Lead telluride is one of the most attractive state-of-the-art thermoelectric (TE) materials. Therefore, any improvement of its average thermoelectric figure of merit (ZT)_av over a broad temperature range is a great challenge. Here, we show that advanced electronic structure engineering using the In quasi-local level with simultaneously optimized E_F leads to a significant enhancement of energy conversion. Electronic structure calculations confirmed the formation of a half-occupied indium quasi-local level in PbTe, which, together with I-implemented electrons, provides the optimal chemical potential in terms of energy conversion, near the conduction band edge over the entire temperature range. As a result, an extremely high average thermoelectric figure of merit (ZT)_av of up to ∼1.05 in the temperature range of 298 K to 773 K was achieved. Thermoelectric efficiency η was enhanced up to 14.2% for an n-type PbTe-based leg, which is one of the record-high values for a single-phase TE material. The performed theoretical analyses and experimental data prove that the large (ZT)_av found in our system comes from the electronic structure and chemical potential engineering, offering a novel route for improving the efficiency of thermoelectric devices.

Because of the large Seebeck coefficient, low thermal conductivity, and earth-abundant nature of components, tetrahedrites are promising thermoelectric materials. DFT calculations reveal that the additional copper atoms in Cu-rich Cu₁₄Sb₄S₁₃ tetrahedrite can effectively engineer the chemical potential towards high thermoelectric performance. Here, the Cu-rich tetrahedrite phase was prepared using a novel approach, which is based on the solvothermal method and piperazine serving both as solvent and reagent. As only pure elements were used for the synthesis, the offered method allows us to avoid the typically observed inorganic salt contaminations in products. Prepared in such a way, Cu₁₄Sb₄S₁₃ tetrahedrite materials possess a very high Seebeck coefficient (above 400 μVK⁻¹) and low thermal conductivity (below 0.3 Wm⁻¹K⁻¹), yielding to an excellent dimensionless thermoelectric figure of merit ZT ≈ 0.65 at 723 K. The further enhancement of the thermoelectric performance is expected after attuning the carrier concentration to the optimal value for achieving the highest possible power factor in this system.

Copper‐based chalcogenides are cost‐effective and environmentally friendly thermoelectric (TE) materials for waste heat recovery. Despite demonstrating excellent thermoelectric performance, binary Cu₂X (X = S, Se, and Te) chalcogenides undergo superionic phase transitions above room temperature, leading to microstructural evolution and unstable properties. In this work, a new γ‐phase of Cu₆Te_3‐xS_1+x (0 < x ≤ 1) is discovered, a narrow‐bandgap semiconductor with outstanding thermoelectric performance and high stability. By substituting Te with S in metallic Cu₆Te₃S, the crystal symmetry is modified and structural phase transitions are eliminated. The γ‐phase exhibits a significantly higher Seebeck coefficient of up to 200 µVK⁻¹ compared to 8.8 µVK⁻¹ for Cu₆Te₃S at room temperature due to optimized carrier concentration and increased effective mass. Cu₆Te_3‐xS_1+x materials also demonstrate ultralow thermal conductivity (≈0.25 Wm⁻¹K⁻¹), which, in concert with improved power factors, enables a high zT of ≈1.1 at a relatively low temperature of 500 K. Unlike most Cu‐based chalcogenides, the γ‐phase exhibits excellent transport property stability across multiple thermal cycles, making it a cost‐effective and eco‐friendly alternative to Bi₂Te₃‐based materials. The developed Cu₆Te_3‐xS_1+x is a promising candidate for thermoelectric converters in waste heat recovery, and its potential can be further extended to cooling applications through carrier concentration tuning.

Cu‐based argyrodites have attracted attention as thermoelectric materials for energy harvesting. However, phase transitions and low energy conversion performance limit their applications.This work explores the great thermoelectric performance of Cu ₈ SiS _x Se _{6‐ x} argyrodite with stabilized high‐symmetry cubic phase at above 282 K. The suppression of the phase transition below room temperature is realized due to the increase of the configurational entropy caused by the substitution of Se by S in Cu ₈ SiS _x Se _{6‐ x} . As a result, a significant enhancement of weighted mobility is achieved for materials without the phase transition due to clean grain boundary contacts. In turn, samples with disordered or dirty grain boundaries lead to a lower and increasing trend of weighted mobility for materials with phase transition in the operating temperature range. The developed argyrodites exhibit excellent thermoelectric performance due to the improved power factor of up to 4.2 µWcm ⁻¹ K ⁻² and ultralow lattice thermal conductivity of 0.18 Wm ⁻¹ K ⁻¹ at 773 K, which is derived from the peculiar chemical bonding with a large number of different Si‐Se, Se‐Cu, and Cu‐Cu interactions. The dimensionless thermoelectric figure of merit zT for Cu ₈ SiS ₃ Se ₃ argyrodite reaches a value of 1.24, offering this material as a low‐cost and Te‐free alternative for thermoelectric energy conversion applications.