Thermoelectric (TE) generators provide electrical energy from direct conversion of heat by means of the Seebeck effect; without moving parts, completely silent, and with negligible maintenance. As any other heat engine this conversion exploits only a fraction of the Carnot efficiency (Rowe (ed.), CRC Handbook of Thermoelectrics (CRC Press Inc., 1995), p. 19 1). The TE efficiency is linked to the thermoelectric figure of merit Z, which itself is given by basic material properties: Z = S²σ/κ. These are the electrical conductivity σ, the thermal conductivity κ and the Seebeck coefficient or thermopower S, which is known as the factor of proportionality between voltage output and applied temperature difference in a given TE sample. A distinct sensitivity to the carrier concentration and structural variations make the control and stabilisation of thermopower very challenging in complex material structures since degradation by diffusion, decomposition or evaporation can be observed in many cases during synthesis, operation and even in the process of characterisation of TE semiconductors; particularly at elevated temperatures. Investigating structural and compositional properties, stability, and performance of TE materials, and consequently aiming to understand their interaction, mainly methods like X‐ray diffraction (XRD), energy‐dispersive X‐ray spectroscopy (EDX), scanning electron microscopy (SEM) or integral temperature dependent measurements of particular transport properties are used. Although TE materials research satisfies highest requirements on accuracy, the above mentioned techniques are not perfectly qualified to investigate promising material classes thoroughly. Against the background of usually complex material structures this article aims to show, that an efficient characterisation of TE materials becomes accessible for several questions by use of a spatially resolved determination of the thermopower.

The thermoelectric figure of merit ZT, which is defined using electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature T, has been widely used as a simple estimator of the conversion efficiency of a thermoelectric heat engine. When material properties are constant or slowly varying with T, a higher ZT ensures a higher maximum conversion efficiency of thermoelectric materials. However, as material properties can vary strongly with T, efficiency predictions based on ZT can be inaccurate, especially for wide-temperature applications. Moreover, although ZT values continue to increase, there has been no investigation of the relationship between ZT and the efficiency in the higher ZT regime. In this paper, we report a counterintuitive situation by comparing two materials: although one material has a higher ZT value over the whole operating temperature range, its maximum conversion efficiency is smaller than that of the other. This indicates that, for material comparisons, the evaluation of exact efficiencies as opposed to a simple comparison of ZTs is necessary in certain cases.

Charge carrier transport and corresponding thermoelectric properties are often affected by several parameters, necessitating a thorough comparative study for a profound understanding of the detailed conduction mechanism. Here, as a model system, we compare the electronic transport properties of two layered semiconductors, Sb₂Si₂Te₆ and Bi₂Si₂Te₆. Both materials have similar grain sizes and morphologies, yet their conduction characteristics are significantly different. We found that phase boundary scattering can be one of the main factors for Bi₂Si₂Te₆ to experience significant charge carrier scattering, whereas Sb₂Si₂Te₆ is relatively unaffected by the phenomenon. Furthermore, extensive point defect scattering in Sb₂Si₂Te₆ significantly reduces its lattice thermal conductivity and results in high zT values across a broad temperature range. These findings provide novel insights into electron transport within these materials and should lead to strategies for further improving their thermoelectric performance.

Decoupling charge and heat transport is essential for optimizing thermoelectric materials. Strategies to inhibit lattice-driven heat transport, however, also compromise carrier mobility, limiting the performance of most thermoelectrics, including Fe₂VAl Heusler compounds. Here, we demonstrate an innovative approach, which bypasses this tradeoff: via liquid-phase sintering, we incorporate the archetypal topological insulator Bi_1−x Sb _x between Fe₂V_0.95Ta_0.1Al_0.95 grains. Structural investigations alongside extensive thermoelectric and magneto-transport measurements reveal distinct modifications in the microstructure, a reduced lattice thermal conductivity and a simultaneously enhanced carrier mobility arising from topologically protected charge transport along the grain boundaries. This yields a huge performance boost, resulting in one of the highest figure of merits among both half- and full-Heusler compounds, z ≈ 1.6 × 10⁻³ K⁻¹ (z T ≈ 0.5) at 295 K. Our findings highlight the potential of topological-insulating secondary phases to decouple charge and heat transport and call for more advanced theoretical studies of multiphase composites.