Two‐step annealing process drives high thermoelectric performance of n‐type Bi ₂ Te ₃ flexible films

Abstract Bismuth telluride (Bi 2 Te 3 ) is a leading material for flexible thermoelectric films, making it suited for low‐grade energy harvesting and sensing applications in wearable devices. However, the performance of Bi 2 Te 3 films suffers from low mobility, a consequence of their small grain size and lack of preferred crystal orientation. Here, we demonstrate that a two‐step annealing process combining in situ and post‐annealing heat treatment can significantly enhance the thermoelectric properties of n‐type Bi 2 Te 3 films prepared by magnetron sputtering. This method promotes pronounced grain growth and strengthens the preferred ( 00l ) crystal texture, increasing charge carrier mobility from 2.84 cm 2 V −1 s −1 to 132.54 cm 2 V −1 s −1 . Meanwhile, the two‐step annealing introduces Bi Te antisite defects, which optimizes the carrier concentration to 1.29 × 10 19 cm −3 . The combination of the tailored carrier concentration and exceptionally high mobility produces a room‐temperature power factor of 14.5 μW cm −1 K −2 in the fabricated Bi 2 Te 3 ‐based films, with a high power density of 545.66 μW cm −2 at a temperature difference of 30 K. A minimal electrical variation (<7%) during the bending test demonstrates the excellent bending resistance and stability of the flexible Bi 2 Te 3 films. This study demonstrates that the two‐step annealing process is an effective method to improve the mobility and performance of Bi 2 Te 3 ‐based films.