Extraordinary Role of Bi for Improving Thermoelectrics in Low-Solubility SnTe–CdTe Alloys

Abstract

As an environment-friendly alternative to traditional PbTe, many attempts have recently been made to improve thermoelectric SnTe. Effective strategies are mainly focused on valence band convergence, nanostructuring, interstitial defects, and alloying solubility. In particular, alloying SnTe with CdTe/GeTe triggers an inherent decline of valence band offset effectively owing to a high solubility of ∼20% of CdTe. However, to what level an additional element doping in low-solubility SnTe−CdTe alloys can play a role in enhancing the thermoelectric performance still remains a mystery. Here, a new strategy is shown that unexpected Bi doping, by alloying with only ∼3% CdTe, induces a significant enhancement of the thermoelectric figure of merit ZT to be ∼240% (ZT up to ∼1.3) at 838 K, which is mainly determined by the dramatically reduced lattice thermal conductivity above 800 K deriving from the exotic Bi doping and Cu-interstitial defects. More interestingly, combining experimental and theoretical evidences, the Bi-doping-driven band convergence is also beneficial to the improvement of thermoelectric performance below 800 K. The present findings demonstrate the extraordinary role of Bi for advancing the thermoelectric performance in SnTe alloys.

Physical Insights on the Lattice Softening Driven Mid‐Temperature Range Thermoelectrics of Ti/Zr‐Inserted SnTe—An Outlook Beyond the Horizons of Conventional Phonon Scattering and Excavation of Heikes’ Equation for Estimating Carrier Properties

“…[26][27][28][29][30][31] Due to larger anharmonicity and effective phonon scattering, PbTe exhibits lower thermal conductivity than SnTe. [32][33][34][35] Various attempts and strategies have been made in the past to improve the thermoelectric performance in SnTe, which includes selfcompensation in the composition with some additional Sn content; [36,37] electronic bandstructure engineering-fostering resonance state in the vicinity of fermi-level (mostly induced by In as a dopant in SnTe), [27,38] simultaneous increase of principal bandgap and convergence of the light hole and heavy hole valence bands (realized with dopants such as Cd, Hg, Mn, Mg, Ca, and few more), [27,29,36,37,[39][40][41][42][43][44] and this also includes valence band inversion or crossing effects; and a diverse nano-structuration approaches to engineer the dense interstitials, stacking faults, point defects, nano-precipitates and semi-coherent interfaces, dislocations, strain clusters, and so forth, [45][46][47][48][49][50][51][52] (by alloying with Cu 2 Te, CdS, SrTe, ZnS, and a few more). [53] All these approaches and dopants in SnTe, though they yielded an improved thermoelectric performance (zT max > 1), are limited for any practical applications, as their improved performance happened only at higher temperature ranges (usually between 823 and 873 K, or higher) for SnTe-based materials.…”

Bi and Zn co-doped SnTe thermoelectrics: interplay of resonance levels and heavy hole band dominance leading to enhanced performance and a record high room temperature ZT

“…[3][4][5][6] Doping is widely used to tune the carrier concentration, introduce resonance levels, increase the band gap, converge the valence sub-bands or introduce defects. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Resonant dopants like In and Bi have been mainly used to improve the Seebeck coefficient at low temperatures but their high temperature ZT is only B1. 7,8 Zn has also been recently added to the resonant family of the SnTe class of materials to improve the room temperature performance with a high temperature ZT of B1.49.…”

“…9 Dopants like Ag, Ca, Cd, Ce, Hg, Li, Mg, Mn, Pd, and Sr are used to increase the band gap and cause valence band convergence to improve the Seebeck values at higher temperatures. [10][11][12][13][14][15][16]18,[20][21][22][23] Since the application of thermoelectric materials in real world devices requires them to perform well for a range of temperature, a material that has a higher ZT throughout the temperature range is highly sought after. Hence, co-doping is implemented with one of the dopants being a resonant dopant.…”

Lattice Dynamics and Thermodynamic Properties of Bulk Phases and Monolayers of GaTe and InTe: A Comparison from First‐Principles Calculations

“…The hexagonal GaTe hα structure exhibits superior thermoelectric characteristics because of lower thermal and higher electrical conductivities [48] . Thermoelectric materials, which enable direct energy conversion between heat and electricity, have attracted considerable attention for clean power generation [56–58] . Gallium chalcogenides not only demonstrate thermoelectric properties themselves [21,32,48,53] but can also be used as dopants to improve thermoelectrics of other compounds, such as SnTe [57] …”

“…[48] Thermoelectric materials, which enable direct energy conversion between heat and electricity, have attracted considerable attention for clean power generation. [56][57][58] Gallium chalcogenides not only demonstrate thermoelectric properties themselves [21,32,48,53] but can also be used as dopants to improve thermoelectrics of other compounds, such as SnTe. [57] Zólyomi et al [51] explicitly compared the stability of hα and hβ InTe monolayers polymorphs.…”