Dense dislocations enable high-performance PbSe thermoelectric at low-medium temperatures

Thermoelectric materials enable direct heat-to-electricity conversion, with device efficiency determined by the dimensionless figure of merit ZT = S²σT/κtot, where S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κtot the total thermal conductivity (sum of electronic κele and lattice κlat). Achieving high performance requires decoupling interdependent transport parameters to maximize power factor (S²σ) and/or reduce κtot. Lead chalcogenides (notably PbTe and PbSe) are prominent TE materials. While numerous band and microstructure engineering strategies have advanced PbTe, PbSe is an attractive, lower-cost alternative with balanced carrier and phonon transport. However, near-room-temperature performance of PbSe has lagged, limiting average ZT at low-medium temperatures. This study targets enhancing near-room-temperature and low-medium-temperature performance of n-type PbSe by introducing dense dislocations via heavy Te/S alloying and Cu interstitial doping to suppress κlat while maintaining favorable carrier transport.

Prior work in lead chalcogenides (especially PbTe) has leveraged electronic band convergence, resonant states, and nanostructuring to boost ZT. For PbSe, reported approaches include band flattening to increase effective mass, band sharpening to optimize mobility, dynamic doping, and nanoscale defect engineering. Dislocation engineering has been shown to strongly scatter phonons and reduce lattice thermal conductivity; prior studies introduced dislocations via vacancies or defect clustering, though impacts on carrier transport can differ depending on defect type (interstitial vs vacancy). Additional strategies include alloying with CdSe or GeSe to tune bands and phonons, and forming nanocomposites. Despite these advances, achieving high ZT near room temperature in n-type PbSe remains challenging, motivating the present approach combining heavy Te/S alloying with Cu interstitial doping to generate dense dislocation networks while optimizing electronic transport.

Synthesis: High-purity Pb (99.999%), Se (99.999%), Te (99.999%), S (99.99%), and Cu (99.99%) were weighed to stoichiometry, sealed in evacuated silica tubes (<10⁻1 Torr), heated to 1323 K over 12 h, held 6 h, then furnace-cooled. Ingots were ground and hot-pressed at 773 K for 40 min in a 15 mm die under 50 MPa in vacuum to form dense disks, followed by annealing at 773 K for 6 h. Structural characterization: Powder X-ray diffraction (Cu Kα, λ=1.5418 Å; 40 kV/40 mA) was used for phase identification; lattice parameters were refined using MAUD. Dislocation structures were examined by TEM/HRTEM and AC-STEM HAADF imaging (Thermo Fisher Titan Themis Z); samples were prepared by FIB (Thermo Fisher Scios 2). Dislocation density was quantified from XRD peak broadening using the modified Williamson–Hall (MWH) method, analyzing ΔK–K plots and contrast factors. Geometric phase analysis (GPA) provided strain maps (exx, exy) around dislocations; EDS mapping identified elemental distributions (Pb, Se, Te, S, Cu) near dislocation cores to elucidate interstitial behavior. Transport measurements: Bars (≈12×4×4 mm) were used to measure σ and S from 300–773 K with CTA equipment (Cryoall) under low-pressure He. Squares (≈8×8×1.5 mm) were used for thermal diffusivity D via LFA-467 (Netzsch) and CLA-1000 (Cryoall); samples were graphite-coated. Density was determined from dimensions and mass; heat capacity Cp was estimated by the Debye model; thermal conductivity κtot was obtained from κtot = D·Cp·ρ, and κlat was calculated by subtracting κele (from σ, with Lorenz number approach; details in Supplementary). Weighted mobility μw was calculated from measured σ and S using the weighted mobility framework (Snyder et al.), with acoustic phonon scattering (r = −1/2) and Fermi integrals to extract μw and assess the balance of electrical transport against κlat (μw/κlat).

