Why is it challenging to improve the thermoelectric properties of n-type Bi2Te3 alloys?

The successful application of nanotechnologies in enhancing thermoelectric properties of n-type Bi2Te3 alloys remains a formidable challenge, despite significant advancements in their p-type counterparts. The distinctive challenges inherent to n-type materials include the complex relationships between defect structures and electron concentration, and the strong anisotropy of thermoelectric properties. Electrons originate from various defect structures, such as impurity dopants, vacancies, antisite defects, and grain/domain boundaries, which sensitively varies depending on material synthesis processes. Moreover, the pronounced anisotropic nature of thermoelectric properties requires grain alignment along specific crystallographic directions. Therefore, the challenges in achieving high-performance n-type Bi2Te3 alloys lie in the difficulties of independently controlling defect structures (electron concentration), textured microstructures (electron/phonon transport property), and nanofeatures. This review aims to provide a comprehensive understanding of the difficulties and challenges associated with these aspects, and to discuss potential routes for realizing high-performance n-type Bi2Te3 alloys.