The coupling nature of thermoelectric properties determines that optimizing the Fermi level is the priority to achieve a net increase in thermoelectric performance. Conventionally, the carrier concentration is used as the reflection of the Fermi level in the band structure. However, carrier concentration strongly depends upon the material's effective mass, leading to that the optimal carrier concentration varies over a large scale for different materials. Herein, inspired by the big data survey, we develop a golden Seebeck coefficient range of 202−230 μV K −1 for thermoelectric semiconductors with lattice thermal conductivity of 0.4−1.5 W m −1 K −1 . When the measured Seebeck coefficient reaches this range, the corresponding figure of merit is maximized. Using this approach, we exemplarily analyze the characteristics of n-type Pb 1−x Bi x Se thermoelectric materials. With detailed electron microscopy and property characterizations, the high densities of dislocations and pores are found to be responsible for a low lattice thermal conductivity. Moreover, Bi substitution significantly tunes the Seebeck coefficient in a wide range. As a result, the Seebeck coefficient of ∼ −230 μV K −1 in Pb 0.98 Bi 0.02 Se is close to the golden range, leading to a figure of merit beyond 1.5. This finding provides an intuitive metric to determine the optimization extent of thermoelectric performance.

No abstract

Thermoelectric materials, which enable the direct conversion between heat and electricity, have attracted worldwide attention for applications in waste heat recovery, wearable devices, space exploration, and refrigeration. [1][2][3][4][5][6][7] The processes for conversion of heat-to-electricity are in favor of high output voltage (large Seebeck coefficient, S), low heat loss (large electrical conductivity, σ), and stable temperature gradients (small thermal conductivity, κ). Accordingly, the characteristics of thermoelectric materials can be evaluated by the dimensionless figure-of-merit ZT = S 2 σT/κ, where T presents the absolute temperature, S 2 σ is known as the power factor, κ is comprised of the electronic κ e and lattice κ l fractions. [4,8] The foremost task of thermoelectric research is to enhance the ZT value, which generally requires enhanced S 2 σ and reduced κ by modulating the transport of carriers and phonons, respectively. For instance, great effort has been devoted to decoupling the trade-off between S and σ through band engineering, including band convergence, [9] resonant state doping, [10] phase transition manipulation, [11] and quantum confinement. [12] While κ (mainly κ l ) can be more independently modulated through reinforcing intrinsic phonon scattering and introducing extrinsic phonon scattering sources via introducing nanostructuring, [13,14] hierarchical architecture, [3,15] and defect engineering. [16,17] GeTe-based thermoelectrics have attracted increasing attention owing to their state-of-the-art mid-temperature performance. [18][19][20][21] Pristine GeTe undergoes a reversible transition from the low-temperature rhombohedral phase (space group R3m, a = 4.172 Å, c = 10.71 Å) to the high-temperature cubic phase (space group Fm3m, a = 5.98 Å) at around 700 K. As shown in Figure 1a,b, the phase transition is accompanied by elongated diagonal and displacement of the central site within the GeTe unit cell. This arrangement favors strengthened optical-acoustic phonon interaction [22] and enhanced band degeneracy. However, due to the low formation energy of Ge vacancies, excessive Owing to the moderate energy offset between light and heavy band edges of the rock-salt structured GeTe, its figure-of-merit (ZT) can be enhanced by the rational manipulation of electronic band structures. In this study, density functional theory calculations are implemented to predict that V is an effective dopant for GeTe to enlarge the bandgap and converge the energy offset, which suppresses the bipolar conduction and increases the effective mass. Experimentally, V-doped Ge 1−x V x Te samples are demonstrated to have an enhanced Seebeck coefficient from ≈163 to ≈191 µV K −1 . Extra alloying with Bi in Ge 1−x−y V x Bi y Te can optimize the carrier concentration to further enhance the Seebeck coefficient up to ≈252 µV K −1 , plus an outstanding power factor of ≈43 µW cm −1 K −2 . Comprehensive structural characterization results also verify the refinement of grain size by V-doping, associated with highly dense ...

