converters was successfully initiated by the development of various highly efficient TE material classes. Such materials require a unique combination of electronic (i.e., Seebeck coeffi cient ( α ), electrical resistivity ( ρ ), electronic thermal conductivity ( κ e ), and lattice (i.e., lattice thermal conductivity ( κ l )) properties, enabling the highest possible TE fi gure of merit ( ZT = α 2 T /[ ρ ( κ e + κ l )], where T is the absolute temperature values, for achieving signifi cant heat-to-electricity conversion effi ciencies. AB (where A is Pb, Sn, and Ge, and B is Te, Se, and S) chalcogenides and their alloys are narrow band-gap semiconductors, known as the most effi cient TE alloys for intermediate working temperatures of up to 600 °C. Yet, due to the fact that the electronic properties (i.e., α , ρ , and κ e ) are strongly coupled and follow opposite trends (i.e., α and ρ are decreased, and κ e is increased) upon increasing the carrier concentration (for example, by introducing the doping elements), most of the recently published highly effi cient chalcogenides were mainly focused on applying the advanced nanostructuring approaches for κ l reduction, and correspondingly enhancement of ZT due to lattice modifi cations. Such approaches included alloying methods (e.g., with SrTe, [ 1,2 ] MgTe, [ 3 ] and CdTe, [ 4 ] generating embedded strained endotaxial nanostructures, for the case of PbTe), the usage of layered structures, effectively scattered phonons (e.g., SnSe, [ 5 ] and approaching phase separation reactions, generating thermodynamic-driven nanoscale modulations (e.g., Ge x Pb 1-x Te [6][7][8] and Ge x (Sn y Pb 1-y ) 1-x Te. [ 9,10 ] All of these approaches resulted in a signifi cant increase of ZT up to ≈2.5 [ 5 ] due to an effective scattering of phonons by the associated generated nanofeatures.Regarding electronic optimization, for maximizing the α 2 / ρκ e component of ZT , besides using standard doping elements (e.g., PbI 2 and Bi as donors, and Na as an acceptor) for a moderate tuning of the carrier concentrations toward TE optimal values in the range of 10 19 cm −3 , attempts for TE electronic optimization of chalcogenides were so far focused on increasing the carrier effective mass by the convergence of electronic bands (e.g., enhancing the effect of heavy holes on account of light holes in degenerate PbSe [ 11 ] and GeTe [ 12 ] alloys). Distortion of the electronic density of states by the generation of resonant states and pinning of Fermi energy at TE optimal energetic locations (e.g., Tl- [ 13 ] and In-doped

methods to enhance the materials' thermoelectric properties, practical design considerations including geometrical optimization and minimization of contact resistances and long-term properties degradation should be considered. For many years, thermoelectric power generation has enjoyed its greatest success in special and exotic applications, such as the space missions, in which the chalcogenides class of thermoelectric materials (e.g., GeTe, [1][2][3][4] PbTe, [5][6][7][8][9][10] PbS, [11][12][13][14] SnTe, [15][16][17][18][19][20][21] or their alloys) is the most thermoelectrically effi cient in the intermediate temperature range of ≈500 °C, but still the system conversion effi ciency for a state-ofpractice NASA RTG (radioisotope thermoelectric generator) is about 6%, [ 22 ] where in this type of application environmental issues and cost are not the main concerns. In this thermoelectric materials class, it was recently reported that upon proper compositional investigation for systems exhibiting a miscibility gap between two individual chalcogenide components, sub-micron thermodynamic-driven phase separation features are expected, leading to dramatically reduced thermal conductivity values and enhanced ZT s. Two examples are the p-type Ge 0.87 Pb 0.13 Te, [ 1 ] shown in Figure 1 a, and the n-type 0.055% PbI 2 doped (PbSn 0.05 Te) 0.92 (PbS) 0.08 [ 11 ] compositions, exhibiting very high maximal ZT s of ≈2.2 and ≈1.5, respectively, and correspondingly a very high thermoelectric potential.For the latter, investigation of the PbS-PbTe quasi-binary phase diagram, Figure 1 b reveals an extended miscibility gap at temperatures below 800 °C (1073 K in Figure 1 b), in which a higher-temperature solution-treated single phase (the upper scanning electron microscopy, SEM image) is expected to follow a rapid phase separation to two individual PbTe-and PbS-rich phases (the lower SEM image) by either the spinodal decomposition or nucleation and growth mechanisms, upon cooling, depending on the specifi c composition. Due to the rapid nature of these phase separation reactions, the apparent alternating phases are usually fi nely dispersed, making them very useful phonon scattering centers for thermal conductivity reduction. Due to its thermodynamic origin, such a submicron phase generation is considered much more stable at elevated operating temperatures, compared to any other mechanical nano-features generation approaches such as ball-milling, which are expected to follow coarsening into the micron scale with long-term operations.

In an attempt to reduce our reliance on fossil fuels, associated with severe environmental effects, the current research is focused on the identification of the thermoelectric potential of p-type (GeTe)1−x(Bi2Te3)x alloys, with x values of up to 20%. Higher solubility limit of Bi2Te3 in GeTe, than previously reported, was identified around ∼9%, extending the doping potential of GeTe by the Bi2Te3 donor dopant, for an effective compensation of the high inherent hole concentration of GeTe toward thermoelectrically optimal values. Around the solubility limit of 9%, an electronic optimization resulted in an impressive maximal thermoelectric figure of merit, ZT, of ∼1.55 at ∼410 °C, which is one of the highest ever reported for any p-type GeTe-rich alloys. Beyond the solubility limit, a Fermi Level Pinning effect of stabilizing the Seebeck coefficient was observed in the x = 12%–17% range, leading to stabilization of the maximal ZTs over an extended temperature range; an effect that was associated with the potential of the governed highly symmetric Ge8Bi2Te11 and Ge4Bi2Te7 phases to create high valence band degeneracy with several bands and multiple hole pockets on the Fermi surface. At this compositional range, co-doping with additional dopants, creating shallow impurity levels (in contrast to the deep lying level created by Bi2Te3), was suggested for further electronic optimization of the thermoelectric properties.

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