Enhanced zT of highly flexible freestanding Ag₂Se films via Cu₂Se nanoparticle doping for wearable thermoelectric generator applications

We report a dual advance in wearable thermoelectrics by combining a materials-level doping strategy with a device-level architectural innovation. Freestanding, ultra-flexible Ag₂Se films are doped with trace amounts (tens of ppm) of Cu₂Se nanoparticles, yielding a dramatic enhancement in thermoelectric performance. At an optimal Cu₂Se doping of 50 ppm, the n-type Ag₂Se films achieve a room temperature zT of ~0.55 (versus ~0.46 for undoped films) while maintaining an extraordinary bending radius of 0.4 mm. This improvement is attributed to a finely tuned carrier concentration and increased phonon scattering imparted by the Cu₂Se nanoparticle inclusions, leading to a favorable balance of Seebeck coefficient, electrical conductivity, and thermal conductivity. Building on this high-performance flexible freestanding film, we design and experimentally demonstrate a novel vertical thermoelectric generator (TEG) architecture optimized for the predominantly out-of-plane temperature gradients of human body heat. The wearable TEG generates a maximum power/area of ~2.6 μW cm⁻² from a ΔT of ~10 °C on a human forearm, substantially outperforming conventional in-plane designs. Our work highlights a synergistic approach to advance wearable energy harvesting: enhancing the intrinsic zT of flexible thermoelectric materials while pioneering device architectures that bridge the gap between material innovation and practical energy harvesting from the human body.