Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence

Abstract

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of ∼0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.

Lead-free SnTe-based thermoelectrics: enhancement of thermoelectric performance by doping with Gd/Ag

“…It clearly shows that our sample has a medium electrical conductivity and a Seebeck coefficient, and the lowest lattice thermal conductivity in comparison with those previously reported SnTe-based materials. It has a ZT ($1.1 at 873 K) comparable to that of In-doped SnTe ($1.1 at 873 K), 7 Cd/CdS-doped SnTe ($1.3 at 873 K), 10 In/ Cd-doped SnTe ($1.4 at 873 K) 24 at the same temperature. …”

On the tuning of electrical and thermal transport in thermoelectrics: an integrated theory–experiment perspective

“…35,36 Such dopants usually have their electronic configuration very close to that of the host, such as when doping by elements from the neighbouring columns of the periodic table. Examples are IIIA elements doping in the rock-salt IV-VI structure and functioning as p-type dopants, [36][37][38][39] and IVA elements in V 2 VI 3 compounds. 40 Pb doping on the Bi-site in BiCuSeO, 41 Sb-doping on the Te-site in CuGaTe 2 , 42 and even antisite defects in ZrNiSn 43 are all known to form resonant levels at the respective band edges, illustrating a variety of resonant levels one can achieve in TE materials.…”

Cation Exchange Combined with Kirkendall Effect in the Preparation of SnTe/CdTe and CdTe/SnTe Core/Shell Nanocrystals

“…A similar picture has been reported elsewhere. 76,86,87 Figure 4 displays a schematic drawing of the stages leading to the formation of an inverted core/shell: the formation of a straight SnTe/CdTe core/shell structure via cation exchange reaction (second circle from left); generation of an intermediate structure composed of bridges for the transport of Sn-cations outward (third circle); and a complete occupation of the core region by Cd-cations (fourth circle), stemming from a Kirkendall effect with efficient cationic diffusion processes. As a whole, the sequence of events shown in the scheme leads to dramatic elemental redistribution across the entire NC volume, culminating in the conversion of straight SnTe/CdTe into inverted CdTe/SnTe core/shell heterostructures.…”