Simultaneous enhancement of thermoelectric performance and mechanical flexibility in S-doped Bi2Te3 thin films

Flexible thermoelectric (TE) materials are promising candidates for powering wearable electronics and self-sustained microdevices. Here, we report the synthesis of S-doped Bi2Te3 flexible thin films (f-TFs) on polyimide (PI) substrates via a two-step method combining physical vapor deposition and subsequent thermal diffusion. Sulfur incorporation effectively substitutes Te in the Bi2Te3 lattice, leading to lattice contraction and a moderate increase in carrier concentration. Structural analyses reveal that S doping shortens the Bi-Te/S bonds and weakens interlayer van der Waals interactions, facilitating strain relaxation through nanoscale slip. As a result, the 3.8 % S-doped film achieves a peak power factor of 14.8 mu W cm-1 K-2 at 300 K and exhibits moderate electrical-conductivity stability under repeated bending, with resistance variation below 10 % after 1000 cycles. These findings demonstrate that thermal diffusion-driven compositional tuning provides a practical approach to enhancing both thermoelectric performance and mechanical flexibility in Bi2Te3-based f-TFs.