Thermoelectric materials have attracted significant research interest in recent decades due to their promising application potential in interconverting heat and electricity. Unfortunately, the strong coupling between the material parameters that determine thermoelectric efficiency, i.e., the Seebeck coefficient, electrical conductivity, and thermal conductivity, complicates the optimization of thermoelectric energy converters. Main‐group chalcogenides provide a rich playground to alleviate the interdependence of these parameters. Interestingly, only a subgroup of octahedrally coordinated chalcogenides possesses good thermoelectric properties. This subgroup is also characterized by other outstanding characteristics suggestive of an exceptional bonding mechanism, which has been coined metavalent bonding. This conclusion is further supported by a map that separates different bonding mechanisms. In this map, all octahedrally coordinated chalcogenides with good performance as thermoelectrics are located in a well‐defined region, implying that the map can be utilized to identify novel thermoelectrics. To unravel the correlation between chemical bonding mechanism and good thermoelectric properties, the consequences of this unusual bonding mechanism on the band structure are analyzed. It is shown that features such as band degeneracy and band anisotropy are typical for this bonding mechanism, as is the low lattice thermal conductivity. This fundamental understanding, in turn, guides the rational materials design for improved thermoelectric performance by tailoring the chemical bonding mechanism.

Thermoelectric materials provide a challenge for materials design, since they require optimization of apparently conflicting properties. The resulting complexity has favored trial-and-error approaches over the development of simple and predictive design rules. In this work, the thermoelectric performance of IV-VI chalcogenides on the tie line between GeSe and GeTe is investigated. From a combination of optical reflectivity and electrical transport measurements, it is experimentally proved that the outstanding performance of IV-VI compounds with octahedral-like coordination is due to the anisotropy of the effective mass tensor of the relevant charge carriers. Such an anisotropy enables the simultaneous realization of high Seebeck coefficients, due to a large density-of-states effective mass, and high electrical conductivity, caused by a small conductivity effective mass. This behavior is associated to a unique bonding mechanism by means of a tight-binding model, which relates band structure and bond energies; tuning the latter enables tailoring of the effective mass tensor. The model thus provides atomistic design rules for thermoelectric chalcogenides.

The thermoelectric compound (GeTe) x (AgSbTe 2 ) 1−x , in short (TAGS-x), is investigated with a focus on two stoichiometries, i.e., TAGS-50 and TAGS-85. TAGS-85 is currently one of the most studied thermoelectric materials with great potential for thermoelectric applications. Yet, surprisingly, the lowest thermal conductivity is measured for TAGS-50, instead of TAGS-85. To explain this unexpected observation, atom probe tomography (APT) measurements are conducted on both samples, revealing clusters of various compositions and sizes. The most important role is attributed to Ag 2 Te nanoprecipitates (NPs) found in TAGS-50. In contrast to the Ag 2 Te NPs, the matrix reveals an unconventional bond breaking mechanism. More specifically, a high probability of multiple events (PME) of ≈60% is observed for the matrix by APT. Surprisingly, the PME value decreases abruptly to ≈20-30% for the Ag 2 Te NPs. These differences can be attributed to differences in chemical bonding. The precipitates' PME value is indicative of normal bonding, i.e., covalent bonding with normal optical modes, while materials with this unconventional bond breaking found in the matrix are characterized by metavalent bonding. This implies that the interface between the metavalently bonded matrix and covalently bonded Ag 2 Te NP is partly responsible for the reduced thermal conductivity in TAGS-50.

Annealing process is crucial in obtaining high-quality perovskite layers used for highly efficient planar perovskite solar cells. In this study, we have investigated annealing-induced chemical and structural changes of tri-iodide (TI) and mixed-halide (MH) organometal perovskite layers using infrared absorption spectroscopy, scanning electron microscopy and x-ray diffraction measurements. For TI layers, the solvent molecule, dimethylformamide (DMF), remained in the form of PbI2/ DMF compound after drying at room temperature. During annealing, the DMF evaporated to form PbI2 crystals. When the MH perovskite film was annealed, both CH3NH3PbCl3 and CH3NH3PbI3 crystals were initially formed from an amorphous phase. With further annealing, the CH3NH3PbI3 crystals gradually grew through the incorporation of source materials supplied from the CH3NH3PbCl3 crystals and the amorphous phase and the slow evaporation of methylammonium (MA) and chloride ions. The resultant MH perovskite layer after annealing was mainly composed of large CH3NH3PbI3 grains with a trace of chloride ions. We suggest that the difference in composition and structure leads to different charge transporting properties of the TI and MH perovskite layers.