Abstract: The commercial viability of thermoelectric (TE) devices relies heavily on two factors: cost reduction and efficiency enhancement. In this study, we first produce p-type Cu₁₂Sb₄S_16-x (x = 0, 3, 4) using a low-temperature bottom-up approach and demonstrate Cu₁₂Sb₄S₁₃ to show the best TE performance among the three tested compositions. Subsequently, the TE energy conversion efficiency of Cu₁₂Sb₄S₁₃ is further enhanced by optimizing its electronic band structure through the incorporation of small amounts of tellurium. At an optimal Te content of 5 mol%, more than a twofold increase in the TE figure of merit (zT) is obtained. To gain insight into the mechanism of improvement on the transport properties of the material, we compare the interphase transport mechanism by incorporating nanodomains of different metals (Ag and Cu) into the Cu₁₂Sb₄S₁₃ matrix. The improved electrical conductivity obtained with Cu₁₂Sb₄S₁₃-Te nanocomposites is attributed to a charge flooding of the Cu₁₂Sb₄S₁₃ surface. In contrast, excessive downward band-bending at the interphases of Ag/Cu metal-semiconductor drastically reduces the electrical conductivity. Besides, a weighted mobility (μ_w) analysis shows a dominant thermal activation of carriers in Cu₁₂Sb₄S₁₃-Te nanocomposites. In this material, a strong decrease in lattice thermal conductivity is also found, which is associated with a phonon-carrier scattering mechanism. Our work shows the importance of proper band-engineering in TE nanocomposites to decouple electrical and thermal transport to enhance TE performance, and the efficacy of μ_w for electrical and thermal transport analysis.

Key words: Modulation doping, Thermoelectric, Interphase transport, Charge flooding, Phonon-carrier scattering