Ultrahigh Power Factor and Electron Mobility in n-Type Bi₂Te₃–x%Cu Stabilized under Excess Te Condition

Abstract

The thermoelectric (TE) community has mainly focused on improving the figure of merit (ZT) of materials. However, the output power of TE devices directly depends on the power factor (PF) rather than ZT. Effective strategies of enhancing PF have been elusive for Bi2Te3-based compounds, which are efficient thermoelectrics operating near ambient temperature. Here, we report ultrahigh carrier mobility of ∼467 cm2 V–1 s–1 and power factor of ∼45 μW cm–1 K–2 in a new n-type Bi2Te3 system with nominal composition Cu x Bi2Te3.17 (x = 0.02, 0.04, and 0.06). It is obtained by reacting Bi2Te3 with surplus Cu and Te and subsequently pressing powder products by spark plasma sintering (SPS). The SPS discharges excess Te but stabilizes the high extent of Cu in the structure, giving unique SPS Cu x Bi2Te3.17 samples. The analyzed composition is close to “Cu x Bi2Te3”. Their charge transport properties are highly unusual. Hall carrier concentration and mobility simultaneously increase with the higher mole fraction of Cu contrary to the typical carrier scattering mechanism. As a consequence, the electrical conductivity is considerably enhanced with Cu incorporation. The Seebeck coefficient is nearly unchanged by the increasing Cu content in contrast to the general understanding of inverse relationship between electrical conductivity and Seebeck coefficient. These effects synergistically lead to a record high power factor among all polycrystalline n-type Bi2Te3-based materials.

Strategies to Improve the Thermoelectric Figure of Merit in Thermoelectric Functional Materials

“…Studies have shown that the two-step sintering technology is an effective method, which can control the existence and distribution of element inhomogeneity, so that the carrier transport and phonon transport have a special directionality. This has a specific guiding significance for other thermoelectric material systems with similar characteristics ( Cha et al, 2019 ; Pan et al, 2019 ).…”

“…Most high-performance thermoelectric materials are heavily doped semiconductors, and the optimal carrier concentration n opt varies with ( m*T ) 3/2 . Therefore, n opt can also be achieved by tuning the effective quality m d * ( Cha et al, 2019 ). According to m d * = N v 2/3 m b * , degeneracy ( N v ) and effective mass ( m b * ) play a decisive role in m d * .…”

Effects of Synthesis Parameters and Thickness on Thermoelectric Properties of Bi2Te3 Fabricated Using Mechanical Alloying and Spark Plasma Sintering

“…The resulting increases in the Seebeck coefficient and electrical conductivity enhanced the ZT value. Cha et al 15 reported that addition of Cu to Bi 2 Te 3 compound increases the carrier concentration and mobility as well as the electrical conductivity, and thus the ZT value.…”

Concentration‐dependent excess Cu doping behavior and influence on thermoelectric properties in Bi 2 Te 3

“…reported improved reproducibility and performance in Cu-incorporated Bi 2 Te 3 . [3][4][5][6][7][8] However, the incorporated Cu was found at different sites such as intercalation or substitution sites within Bi 2 Te 3 lattice, and could even be precipitated. [9][10][11][12][13][14][15][16][17] The intercalation mechanism in Bi 2 Te 3 -based systems were also investigated with different dopants like Sn and CuI.…”

“…[18][19][20] The improved reproducibility and enhanced zT was reported in Cu-intercalated n-type ternary Bi 2 Te 2.7 Se 0.3 . 5 Recently, doping behavior of excess Cu in the ternary Bi 2 Te 2.7 Se 0.3 was observed via atomicscale chemical mapping. 17 Excess Cu doping behavior in ternary Bi 2 Te 2.7 Se 0.3 has been directly demonstrated by atomic-scale chemical mapping: excessive Cu atoms in Bi 2 Te 2.7 Se 0.3 occupied (1) Bi-site at a low content of 0.2 at % (Cu 0.01 Bi 2 Te 2.7 Se 0.3 ) and (2) three crystallographic sites (Bi, Te, (1) and Te (2) ) and van der Waals gap (intercalation) at high content of 1.2 at% (Cu 0.06 Bi 2 Te 2.7 Se 0.3 ).…”