Tailoring Nanoporous Structures in Bi₂Te₃ Thin Films for Improved Thermoelectric Performance

Thin-film thermoelectrics (TEs) with unique advantages have triggered great interest in thermal management and energy harvesting technology for ambient temperature microscale systems. Although they have exhibited a good prospect, their unsatisfactory performances still seriously hamper their widespread application. Tailoring the porous structure has been demonstrated to be a facile strategy to significantly reduce thermal conductivity and enhance the figure of merit (ZT) of bulk TE materials; however, it is challenging for thin-film TEs. Here, the nanoporous Bi₂Te₃ thin films with faceted pore shapes and various porosities, pore sizes, and pore intervals are carefully designed and fabricated by evacuating the over-stoichiometry Te atoms. The dependence of the carrier mobility and lattice thermal conductivity on the pore characteristics is investigated. In the case of the pore interval longer than the electron mean free path, the porous structure greatly reduces the lattice thermal conductivity without affecting the electrical conductivity obviously. Phonon specular backscattering that is highly related to the pore characteristics is suggested to be mainly responsible for thermal conductivity reduction, resulting in ∼60% enhancement in ZT at room temperature, that is, from ∼0.42 for the dense film to ∼0.67 for the nanoporous film. The enhanced ZT value is comparable to that of commercial bulk TEs and can be further improved by optimizing the carrier concentrations. This work provides a general approach to fabricate high-performance chalcogenide TE thin-film materials.