f.

To quantify the intrinsic electrical transport properties of SnCl₂-doped SnS_0.925, we calculated the weighted mobility (μ_w) using

$${\mu }_{\mathrm{w}}=\frac{3h^3\sigma }{8\pi e{(2m_{\mathrm{e}}{K}_{\mathrm{B}}T)}^{3/2}}[\frac{\exp [\frac{|S|}{K_{\mathrm{B}}/e}-2]}{1+\exp [-5(\frac{|S|}{K_{\mathrm{B}}/e}-1)]}+\frac{\frac{3}{{\pi }^2}\frac{|S|}{K_{\mathrm{B}}/e}}{1+\exp [5\frac{|S|}{K_{\mathrm{B}}/e}-1]}]$$ 2

Among all compositions, the SnS_0.925 + 0.03 mol % SnCl₂ sample demonstrated the highest μ_w, as shown in Figure a, which confirms its enhanced electrical transport properties. The μ_w/κ_l ratios of SnS_0.925 + x mol % SnCl₂ consistently surpassed those of undoped SnS_0.925 throughout the measured temperature range (Figure b), demonstrating that SnCl₂ doping successfully optimizes both electron and phonon transport mechanisms. To demonstrate the decoupling of electrical and thermal transport properties in n-type SnS polycrystals, we calculated the quality factor, $B={\left(\frac{k_{\mathrm{B}}}{e}\right)}^2\frac{8\pi e{(2m_{\mathrm{e}}{k}_{\mathrm{b}}T)}^{3/2}}{3h^3}·\frac{{\mu }_{\mathrm{w}}}{k_{\mathrm{L}}}·(T)$ . Since quality factor B is linearly proportional to μ_w/κ_L, both parameters showed significant improvement in the SnS_0.475Se_0.45 + 0.03 mol % SnCl₂ sample, as shown in Figure c. Moreover, K _l of SnS_0.475Se_0.45 + 0.03 mol % SnCl₂ sample was largely reduced to an ultralow value of ∼0.24 Wm^–1 K^–1 at 823 K, which is relatively lower than other reported values, ,, as shown in Figure c. The figure of merit is a crucial parameter for evaluating the performance of thermoelectric materials, and its value is temperature-dependent. Since these materials typically operate across various temperatures, calculating the average ZT becomes essential. The ZT _ave is calculated using the following formula

$${ZT}_{\mathrm{a}\mathrm{v}\mathrm{e}}=\frac{1}{T_{\mathrm{h}}-T_{\mathrm{c}}}{\int }_{T_{\mathrm{c}}}^{T_{\mathrm{h}}}ZT\mathrm{d}T$$ 3

6.

(a) Temperature-dependent weighted mobility (μ_w) and (μ_w/K _l) of polycrystalline SnS_0.475Se_0.45 + x mol % SnCl₂ samples (x = 0.01, 0.02, 0.03). (b) Quality factor. (c) Comparison of lattice thermal conductivity for n-type polycrystal SnS_0.475Se_0.45 + 0.03SnCl₂ samples in this study and other n-type SnS. ,, (d) Average ZT (ZT _avg) comparison with reported polycrystalline n-type SnS. ,, (e) comparison of ZT for n-type SnS_0.475Se_0.45 + 0.03SnCl₂ samples with reported values. ,, (f) Calculated efficiency (η_e) with reported values. ,,

The calculated ZT _ave value of 0.2 for the SnS_0.475Se_0.45 + 0.03 mol % SnCl₂ sample was obtained over the 308–823 K temperature range, as shown in Figure d. The theoretical thermoelectric conversion efficiency was then determined using the Synder approach

$${\eta }_{\mathrm{e}}=\frac{T_{\mathrm{h}}-T_{\mathrm{c}}}{T_{\mathrm{H}}}\frac{\sqrt{1+{ZT}_{\mathrm{a}\mathrm{v}\mathrm{e}}}-1}{\sqrt{1+{ZT}_{\mathrm{a}\mathrm{v}\mathrm{e}}+\frac{T_{\mathrm{c}}}{T_{\mathrm{h}}}}}$$ 4

where T _h and T _c are the temperatures of the hot side and cold side temperature of the TE module, respectively, with T _c fixed at 323 K. As shown in Figure e, the calculated maximum ZT efficiency reaches approximately 4.01% for SnS_0.475Se_0.45 + 0.03SnCl₂ samples. These samples demonstrate superior thermoelectric performance across the entire temperature range compared to previously reported n-type SnS. ,, Figure f shows enhanced ZT values in the 323–623 K range compared to polycrystalline SnS materials with single metal halide or elemental metal dopants. ,, This innovative codoping strategy using metal halides and metal elements to optimize thermoelectric properties can be applied to other thermoelectric systems. The approach shows promising potential for thermoelectric applications in medium-temperature ranges.

