Study indicates interstitial Cu reduces the defect density in matrix and suppresses the donor-like effect

Due to the capacity to directly and reversibly convert heat into electricity, thermoelectric (TE) material has potential applications in solid-state heat pumping and exhaust heat recuperation, thus attracting worldwide attention. Bi₂Te₃ stands out for its excellent thermoelectric properties and has been used in commercial thermoelectric devices.

However, the development of Bi₂Te₃-based thermoelectric devices is seriously hindered by the weak mechanical properties and low TE properties of n-type Bi₂(Te, Se)₃. Therefore, it is important to develop a high-performance n-type Bi₂Te₃ polycrystalline material.

To address this issue, a study, published in the journal Science Bulletin, introduced extra Cu into the classical n-type Bi₂Te_2.7Se_0.3 to optimize its local defect state, and a two-step hot deformation process was employed to construct the high textured polycrystalline Bi₂Te_2.7Se_0.3 material.

This research reveals that the extra Cu is able to enter the van der Waals gaps between the Te⁽¹⁾-Te⁽¹⁾ layers in Bi₂Te_2.7Se_0.3 matrix, suppressing the formation of the anionic vacancies. This reduction in defect density contributes to lattice plainification in Cu_0.01Bi₂Te_2.7Se_0.3, improving the carrier mobility of Bi₂Te_2.7Se_0.3 from 174 cm² V^–1 s^–1 to 226 cm² V^–1 s^–1 with the 1% additional Cu, resulting in a maximum ZT of 1.10 at 348 K.

Subsequently, the SPS-sintered Cu_0.01Bi₂Te_2.7Se_0.3 bulk material underwent a two-step hot deformation process. Since the interstitial Cu can stabilize the lattice and effectively suppress the donor-like effect. The carrier concentration of hot deformation sample remains almost unchanged, while its grain orientation and grain size have significantly increased, which dramatically boosts the carrier mobility, from the initial 174 cm² V^–1 s^–1 to 333 cm² V^–1 s^–1, representing a 91% increase after the hot deformation process.

This significant improvement in electronic properties contributes to a substantial enhancement in ZT for hot deformation sample. The ZT_max of the textured Cu_0.01Bi₂Te_2.7Se_0.3 reaches 1.27 at 373 K, and its average ZT value is 1.22 in the range of 300-425 K, nearly twice as much as the initial Bi₂Te_2.7Se_0.3.

Furthermore, a 127-pair thermoelectric cooling device (TEC) was fabricated by using the textured Cu_0.01Bi₂Te_2.7Se_0.3 sample coupled with commercial p-type BST. The TEC module achieved cooling temperature differentials of 65 K and 83.4 K at hot-end temperatures (T_h) of 300 K and 350 K, respectively, which is superior to the commercial Bi₂Te₃-based TEC modules. And a 7-pair thermoelectric generator module (TEG) was constructed by using the same materials.

The TEG module demonstrated a significantly high conversion efficiency of 6.5% at a temperature different of 225 K, which is comparable to other state-of-the-art Bi₂Te₃-based TEG modules.