image: (a) The schematic diagram illustrates the optimization mechanism of lattice plainification and band engineering for the n-type BTS+Cu in this study. Comparison of (b) maximum cooling temperature difference ΔTmax and (c) power generation efficiency η between optimized thermoelectric devices and traditional commercial devices.
Credit: ©Science China Press
Thermoelectric technology achieves direct and reversible conversion between thermal and electrical energy through the carrier transport within thermoelectrics, making it a crucial green energy conversion technology with broad prospects in power generation and cooling fields. Particularly in high-precision areas such as deep-space and deep-sea exploration energy supply and precise temperature control of sophisticated instruments, thermoelectric devices are increasingly playing a significant role.
The conversion efficiency of thermoelectric power generation and cooling devices is primarily determined by the dimensionless thermoelectric figure of merit (ZT) of thermoelectrics. As defined by the equation ZT = (S²σ/κ)T, for a given temperature T, high-performance thermoelectrics should possess a large Seebeck coefficient S (to generate a high voltage), high electrical conductivity σ (to reduce joule heating losses), and low thermal conductivity κ (to create a large temperature difference). However, the complex interconnections between these physical parameters form tight phonon-electron coupling, making the optimization of thermoelectric performance extremely challenging. Regulating these strongly coupled and complex thermoelectric parameters is key to improving the ZT value and the device efficiency.
Recently, new thermoelectrics have emerged, but bismuth telluride (Bi2Te3)-based alloys remain the most mature thermoelectrics currently in commercial use. Within this material system, the development of p-type (Bi,Sb)₂Te₃ (BST) materials has already matured, with stable room-temperature ZT values reaching ~1.0 or higher. In contrast, the performance optimization of n-type Bi₂(Te,Se)₃ (BTS) materials has lagged behind. The commercial n-type BTS materials widely used in the market are mostly grown by zone melting, which presents issues such as easy grain boundary separation during subsequent processing of thermoelectric devices. This not only increases production costs but also negatively impacts the overall performance of the devices. Therefore, enhancing the thermoelectric performance and mechanical processing properties of n-type BTS materials is particularly important.
This study adopts lattice plainification and band engineering as the optimization strategy, introducing trace amounts of Cu atoms into commercial n-type BTS materials to effectively regulate intrinsic lattice defects, achieving fine tuning of the conduction band structure, and significantly enhance the carrier mobility μ. Specifically, after Cu enters the lattice, it occupies intrinsic Bi vacancies, thereby reducing point defect scattering for carriers. Additionally, Cu occupying Bi sites can also regulate the band structure, promoting band sharpening and divergence, thereby reducing the effective mass m* and further enhancing μ. Furthermore, interstitial Cu atoms located in van der Waals gaps and (-Te-Bi-Te-Bi-Te-) layers also bond with neighboring Te atoms, forming additional carrier transport channels and further improving μ. Through the synergistic optimization of trace amounts of Cu atoms, the BTS+0.2%Cu sample achieved excellent μ of ~285 cm2 V-1 s-1 and a high power factor of ~60 μW cm-1 K-2 at 300K, with a room-temperature ZT value of ~1.3 and an average ZTave of ~1.2 in the temperature range of 300-523 K. Additionally, the formation of Cu-Te bonds significantly improved the mechanical processing properties of the BTS+0.2%Cu crystal ingots, providing great convenience for large-scale production. Full-size thermoelectric devices prepared based on the optimized BTS+0.2%Cu crystal ingots and commercial p-type BST materials achieved a power generation efficiency of ~6.4% under the temperature difference of 223 K and a maximum cooling temperature difference of ~70.1 K at room temperature.
This study provides a detailed investigation into the atomic occupancy and role of trace Cu atoms in regulating the thermoelectric and mechanical processing properties of commercial N-type BTS, which will help promote the practical application of Bi₂Te₃-based commercial thermoelectric devices in power generation and cooling.