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Decades of studies on thermoelectric materials have enabled the design of high‐performance materials based on basic materials properties, such as bandgap engineering. In general, bandgap energies correspond to the temperature at which the peak thermoelectric performance occurs. For instance, CuGaTe2 with a relatively wide bandgap of 1.2 eV has its peak zT > 1 at > 900 K. On the other hand, the zT is usually very low (<0.1) for this material at room temperature. This severely limits its average zT and hence overall performance. In this study, a phase diagram‐guided Sb alloying strategy to improve the low‐temperature zT of CuGaTe2 is used, by leveraging on the solubility limits to control the formation of the microstructural defects. The addition of Sb simultaneously improves the electrical conductivity and decreases the lattice thermal conductivity. For a low‐temperature range of 300–623 K, this Sb‐alloying strategy enables the achievement of a record high average zT of 0.33. The strategy developed in this study targets the improvement of the low‐temperature range of CuGaTe2, which is rarely focused on for wide‐bandgap ABX2 compounds, opening up more opportunities for holistic performance improvements, potentially enabling ultrahigh‐performance thermoelectrics over a wide temperature range. [ABSTRACT FROM AUTHOR]

Cu 3 SbSe 3 , with its ultralow thermal conductivity and Earth-abundant elements, has emerged as a promising thermoelectric material. However, synthesizing its pure phase and achieving effective doping have proven to be challenging. In this study, we present our findings on the investigation of thermoelectric properties of stoichiometric and off-stoichiometric Cu 3 SbSe 3 compositions along with various doping strategies. Our results demonstrate that samples with non-stoichiometric compositions and Bi doping further reduces the lattice thermal conductivity of 0.16 W/mK. The consistently moderate Seebeck coefficient leads to a zT value of 0.25 at 650 K across all cation-deficient samples. Furthermore, we observed that 5% Bi doping further enhances the zT value to 0.35, representing a 52% improvement over pristine Cu 3 SbSe 3. We believe that the compositional insensitivity of zT and the enhanced performance achieved through Bi doping offer new opportunities for exploring materials with ultra-low thermal conductivity in the field of thermoelectrics. (a) The effect of stoichiometry control on the ultralow thermal conductivity of Cu 3 SbSe 3. (b) The effect of doping on zT of Cu 3 SbSe 3. • Cu-deficiency effectively reduces lattice thermal conductivities down to 0.16 W/mK. • Cu 3 Sb 0.95 Bi 0.05 Se 3 exhibits a 52% improvement in zT compared to pristine Cu 3 SbSe 3. • Cu excess results in the formation of 100% pure Cu 3 SbSe 3. [ABSTRACT FROM AUTHOR]