for cooling has been predicted to increase from 2020 terawatt hours (TWh) in 2016 to 6200 TWh in 2050. [1] Traditional centralized climate control systems are still the mainstream cooling technology and this approach has been criticized for two reasons: i) the traditional centralized climate control system tends to cover large spaces, leading to wastage of up to ≈70% of energy to the environment; [2] ii) refrigerants are widely used in traditional centralized climate control systems and their leakage induces global warming. [3] Therefore, personalized cooling has been proposed to deliver a precise cooling dose to target locations on individuals to save energy. [4,5] Particularly, personalized cooling has three major advantages: i) less energy wasted to the environment and a low level of carbon emissions; [6] ii) adjustable temperature control to ensure conditions of thermal comfort for each individual; [7] iii) potential application in outdoor environments or healthcare applications. [8] In fact, personalized cooling has found niche applications in order to improve the comfort of outdoor athletes, which can by no means be realized using a centralized cooling system. Moreover, personalized cooling devices can also be incorporated into medical applications in order to manage symptoms including fever and burns. [9] Unlike traditional centralized cooling, personalized cooling technology is still in its early stages. The commercial cooling vest could be an option for personalized cooling technology. [10] However, the use of bulky fluidic channels for coolant circulation in order to pump heat from target areas is inconvenient for daily life. Except for the fluidic-based cooling vest, solid-state cooling technologies, such as electrocaloric, [11,12] magnetocaloric, [13,14] and mechanocaloric [15,16] cooling systems, have also been primarily studied with prospective applications demonstrated. These technologies require extreme working conditions (ultrahigh applied voltage of 10 kV, [11] magnetic field amplitude of several T, [13] and pressure in GPa level [16] ), presenting an overwhelming roadblock to their practical application in personalized cooling systems. Unlike the caloric materials which show inherent shortages, previous research on thermoelectric (TE) materials [17][18][19][20][21][22][23] revealed that a considerable temperature difference can be achieved simply by supplying electrical current through the thermoelectric coolers (TECs), known as the Peltier effect. [24][25][26][27] This solid-state cooling technology provides an alternative pathway to satisfy personalized cooling because In this study, flexible thermoelectric coolers (FTECs) are used to develop an alternative personalized cooling technology to achieve a large temperature drop of 10 °C and cooling capacity of 256 W m -2 . Such an excellent cooling performance is attributed to the innovative design of the quadra-layered Ag 2 Se/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate structure in FTECs and the induced air vortices by the vortex ge...

Owing to high intrinsic figure‐of‐merit implemented by multi‐band valleytronics, GeTe‐based thermoelectric materials are promising for medium‐temperature applications. Transition metals are widely used as dopants for developing high‐performance GeTe thermoelectric materials. Herein, relevant work is critically reviewed to establish a correlation among transition metal doping, electronic quality factor, and figure‐of‐merit of GeTe. From first‐principle calculations, it is found that Ta, as an undiscovered dopant in GeTe, can effectively converge energy offset between light and heavy conduction band extrema to enhance effective mass at high temperature. Such manipulation is verified by the increased Seebeck coefficient of synthesized Ge1−x−yTaxSbyTe samples from 160 to 180 µV K−1 at 775 K upon doping Ta, then to 220 µV K−1 with further alloying Sb. Characterization using electron microscopy also reveals the unique herringbone structure associated with multi‐scale lattice defects induced by Ta doping, which greatly hinder phonon propagation to decrease thermal conductivity. As a result, a figure‐of‐merit of ≈2.0 is attained in the Ge0.88Ta0.02Sb0.10Te sample, reflecting a maximum heat‐to‐electricity efficiency up to 17.7% under a temperature gradient of 400 K. The rationalized beneficial effects stemming from Ta doping is an important observation that will stimulate new exploration toward high‐performance GeTe‐based thermoelectric materials.