n-type bismuth telluride-based thermoelectric materials, devices, and applications

<p indent="0mm">Thermoelectric (TE) materials can directly convert electricity into thermal energy or vice versa based on Seebeck effect and Peltier effect, providing a feasible scheme to settle the energy shortage problems. To better elucidate TE properties of materials, the figure of merit, <italic>ZT</italic>=(<italic>S</italic>²<italic>σ</italic>/<italic>κ</italic>)<italic>T</italic> is introduced, where the <italic>S</italic>, <italic>σ </italic>and <italic>κ </italic>represent the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. From the presented expression, a satisfactory TE material is supposed to have a relatively high <italic>ZT</italic> value, that is, a large Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity. Bismuth telluride (Bi₂Te₃)-based thermoelectric materials, as the only thermoelectric system that has been commercialized at present, are widely used in the fields of near-room temperature refrigeration and low-grade thermoelectric power generation due to their excellent thermoelectric performance in near-room temperature regions. However, due to the narrow band gap, Bi₂Te₃ thermoelectric materials will undergo bipolar diffusion with rising temperature, which will worsen the Seebek coefficient and thus affect their thermoelectric properties. Therefore, for the Bi₂Te₃ system, how to increase the band gap and increase the carrier concentration to suppress bipolar diffusion has always been the key research direction. For the improvement of thermoelectric performance, the main difficulty lies in the coupling relationship between the thermoelectric performance parameters that determine <italic>ZT</italic> values, so it is a great challenge to find the decoupling strategy between parameters. In recent twenty years, researchers have explored various strategies to optimize the performance of Bi₂Te₃-based thermoelectric materials, including carrier adjustment, texture adjustment, energy band engineering, and phonon engineering, etc. In general, because n-type Bi₂Te₃-based thermoelectric materials are mostly “single crystal-like” obtained by zone melting or directional solidification, they are more anisotropic than p-type materials obtained usually by powder sintering. Therefore, some strategies used to improve the p-type Bi₂Te₃ may not be suitable for the optimization of n-type materials. In recent years, researchers have successfully raised the maximum <italic>ZT</italic> value of the p-type Bi₂Te₃ to ~1.5. Nevertheless, the <italic>ZT</italic> value of the matching n-type Bi₂Te₃ is only about 1.0, which limits the further development of thermoelectric devices. Therefore, it is of great significance to further improve the thermoelectric performance of the n-type Bi₂Te₃ through a reasonable optimization strategy. Moreover, the mechanical properties should also be taken into account, so as to ensure the stable function of high-performance thermoelectric devices. In this paper, the recent research achievements in the n-type Bi₂Te₃ thermoelectric system are summarized in detail, and the latest research progress in thermoelectric properties, mechanical properties, and thermoelectric device optimization of this system is comprehensively reviewed. The current challenges and future development trends of Bi₂Te₃-based thermoelectric materials are discussed in depth. Not only can this work give a comprehensive understanding of the n-type Bi₂Te₃ thermoelectric system, but also it provides a reference for promoting further applications of Bi₂Te₃-based thermoelectric materials and devices.