3. Conclusion

In this study, we systematically enhanced the thermoelectric efficiency of n-type SnS through a multistep approach, achieving significant improvements in its figure of merit (ZT). By generating sulfur vacancies and implementing strategic dopants, including aliovalent Cl and isoelectronic Se, we effectively reduced the band gap and increased carrier concentration. This led to a notable ZT value of 0.45 at 823 K in SnS_0.455Se_0.45Cl_0.02. Further optimization, addressing intrinsic Sn and S deficiencies through SnCl₂ incorporation, yielded a remarkable peak ZT of 0.7 at 823 K in SnS_0.475Se_0.45 + 0.03 mol % SnCl₂. The synergistic effects of enhanced phonon scattering and suppressed hole carrier generation resulted in a low thermal conductivity (κ_L) of 0.24 W m^–1 K^–1. This comprehensive methodology not only underscores the potential of n-type SnS for high-performance thermoelectric applications but also establishes a robust framework for future advancements in SnS-based materials. The integration of metal chlorides within the SnS matrix opens new avenues for optimizing thermoelectric properties, paving the way for environmentally friendly and efficient energy conversion technologies.

4. Experimental Procedure

Polycrystalline SnS_1−δ (X = 0.05, 0.075), SnS_0.905–x SeCl_0.02 (x = 0.25, 0.35, 0.45), and SnSe_0.475Se_0.45 + x mol % SnCl₂ (where x = 0.01, 0.02, 0.03) samples were meticulously synthesized through the combination of stoichiometric ratios of elemental Sn (99.999%), S (99.999%, Alfa Aesar), Se (99.999%) and powdered SnCl₂ (99.99%) powder were vacuum sealed in a quartz tube. These tubes were then vacuum sealed at 10^–6 Torr. The sealed tubes underwent heating to 1223 K over 15 h, maintained at 12 h, and then cooled down naturally. The synthesized ingots underwent a process of comminution using a mortar and pestle, followed by granulometric separation via sieving to yield a fine powder. This prepared powder was subsequently consolidated into dense compacts through spark plasma sintering (SPS), utilizing an SPS211-LX apparatus manufactured by Dr. Sinter Lab, in preparation for the characterization of thermoelectric properties. The finely divided samples were molded into cylindrical forms, exhibiting nominal dimensions of 12.7 mm in diameter and 8 mm in axial thickness, achieved through SPS processing employing graphite dies under an applied uniaxial pressure of 50 MPa at a sintering temperature of 570 °C. Postconsolidation density measurements (ρ) indicated a relative density of approximately 95% with respect to the theoretical maximum density. Concluding the preparation, the consolidated samples were meticulously sectioned and surface-treated through polishing along a predetermined parallel orientation, designed to enable precise measurement of anisotropic electrical and thermal transport properties inherent to the n-type SnS material.

4.1. Thermoelectric Property Measurement

Thermoelectric measurements were conducted in a parallel orientation relative to the direction of applied pressure during sample preparation. Electrical resistivity and Seebeck coefficient values were ascertained utilizing a ULVAC-Riko ZEM 3 apparatus, operating under conditions of low pressure and within a helium-enriched atmosphere, spanning a temperature interval from 300 to 823 K. Determination of carrier concentration was achieved through Hall effect measurements, performed on rectangular specimens consistent with those employed for electrical transport assessments within a Physical Property Measurement System (PPMS) at magnetic field strengths up to 0.9 T at ambient temperature. Thermal diffusivity (D) as a function of temperature was evaluated via a laser flash method, employing a Netzsch LFA-467 instrument. Subsequently, total thermal conductivity (κ) was derived from the relation κ = D × C _p × ρ, where C _p denotes specific heat capacity. Dimensional specifications for samples utilized in κ assessments were established at 12.7 mm in diameter and 2 mm in thickness, with a parallel alignment. The cumulative uncertainty associated with all measurement parameters contributing to determining the figure of merit (ZT) was maintained below a threshold of 15%.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.5c21755.

• Log scale XRD pattern, Rietveld refinement results, SEM–EDS characterization, lattice parameter table, and additional thermoelectric transport data (PDF)

Sidharth Duraisamy: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Yang-Yuan Chen: Resources, Conceptualization. Kuei-Hsien Chen: Resources, Conceptualization. Maw-Kuen Wu: Resources, Conceptualization. G. Peramaiyan: Investigation, Data curation, Conceptualization. V. K. Ranganayakulu: Formal analysis, Data curation, Conceptualization. Muluken Biadgelegn Wollele: Data curation, Conceptualization. Min-Nan Ou: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Supervision, Resources, Project administration, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

The authors declare no competing financial interest.