High-Performance GeTe Thermoelectrics in Both Rhombohedral and Cubic Phases

Abstract

GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV–VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.

Manipulating the Ge Vacancies and Ge Precipitates through Cr Doping for Realizing the High‐Performance GeTe Thermoelectric Material

“…Temperature‐dependent thermoelectric transport properties for Cr doping and Cr‐Bi/Sb codoping samples: a) total thermal conductivity, b) lattice thermal conductivity, c) figure of merit (ZT), and c) average ZT with a comparison to the literatures. [ 39–41,47,63–65 ] …”

“…[ 38 ] Therefore, many previous studies for improving thermoelectric GeTe focus on optimizing the carrier concentration. [ 39–45 ] An obvious solution is doping of elements acting as electron donors to decrease the hole carrier concentration. For example, Bi and Sb have been reported to be effective donors to decrease the carrier concentration.…”

Promising and Eco‐Friendly Cu2X‐Based Thermoelectric Materials: Progress and Applications

“…Hong et al reported a peak zT value higher than 2 at ≈580 K. From ≈650 to ≈800 K, the peak zT values of GeTe‐based thermoelectric materials are higher than 1 . Further studies have enhanced the peak zT values of GeTe‐based thermoelectric materials to a level higher than 2 . Under this temperature range (from ≈650 to ≈800 K), other p‐type thermoelectric materials, such as SnSe and higher manganese silicide have also drawn extensive research interests due to either low cost or high performance.…”

Positive Effect of Ge Vacancies on Facilitating Band Convergence and Suppressing Bipolar Transport in GeTe‐Based Alloys for High Thermoelectric Performance

“…The Bi and Sb have been widely used to reduce the hole concentration of GeTe due to their donor dopant nature. [ 29–33 ] Other doping or alloying with Pb, [ 25,34–36 ] Se, [ 30,37–39 ] Bi‐Sb, [ 40 ] Bi‐Cu, [ 41 ] Mn‐Bi, [ 42 ] Mn‐Sb, [ 43 ] Pb‐Sb, [ 28,44 ] Pb‐Bi, [ 45 ] Cd‐Bi, [ 46,47 ] Sb‐Zn, [ 48 ] Sb‐In, [ 49 ] Bi 2 Te 3 , [ 23,26 ] Sb 2 Te 3 , [ 50 ] and AgSbTe 2 [ 51,52 ] have also been widely applied to optimize the carrier density and to reduce κ lat for enhancing the ZT of GeTe‐based alloys. Combined with the synergic effects of carrier‐density optimization, band engineering and phonon engineering strategies, many GeTe‐based alloys with peak ZT of around 2 have been reported recently.…”

“…Many studies have considered Ge vacancies as negative effects for enhancing ZT of GeTe‐based alloys, [ 24,42,44,53–56 ] and have also proposed various approaches for suppressing Ge vacancies. Dong et al have reported that excess Ge can be effective in eliminating the Ge vacancies in pristine GeTe for reducing carrier density and increasing carrier mobility, [ 24 ] resulting in a peak ZT of ≈1.6 around 650 K. As reported by Li et al, [ 53 ] alloying with PbSe can elevate the formation energy of Ge vacancies by increasing the mean size of cations and decreasing the average size of anions, resulting in suppressed Ge vacancies.…”