Engineering the electronic band structures upon doping is crucial to improve the thermoelectric performance of materials. Understanding how dopants influence the electronic states near the Fermi level is thus a prerequisite to precisely tune band structures. Here, we demonstrate that the Sn-s states in SnTe contribute to the density of states at the top of the valence band. This is a consequence of the half-filled p-p σ-bond (metavalent bonding) and its resulting symmetry of the orbital phases at the valence band maximum (L point of the Brillouin zone). This insight provides a recipe for identifying superior dopants. The overlap between the dopant s- and the Te p-state is maximized, if the spatial overlap of both orbitals is maximized and their energetic difference is minimized. This simple design rule has enabled us to screen out Al as a very efficient dopant to enhance the local density of states for SnTe. In conjunction with doping Sb to tune the carrier concentration and alloying with AgBiTe₂ to promote band convergence, as well as introducing dislocations to impede phonon propagation, a record-high average ZT of 1.15 between 300 and 873 K and a large ZT of 0.36 at 300 K is achieved in Sn_0.8Al_0.08Sb_0.15Te-4%AgBiTe₂.

Thermoelectric technology exhibits significant potential for power generation and electronic cooling. In this study, we report the achievement of exceptional thermoelectric performance and high plasticity in stable Cu₂Se/SnSe composites. A novel matrix plainification strategy was employed to eliminate lattice vacancies within the Cu₂Se matrix of the Cu₂Se/SnSe composites, resulting in a marked improvement in carrier mobility and power factor. The presence of quasi-coherent interfaces induces phonon scattering, reducing lattice thermal conductivity without compromising carrier mobility. Consequently, a high figure of merit (ZT) of 3.3 was attained in the Cu₂Se/5 wt.% Sn_0.96Pb_0.01Zn_0.03Se composite. Additionally, the presence of high-density nanotwins imparts remarkable plasticity to the composite, yielding a compressive strain of 12%. The secondary phase contributes to the stability of the composite by hindering the extensive migration of Cu ions through bonding interactions. Our findings present a novel strategy for enhancing the thermoelectric performance of composite semiconductors, with potential applicability to other thermoelectric systems.

Discrepancies between experimental and theoretical results in the study of thermoelectric generators (TEGs) have been a major long-standing problem in thermoelectric technology. In this Letter, we report that, besides interfacial resistance, the inevitable heat leap caused by the Peltier effect is the main factor affecting the conversion efficiency of TEGs. In fact, the heat leap is proven to have an impact of approximately 10% on the conversion efficiency of common TEGs. In addition, we enhance the formula for maximum conversion efficiency with heat leap from the classical expression to allow for the prediction of the performance of advanced materials in TEGs. For the first time, the experimental data from conversion efficiency corresponds exactly to that obtained theoretically by considering both the heat leap and interfacial resistivity.