Realization of Both n- and p-Type GeTe Thermoelectrics: Electronic Structure Modulation by AgBiSe₂ Alloying

Abstract

Successful applications of a thermoelectric material require simultaneous development of compatible n-and p-type counterparts. While the thermoelectric performance of p-type GeTe has been improved tremendously in recent years, it has been a challenge to find a compatible n-type GeTe counterpart due to the prevalence of intrinsic Ge vacancies. Herein, we have shown that alloying of AgBiSe 2 with GeTe results in an intriguing evolution in its crystal and electronic structures, resulting in n-type thermoelectric properties. We have demonstrated that the ambient rhombohedral structure of pristine GeTe transforms into cubic phase in (GeTe) 100−x (AgBiSe 2 ) x for x ≥ 25, with concurrent change from its p-type electronic character to n-type character in electronic transport properties. Such change in structural and electronic properties is confirmed from the nonmonotonic variation of band gap, unit cell volume, electrical conductivity, and Seebeck coefficient, all of which show an inflection point around x ∼ 20, as well as from the temperature variations of synchrotron powder X-ray diffractions and differential scanning calorimetry. First-principles density functional theoretical (DFT) calculations explain that the shift toward n-type electronic character with increasing AgBiSe 2 concentration arises due to increasing contribution of Bi p orbitals in the conduction band edge of (GeTe) 100−x (AgBiSe 2 ) x . This cubic n-type phase has promising thermoelectric properties with a band gap of ∼0.25 eV and ultralow lattice thermal conductivity that ranges between 0.3 and 0.6 W/mK. Further, we have shown that (GeTe) 100−x (AgBiSe 2 ) x has promising thermoelectric performance in the mid-temperature range (400−500 K) with maximum thermoelectric figure of merit, zT, reaching ∼1.3 in p-type (GeTe) 80 (AgBiSe 2 ) 20 at 467 K and ∼0.6 in n-type (GeTe) 50 (AgBiSe 2 ) 50 at 500 K.

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

“…As a result of aliovalent doping and subsequent n p optimization, the ZT values of 1.7 and 1.65 can be obtained at ≈700K for Ge 1− x Bi x Te [ 117,118 ] and Ge 1− x Sb x Te, [ 77 ] respectively. Additionally, Samanta et al [ 119 ] discovered that p‐type GeTe transfers into n ‐type (GeTe) 100− x (AgBiSe 2 ) x with increasing x to ≈25 (Figure 6d). [ 119 ] With increasing the AgBiSe 2 alloying level, the room‐temperature crystal structure of (GeTe) 100− x (AgBiSe 2 ) x changed from rhombohedral to cubic.…”

“…Additionally, Samanta et al [ 119 ] discovered that p‐type GeTe transfers into n ‐type (GeTe) 100− x (AgBiSe 2 ) x with increasing x to ≈25 (Figure 6d). [ 119 ] With increasing the AgBiSe 2 alloying level, the room‐temperature crystal structure of (GeTe) 100− x (AgBiSe 2 ) x changed from rhombohedral to cubic. [ 119 ] Correspondingly, the bandgap of the (GeTe) 100− x (AgBiSe 2 ) x alloy reduced and approached to CBs/VBs overlap when the AgBiSe 2 alloying level increased from 20% to 40%, which is the reason for the p–n transition.…”

“…[ 119 ] With increasing the AgBiSe 2 alloying level, the room‐temperature crystal structure of (GeTe) 100− x (AgBiSe 2 ) x changed from rhombohedral to cubic. [ 119 ] Correspondingly, the bandgap of the (GeTe) 100− x (AgBiSe 2 ) x alloy reduced and approached to CBs/VBs overlap when the AgBiSe 2 alloying level increased from 20% to 40%, which is the reason for the p–n transition. [ 119 ] This can be attributed to the substitution of Bi 3+ on Ge 2+ site, which has significant contribution to the conduction band edge states.…”

“…[ 119 ] Correspondingly, the bandgap of the (GeTe) 100− x (AgBiSe 2 ) x alloy reduced and approached to CBs/VBs overlap when the AgBiSe 2 alloying level increased from 20% to 40%, which is the reason for the p–n transition. [ 119 ] This can be attributed to the substitution of Bi 3+ on Ge 2+ site, which has significant contribution to the conduction band edge states. [ 119–121 ] The n ‐type (GeTe) 50 (AgBiSe 2 ) 50 has the peak ZT value of ≈0.6 at ≈500 K and the corresponding S of ≈−160 µV K −1 (Figure 6d).…”

“…[ 119 ] This can be attributed to the substitution of Bi 3+ on Ge 2+ site, which has significant contribution to the conduction band edge states. [ 119–121 ] The n ‐type (GeTe) 50 (AgBiSe 2 ) 50 has the peak ZT value of ≈0.6 at ≈500 K and the corresponding S of ≈−160 µV K −1 (Figure 6d).…”