• Achieved high average figures of merit in n-type PbSe-based composition Pb1.02Se0.72Te0.20S0.08-0.3%Cu: ZTave = 0.90 over 300–573 K and ZTave = 0.96 over 300–773 K; room-temperature ZT ≈ 0.62.
• Dense dislocations generated via heavy Te/S alloying and Cu interstitial doping suppress lattice thermal conductivity while preserving favorable carrier transport. Dislocation density from MWH analysis reaches ≈5.4 × 10^16 m⁻², with both screw and edge dislocations observed (length scales from several to hundreds of nanometers).
• Lattice thermal conductivity is strongly reduced: κlat at room temperature ≈ 0.42 W m⁻¹ K⁻¹; minimal κlat ≈ 0.29 W m⁻¹ K⁻¹ across the measured range in Pb1.02Se0.72Te0.20S0.08-0.3%Cu. Comparison against literature shows this κlat is among the lowest for n-type PbSe systems and improves with Cu interstitial content.
• Electrical transport remains high despite dense dislocations. Cu interstitials act as donor dopants, increasing σ and decreasing |S| in a temperature-dependent manner consistent with dynamic doping behavior. The maximum power factor doubles from 7.73 to 14.6 μW cm⁻¹ K⁻² upon 0.3% Cu interstitial doping.
• Weighted mobility μw is enhanced by Cu interstitials, reaching ≈146.9 cm² V⁻¹ s⁻¹ at room temperature for the 0.3% Cu sample. The μw/κlat ratio increases with Cu doping, indicating synergistic improvement of electrical performance relative to phonon transport.
• Microstructural analyses (HAADF-STEM, GPA) reveal both edge (b = 1/2<111>) and screw (b = 1/3<111>) dislocations; screw dislocations exhibit larger strain fields and are inferred to scatter phonons more strongly than edge dislocations.
• Relative to other n-type PbSe systems (e.g., PbSe–SnS–Cu, PbSe–Cd–Cu, PbSe–Cu–Te, high-entropy PbSe, PbSe–Br–Cu2Se), the present composition exhibits superior near-room-temperature performance, with ZTRT ≈ 0.62 and ZTave (300–573 K) ≈ 0.90.
• Cycling tests indicate good repeatability and reliability of the high performance.
• Overall thermoelectric performance at low-medium temperatures is comparable to typical n-type Bi2Te3-based materials (e.g., Bi2Te2.5Se0.5, Bi2Te2.7Se0.3 + 1 wt% Bi2S3, Bi2Te3 + 1 wt% Ru).

The study demonstrates that engineering dense dislocation networks via heavy alloying (Te/S) combined with Cu interstitial doping effectively decouples thermal and electrical transport in n-type PbSe. Dislocations introduce strong phonon scattering centers, markedly lowering κlat across 300–773 K. Crucially, Cu interstitials serve a dual role: they promote dislocation formation (enhancing phonon scattering) and donate electrons, preserving or enhancing carrier transport. Weighted mobility analysis shows that μw remains high, and its ratio to κlat improves, aligning with the observed increase in power factor. Microstructural evidence indicates both edge and screw dislocations; the larger strain fields of screw dislocations likely account for stronger phonon scattering, offering mechanistic insight into dislocation-type-specific effects. Comparative analysis suggests interstitial-induced dislocations are more compatible with n-type conductivity than vacancy-induced ones, which may introduce local hole carriers and energy barriers. The combined effects yield significantly improved ZT near room temperature and high ZTave at low-medium temperatures, moving PbSe performance toward that of established Bi2Te3-based materials for cooling and energy harvesting in this temperature regime.

By integrating heavy Te/S alloying with Cu interstitial doping, the authors realize dense dislocation networks in n-type PbSe that suppress lattice thermal conductivity to as low as ≈0.29–0.42 W m⁻¹ K⁻¹ while retaining high electrical transport (enhanced μw and doubled power factor). This synergy delivers ZTave ≈ 0.90 (300–573 K) and 0.96 (300–773 K), with ZTRT ≈ 0.62, outperforming prior n-type PbSe systems and approaching Bi2Te3-based benchmarks near room temperature. The work underscores the importance of dislocation-type control (screw vs edge) and interstitial vs vacancy defect chemistry for optimizing phonon–carrier decoupling. The strategy of heavy alloying plus interstitial doping to generate dense dislocations provides a generalizable route to improved thermoelectrics and could be extended to other semiconductor systems. Future research may focus on precise control of dislocation character and density, optimizing interstitial concentrations for stability and performance, and integrating such materials into device architectures for low-medium temperature cooling and power generation.