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High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. 5However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential 10 theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction.Increasing the thermoelectric figure of merit (zT) is the most challenging task to enable the widespread use of this method to directly convert heat into electricity. The 15 transport properties including resistivity (ρ), Seebeck coefficient (S), electronic (κ E ) and lattice (κ L ) components of thermal conductivity (κ=κ E +κ L ) determine the figure of merit, zT=S 2 T/ρκ, where T is absolute temperature.Creating phonon scattering centers such as 20nanostructures [1][2][3][4] to lower κ L , has been proven effective for achieving zT > 1 in many instances. However, κ L in such materials already approaches their amorphous limit [3,4], suggesting strategies targeting increases in zT by improvements of the thermoelectric power factor (S 2 /ρ). 25The decoupling of S, ρ and κ E in an effort to achieve high zT has been a longstanding challenge as they are strongly coupled with each other through the carrier concentration, scattering and band structure [5][6][7]. However, it is well known that the optimal electronic performance of a 30 thermoelectric semiconductor depends primarily on the weighted mobility[7-10], µm *3/2 , which includes both the density-of-states effective mass (m * , with a unit of free electron mass m e ) and the nondegenerate mobility (µ) of carriers. 35More generally, each degenerate carrier pocket makes a contribution to m * via m * =N v 2/3 m b * [7][8][9][10][11], where N v and m b * are the number of degenerate carrier pockets and the average band mass (density-of-states effective mass for each pocket), respectively. Without explicitly reducing µ, 40 converging many valence (or conduction) bands to achieve high N v and therefore a high m * has been proposed as an effective approach to high performance in both bulk[12] and low dimensional[13] thermoelectrics. Without modifying N v and in an attempt to increase the 45 power factor, many efforts have been recently devoted to increasing the Seebeck coefficient (i. e. increasing m * through high m b * ) either by designing [14,15] the density of states or manipulating nanostructure...

Thermoelectric power generation is drawing increased interest in engine exhaust heat recovery to improve fuel effi ciency. [ 1 ] The development of useful devices requires materials with high dimensionless fi gure of merit zT ( zT = S 2 T / ρ κ , S being the Seebeck coeffi cient, ρ the electric resistivity, and κ thermal conductivity). [ 2 ] Among the materials suitable for power generation applications PbTe has been the most studied, [ 3 ] including recent results from nanostructuring [ 4 ] and band modifi cation [ 5 ] with exceptional zT in both n-type [ 6 ] and p-type [ 7 ] materials.PbSe, closely related to PbTe, is much less frequently considered for thermoelectrics. This can be traced to the smaller bandgap reported at low temperature and higher thermal conductivity expected from the lighter PbSe compared with PbTe.A recent calculation by Parker and Singh [ 8 ] suggests that heavily doped PbSe may reach zT ∼ 2 at temperatures near 1000 K due to the appearance of band fl attening ∼ 0.35 eV below the valence band edge, despite the small bandgap of 0.065 eV. On the other hand, there is the prevailing notion that PbSe has considerably lower performance compared to PbTe, [ 9 ] also the only high temperature ( > 450 K) experimental data that we know of for heavily doped PbSe reports a peak zT ∼ 0.7 at around 900K. [ 10 ] In this paper we report thermoelectric properties of a series of Na doped PbSe polycrystalline samples from room temperature to 850 K. Na is an effective dopant [ 11 ] as it provides high enough hole concentration and it does not interfere with PbSe valence bands. Samples of Na x Pb 1-x Se with 0 ≤ x ≤ 0.025 were prepared and are identifi ed according to the measured Hall carrier density ( n H = 1/e R H ) at room temperature ( Table 1 ). The composition range of the samples studied spans carrier concentrations typical for good IV-VI thermoelectric materials. [ 12 ] The Seebeck coeffi cients show typical behavior of degenerate semiconductors and increase with temperature for all doped samples, in agreement with data reported by Alekseeva et al. [ 10 ] ( Figure 1 ). In the lightly doped samples, maximum values are reached at high temperature due to the onset of thermal excitation of minority carriers (i.e., the bipolar effect). The reported experimental bandgap E g of PbSe is fairly small at 0 K (0.16 ± 0.02 eV [ 13 ] ) and increases with temperature at a rate of [ 14 ] 4 × 10 − 4 eV K − 1 , such a model gives E g ∼ 0.44 eV at 700K. Whereas the effective E g obtained using E g = 2e T max S max , where S max and T max represents the maximum of Seebeck coeffi cient and the temperature at which this value is achieved, [ 15 ] yields effective bandgaps of 0.33 eV at 630 K ('6E18'), 0.37 eV at 740 K ('1E19') and > 0.43 eV at 850 K ('3E19'), qualitatively consistent with the model. This is also consistent with the Seebeck coeffi cients increasing monotonically up to 850 K without any sign of the bipolar effect in heavily doped samples.Important information about band structure can be revealed with a plot ...

PbSe is a surprisingly good thermoelectric material due, in part, to its low thermal conductivity that had been overestimated in earlier measurements. The thermoelectric figure of merit, zT, can exceed 1 at high temperatures in both p-type and n-type PbSe, similar to that found in PbTe. While the p-type lead chalcogenides (PbSe and PbTe) benefit from the high valley degeneracy (12 or more at high temperature) of the valence band, the n-type versions are limited to a valley degeneracy of 4 in the conduction band. Yet the n-type lead chalcogenides achieve a zT nearly as high as the p-type lead chalcogenides. This effect can be attributed to the weaker electron-phonon coupling (lower deformation potential coefficient) in the conduction band as compared with that in the valence band, which leads to higher mobility of electrons compared to that of holes. This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials.energy | semiconductor | quality factor W aste heat recovery using thermoelectric power generation is attracting considerable interest from the automobile industry (1) as well as from many other areas (2). Large-scale production of bulk materials with high figure of merit, zT, defined as zT ¼ S 2 σT∕ðκ e þ κ L Þ (S is the Seebeck coefficient, σ is the electric conductivity, and κ e and κ L are the electronic and lattice thermal conductivity, respectively), is the key to widespread adaption of thermoelectric technology. The search for good thermoelectric materials has focused on investigating semiconductors that have suitable band structures and low thermal conductivities (3, 4). As one of the first investigated material systems (5), PbTe and its alloys have been extensively studied and remain some of the best (6, 7) thermoelectric materials for applications from 500 to 900 K. Considerable effort has been made to achieve a higher zT in these alloys by reducing the lattice thermal conductivity, κ L , by incorporation of nanometer scale inclusions (8-11). Other strategies approach the challenge of increasing zT from different angles, such as band structure distortion by Tl doping (12), and more recently, increasing band degeneracy by converging two different valence bands in p-type PbTe (13).It can be shown that the material parameter called the thermoelectric quality factor, B,determines the optimized figure of merit (14-16) (N V is the band degeneracy, m à b is the density of states effective mass of a single band, μ 0 is the mobility at nondegenerate limit, and κ L is the lattice thermal conductivity). This expression is derived for semiconductors with single band transport behavior where the carrier concentration can be optimized to achieve maximum zT. For good thermoelectric semiconductors the dominant scattering mechanism at high temperatures, where zT peaks, is typically due to acoustic phonons. The deformation pote...