Higher mobility in bulk semiconductors by separating the dopants from the charge-conducting band – a case study of thermoelectric PbSe

In the rigid band approximation dopants in semiconductors only change the Fermi level and carrier concentration such that different dopants are thought equivalent when fully ionized. In this work we examine the small but significant difference in mobility due to the type of dopant in heavily doped PbSe by studying n-type samples doped with Br, In and Bi. We propose that cation and anion dopants lead to a difference in mobility at high concentrations. This can be understood considering the predominance of cation states to the conduction band and anion states to the valence band. For higher mobility and better performance for most applications of heavily doped semiconductors, dopants should be on the site that is of less influence on the charge-conducting band. This concept can be viewed as an analog of modulation doping on the atomic level. Its physical origin is the random potential due to disorder that perturbs carriers, which is also the origin of Anderson localization at low temperature, a well-studied topic in theoretical physics. In thermoelectric PbSe, the selection of dopant can lead to 10% difference in mobility and in zT

Thermoelectric properties and electronic structure of the Zintl phase Sr_5Al_2Sb_6

The Zintl phase Sr_5Al_2Sb_6 has a large, complex unit cell and is composed of relatively earth-abundant and non-toxic elements, making it an attractive candidate for thermoelectric applications. The structure of Sr_5Al_2Sb_6 is characterized by infinite oscillating chains of AlSb_4 tetrahedra. It is distinct from the structure type of the previously studied Ca_5M_2Sb_6 compounds (M = Al, Ga or In), all of which have been shown to have promising thermoelectric performance. The lattice thermal conductivity of Sr_5Al_2Sb_6 (∼0.55 W mK^(-1) at 1000 K) was found to be lower than that of the related Ca_5M_2Sb_6 compounds due to its larger unit cell (54 atoms per primitive cell). Density functional theory predicts a relatively large band gap in Sr_5Al_2Sb_6, in agreement with the experimentally determined band gap of E_g ∼ 0.5 eV. High temperature electronic transport measurements reveal high resistivity and high Seebeck coefficients in Sr_5Al_2Sb_6, consistent with the large band gap and valence-precise structure. Doping with Zn^(2+) on the Al^(3+) site was attempted, but did not lead to the expected increase in carrier concentration. The low lattice thermal conductivity and large band gap in Sr_5Al_2Sb_6 suggest that, if the carrier concentration can be increased, thermoelectric performance comparable to that of Ca_5Al_2Sb_6 could be achieved in this system

Thermoelectric properties of the Yb_9Mn_(4.2-x)Zn_xSb_9 solid solutions

Yb_9Mn_(4.2)Sb_9 has been shown to have extremely low thermal conductivity and a high thermoelectric figure of merit attributed to its complex crystal structure and disordered interstitial sites. Motivated by previous work which shows that isoelectronic substitution of Mn by Zn leads to higher mobility by reducing spin disorder scattering, this study investigates the thermoelectric properties of the solid solution, Yb_9Mn_(4.2−x)Zn_xSb_9 (x = 0, 1, 2, 3 and 4.2). Measurements of the Hall mobility at high temperatures (up to 1000 K) show that the mobility can be increased by more than a factor of 3 by substituting Zn into Mn sites. This increase is explained by the reduction of the valence band effective mass with increasing Zn, leading to a slightly improved thermoelectric quality factor relative to Yb_9Mn_(4.2)Sb_9. However, increasing the Zn-content also increases the p-type carrier concentration, leading to metallic behavior with low Seebeck coefficients and high electrical conductivity. Varying the filling of the interstitial site in Yb_9Zn_(4+y)Sb_9 (y = 0.2, 0.3, 0.4 and 0.5) was attempted, but the carrier concentration (~10^(21) cm^(−3) at 300 K) and Seebeck coefficients remained constant, suggesting that the phase width of Yb_9Zn_(4+y)Sb_9 is quite narrow

Tuning bands of PbSe for better thermoelectric efficiency

Improving the thermoelectric performance of PbSe over its previously reported maximum zT can be achieved by engineering its electronic band structure. We demonstrate here, using optical absorption spectra, first principles calculations, and temperature dependent transport measurements, that alloying PbSe with SrSe leads to a dramatic change of the band structure that increases the thermoelectric figure of merit, zT. The temperature where the two valence bands converge decreases with Sr addition. The zT value, when the carrier density is optimized, increases with Sr addition in Pb_(1−x)Sr_xSe and when x = 0.08 a maximum zT of 1.5 at 900 K is achieved. The net benefit in zT comes from the band structure tuning even though in other thermoelectric solid solutions it is the thermal conductivity reduction from disorder that leads to net zT improvement

Validity of rigid band approximation of PbTe thermoelectric materials

The tuning of carrier concentration through chemical doping is very important for the optimization of thermoelectric materials. Traditionally, a rigid band model is used to understand and guide doping in such semiconductors, but it is not clear whether such an approximation is valid. This letter focuses on the changes in the electronic density of states (DOS) near the valence band maximum for different p-type dopants (Na, K, Tl, or vacancy on Pb site) maintaining the high symmetry of the NaCl structure. Na-and K-doped, and vacancy-introduced PbTe show a clear rigid-band like change in DOS unlike that concluded from supercell based calculations

Thermoelectric Properties of Co-Substituted Al–Pd–Re Icosahedral Quasicrystals

The practical application of quasicrystals (QCs) as thermoelectric materials makes icosahedral (i-) Al–Pd–Re QC attractive because of its moderate electrical conductivity (~280 Ω−1 cm−1), relatively high Seebeck coefficient (~100 μV K−1), and low thermal conductivity (~1.3 W m−1 K−1) at room temperature. To develop a thermoelectric Π-shaped power generation module, we need both p- and n-type thermoelectric materials. In this work, we aimed to develop an n-type i-Al–Pd–Re-based QC and investigated the effect of Co substitution for Re on the thermoelectric properties, i.e., the electron-doping effect. We synthesized dense bulk samples with nominal compositions of Al71Pd20(Re1−xCox)9 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) via arc-melting, annealing, and sintering methods. We found that Co can produce n-type carriers in dilute substitution amounts of x = 0.1 and 0.2; however, the Seebeck coefficient at 300 K showed an n- to p-type transition with increasing x. This indicates that a simple rigid-band approximation is not applicable for i-Al–Pd–Re QC, which makes it difficult to synthesize an n-type i-Al–Pd–Re-based QC. Although the thermal conductivity was reduced from 1.28 (x = 0) to 1.08 W m−1 K−1 (x = 0.3) at 373 K by lowering of the electron thermal conductivity (electrical conductivity) and the alloying effect via Co substitution, the dimensionless figure of merit was not enhanced because of lowering of the power factor for all samples. The elastic moduli of i-Al–Pd–Re QC decreased by Co substitution, indicating that i-Al–Pd–Re-Co QC had a more ionic and brittle character