Chinese scientists have developed high-performance n-type Mg₃(Sb, Bi)₂-based thermoelectric thin films using a novel dual-defect engineering strategy, addressing key challenges that have long hindered the use of these materials in miniature energy harvesting devices.

Thermoelectric materialscan directly convert heat into electricity and are ideal for powering Internet of Things (IoT) sensors through waste heat recovery and for solid-state cooling applications. Among various material systems, n-type bulk Mg₃(Sb, Bi)₂-based compounds have attracted attention due to their abundant, environmentally friendly constituents and excellent thermoelectric properties.

However, research on their thin-film counterparts has significantly lagged due to two major obstacles. The first is the difficulty of controlling the composition during deposition, which is caused by the high vapor pressure of magnesium. This creates detrimental magnesium vacancies. The second obstacle is the complexity of achieving effective doping in thin films to optimize carrier concentrations. These issues have severely hindered the optimization of Mg₃(Sb, Bi)₂ thin-film materials' thermoelectric performance, thereby limiting their application in IoT.

Researchers led by Prof. TAI Kaiping from the Institute of Metal Research (IMR) of the Chinese Academy of Sciences overcame these obstacles using multi-target magnetron co-sputtering. Their dual-defect engineering strategy combines two synergistic approaches: introducing excess magnesium during deposition to suppress the formation of electron-killing magnesium vacancies and precisely substituting tellurium for bismuth to introduce additional electrons and fine-tune the carrier concentration.

The optimized Mg_3.15Bi_1.46Sb_0.5Te_0.04 thin film achieved a peak thermoelectric figure of merit (zT) of 0.47 at 525 K, which is 114% higher than that of p-type Mg₃(Sb, Bi)₂ films. Atomic-scale characterization revealed that Te doping not only optimizes carrier concentration but also induces lattice strain and dislocations that enhance phonon scattering. This suppresses thermal conductivity by over 53% compared to p-type films.

An all-Mg-based prototype device incorporating the optimized films demonstrated practical potential by generating an output voltage of 73 mV and a power density of 1185 μW/cm² at a temperature difference of 120 K, showcasing its capability for micro-power generation in mid-temperature range applications.

The study, published in Acta Materialia, establishes a universal design principle for thin-film thermoelectric materials and bridges the critical performance gap between bulk and film systems.

Schematic illustration of the fabrication process for n-type Mg₃(Sb, Bi)₂ thin films and the mechanism of p-type to n-type transition. (Image by IMR)

Comparison of microstructural characterization between n-type (a-d) and p-type (e-f) Mg₃(Sb, Bi)₂ thin films. (Image by IMR)