High thermoelectric performance is generally achieved through either electronic structure modulations or phonon scattering enhancements, which often counteract each other. A leap in performance requires innovative strategies that simultaneously optimize electronic and phonon transports. We demonstrate high thermoelectric performance with a near room-temperature figure of merit, ZT ~ 1.5, and a maximum ZT ~ 2.6 at 573 kelvin, by optimizing atomic disorder in cadmium-doped polycrystalline silver antimony telluride (AgSbTe2). Cadmium doping in AgSbTe2 enhances cationic ordering, which simultaneously improves electronic properties by tuning disorder-induced localization of electronic states and reduces lattice thermal conductivity through spontaneous formation of nanoscale (~2 to 4 nanometers) superstructures and coupling of soft vibrations localized within ~1 nanometer around cadmium sites with local strain modulation. The strategy is applicable to most other thermoelectric materials that exhibit inherent atomic disorder.

GeTe and its derivatives constituting Pb-free elements have been well known as potential thermoelectric materials for the last five decades, which offer paramount technological importance. The main constraint in the way of optimizing thermoelectric performance of GeTe is the high lattice thermal conductivity (κ). Herein, we demonstrate low κ (∼0.7 W/m·K) and a significantly high thermoelectric figure of merit (ZT = 2.1 at 630 K) in the Sb-doped pseudoternary (GeTe)(GeSe)(GeS) system by two-step strategies. The (GeTe)(GeSe)(GeS) system provides an excellent podium to investigate competition between an entropy-driven solid solution and enthalpy-driven phase separation. In the first step, small concentrations of Se and S were substituted simultaneously in the position of Te in GeTe to reduce the κ by phonon scattering due to mass fluctuations and point defects. When the Se/S concentration increases significantly, the system deviates from a solid solution, and phase separation of the GeSSe (5-20 μm) precipitates in the GeTeSe matrix occurs, which does not participate in phonon scattering. In the second stage, κ of the optimized sample is further reduced to 0.7 W/m·K by Sb alloying and spark plasma sintering (SPS), which introduce additional phonon scattering centers such as excess solid solution point defects and grain boundaries. The low κ in Sb-doped (GeTe)(GeSe)(GeS) is attributed to phonon scattering by entropically driven solid solution point defects rather than conventional endotaxial nanostructuring. As a consequence, the SPS-processed GeSbTeSeS sample exhibits a remarkably high ZT of 2.1 at 630 K, which is reproducible and stable over temperature cycles. Moreover, Sb-doped (GeTe)(GeSe)(GeS) exhibits significantly higher Vickers microhardness (mechanical stability) compared to that of pristine GeTe.

GeTe and its derivatives have recently attracted wide attention as promising thermoelectric materials. The principle challenge in optimizing the thermoelectric figure of merit, zT, is the low Seebeck coefficient (S) and high thermal conductivity of GeTe. Here, we report a high zT of $2.1 at 723 K in In and Bi codoped GeTe along with an extremely high TE conversion efficiency of $12.3% in a single-leg thermoelectric generator for the temperature difference of 445 K. In and Bi play a distinct but complementary role. In doping significantly enhances the S through the formation of resonance level, which is confirmed with first-principles density functional theory calculations and Pisarenko plot considering two valance band model. However, Bi doping markedly reduces the lattice thermal conductivity due to the formation of extensive solid solution point defects and domain variants. Moreover, a high value of Vickers microhardness ($200 H v , H v = kgf/mm 2 ) reveals excellent mechanical stability.

Realization of high thermoelectric performance in n-type semiconductors is of imperative need on account of the dearth of efficient n-type thermoelectric materials compared to the p-type counterpart. Moreover, development of efficient thermoelectric materials based on Te-free compounds is desirable because of the scarcity of Te in the Earth's crust. Herein, we report the intrinsic ultralow thermal conductivity and high thermoelectric performance near room temperature in n-type BiSe, a Te-free solid, which recently has emerged as a weak topological insulator. BiSe possesses a layered structure consisting of a bismuth bilayer (Bi) sandwiched between two BiSe quintuple layers [Se-Bi-Se-Bi-Se], resembling natural heterostructure. High thermoelectric performance of BiSe is realized through the ultralow lattice thermal conductivity (κ of ∼0.6 W/mK at 300 K), which is significantly lower than that of BiSe (κ of ∼1.8 W/mK at 300 K), although both of them belong to the same layered homologous family (Bi) (BiSe) . Phonon dispersion calculated from first-principles and the experimental low-temperature specific heat data indicate that soft localized vibrations of bismuth bilayer in BiSe are responsible for its ultralow κ. These low energy optical phonon branches couple strongly with the heat carrying acoustic phonons, and consequently suppress the phonon mean free path leading to low κ. Further optimization of thermoelectric properties of BiSe through Sb substitution and spark plasma sintering (SPS) results in high ZT ∼ 0.8 at 425 K along the pressing direction, which is indeed remarkable among Te-free n-type thermoelectric materials near room temperature.

