Vacancy Manipulation for Thermoelectric Enhancements in GeTe Alloys

Abstract

Optimization of carrier concentration plays an important role on maximizing thermoelectric performance. Existing efforts mainly focus on aliovalent doping, while intrinsic defects (e.g., vacancies) provide extra possibilities. Thermoelectric GeTe intrinsically forms in off-stoichiometric with Ge-vacancies and Ge-precipitates, leading to a hole concentration significantly higher than required. In this work, Sb2Te3 having a smaller cation-to-anion ratio, is used as a solvend to form solid solutions with GeTe for manipulating the vacancies. This is enabled by the fact that each substitution of 3 Ge2+ by only 2 Sb3+ creates 1 Ge vacancy, because of the overall 1:1 cation-to-anion ratio of crystallographic sites in the structure and by the charge neutrality. The increase in the overall Ge-vacancy concentration facilitates Ge-precipitates to be dissolved into the matrix for reducing the hole concentration. In a combination with known reduction in hole concentration by Pb/Ge-substitution, a full optimization on hole concentration is realized. In addition, the resultant high-concentration point defects including both vacancies and substitutions strongly scatter phonons and reduce the lattice thermal conductivity to the amorphous limit. These enable a significantly improved thermoelectric figure of merit at working temperatures of thermoelectric GeTe.

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

“…It has been proven that Sb doping could increase formation energy of Ge vacancies and drive the Ge precipitates to be dissolved into matrix for occupying the Ge vacancy sites. [ 54 ] The total thermal conductivity and lattice thermal conductivity decrease more significantly in Bi‐doped samples than those with Sb doping because of the larger difference in atomic mass and size between Bi and Ge. The effects of balancing the Ge vacancies and precipitates as well as donating electrons through Cr, and simultaneously optimization of carrier concentration and enhancing phonon scattering by codoping Bi/Sb lead to further enhancement in the thermoelectric ZT, as shown in Figure 5c.…”

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

“…Like other mid‐temperature thermoelectric materials (such as PbTe, [ 10,13 ] PbSe, [ 14 ] SnTe, [ 15 ] and SnSe [ 16,17 ] ), GeTe‐based alloys have also received great attention recently, [ 18–27 ] due to their special multiple‐band structures, relative low thermal conductivity and high carrier mobility. Ge vacancies can be easily formed in GeTe due to their low formation energy, [ 19,28 ] which lead to high hole concentration (≈10 21 cm −3 ), high κ e and therefore low ZT in un‐doped GeTe. The Bi and Sb have been widely used to reduce the hole concentration of GeTe due to their donor dopant nature.…”

“…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.…”

“…Other effects of Ge vacancies such as phase transition [ 60 ] and lattice hardening [ 61 ] have also been reported in the GeTe‐based materials. Thus, the Ge‐vacancy engineering is significant for enhancing the ZT of GeTe‐based alloys, [ 19,28 ] and more effects of Ge vacancies besides the tuning of carrier density are still worth exploring in GeTe‐based alloys for further enhancing their thermoelectric performance.…”