S2 MethodsReagents. Bismuth nitrate (Bi(NO3)35H2O, Alfa Aesar, 99.9%), selenourea (SeC(NH2)2, Alfa Aesar, 99.9%), potassium hydroxide (KOH, S D Fine-Chem Limited (SDFCL)), sodium hydroxide (NaOH, SDFCL), Disodium EDTA (C10H14O8Na2N2H2O, SDFCL) and ethanol were used without any further purification.Synthesis procedure. 100 mg (0.206 mmol) of Bi(NO3)35H2O, 12.7 mg (0.103 mmol) of SeC(NH2)2 and 306.8 mg (0.824 mmol) of disodium EDTA were sequentially added at a 5 minutes interval into 20 ml water in a glass beaker. The solution was stirred continuously. The addition of Bi(NO3)35H2O into water results in a milky white color solution which turns into an orange color solution after the addition of SeC(NH2)2. The solution becomes clear after the addition of disodium EDTA. Finally, 120 mg (2.14 mmol) of KOH and 320 mg (8 mmol) of NaOH were added into the solution which turns the solution color black. After 10 minutes of stirring, the solution was put to rest which results in precipitation of the dark brown color nanosheets. We observed that nanosheets of similar morphology and thickness can also be obtained without using disodium EDTA, however, in that case, the required amount of water solvent is much higher, 200 ml. These were then washed with alcohol and water and centrifuged to remove disodium EDTA. The purified product was then dried in a vacuum oven at 150 C.Step I: In water, Bi(NO3)3 undergoes hydrolysis to produce BiONO3 and the process of hydrolysis is expedites in alkaline medium:Step II: Selenourea SeC(NH2)2 undergoes decomposition in alkaline medium to generate selenide ions (Se 2-) along with cyanamide (H2NCN): SeC(NH2)2 + OH -→ Se 2-+ H2NCN + H2O

Successful applications of a thermoelectric material require simultaneous development of compatible n-and p-type counterparts. While the thermoelectric performance of p-type GeTe has been improved tremendously in recent years, it has been a challenge to find a compatible n-type GeTe counterpart due to the prevalence of intrinsic Ge vacancies. Herein, we have shown that alloying of AgBiSe 2 with GeTe results in an intriguing evolution in its crystal and electronic structures, resulting in n-type thermoelectric properties. We have demonstrated that the ambient rhombohedral structure of pristine GeTe transforms into cubic phase in (GeTe) 100−x (AgBiSe 2 ) x for x ≥ 25, with concurrent change from its p-type electronic character to n-type character in electronic transport properties. Such change in structural and electronic properties is confirmed from the nonmonotonic variation of band gap, unit cell volume, electrical conductivity, and Seebeck coefficient, all of which show an inflection point around x ∼ 20, as well as from the temperature variations of synchrotron powder X-ray diffractions and differential scanning calorimetry. First-principles density functional theoretical (DFT) calculations explain that the shift toward n-type electronic character with increasing AgBiSe 2 concentration arises due to increasing contribution of Bi p orbitals in the conduction band edge of (GeTe) 100−x (AgBiSe 2 ) x . This cubic n-type phase has promising thermoelectric properties with a band gap of ∼0.25 eV and ultralow lattice thermal conductivity that ranges between 0.3 and 0.6 W/mK. Further, we have shown that (GeTe) 100−x (AgBiSe 2 ) x has promising thermoelectric performance in the mid-temperature range (400−500 K) with maximum thermoelectric figure of merit, zT, reaching ∼1.3 in p-type (GeTe) 80 (AgBiSe 2 ) 20 at 467 K and ∼0.6 in n-type (GeTe) 50 (AgBiSe 2 ) 50 at 500 K.

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit (ZT ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZT ). Herein an ultrahigh ZT of 1.4 for (GeTe) (AgSbSe ) [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZT of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe-x is attributed to extremely low lattice thermal conductivity (κ ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe in GeTe results in κ of ≈0.4 W mK in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (κ ) of GeTe. Additionally, (GeTe) (AgSbSe ) exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics.

Thermoelectric materials can convert untapped heat to electricity and are expected to have an important role in future energy utilization. IV−VI metal chalcogenides are the most promising candidates for mid-temperature thermoelectric power generation. Among them, PbTe and their alloys have been proven to be the superior thermoelectric materials. Unfortunately, the toxicity of lead (Pb) prevents the application of lead chalcogenides and demands the search for lead-free high-performance solids. This perspective discusses the recent progress of thermoelectric property studies on germanium chalcogenides (GeTe, GeSe, and GeS) for midtemperature power generation. Here, we have discussed the crystal structure, chemical bonding, and phonon dispersion of germanium chalcogenides to understand the underlying lattice dynamics and low lattice thermal conductivity from a chemistry perspective. We have also discussed the uniqueness of the electronic structure of GeTe and GeSe, which plays an important role in tailoring thermoelectric properties. Additionally, the implications of the recent state-of-art strategies such as resonant level formation, valence band convergence, slight symmetry breaking of the crystal and electronic structures, point defect, and nanostructure induced phonon scattering on the high thermoelectric performance of the germanium chalcogenides are discussed in detail. In conclusion, we highlight some of the innovative ideas for discovery and designing of new thermoelectric compositions. Finally, we point out the major challenges and opportunities in this field. All the strategies discussed in this perspective not only make germanium chalcogenides as a promising candidate for future thermoelectric applications but also serve as a guide to enhance the thermoelectric performance of other materials.