Highly Flexible Silver Selenide Films for Skin-Conformal and Stretchable Thermoelectric Generator

Thermoelectric materials, which convert temperature gradients into electricity, are vital for applications such as thermoelectric generators and Peltier coolers. Their performance is typically assessed using the dimensionless figure of merit (zT), defined by the Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature. Recent research has made significant advancements in the development of flexible thermoelectric materials with high zT values at room temperature, particularly for wearable devices that harvest body heat. This study highlights the development of highly flexible and bendable silver selenide films with excellent thermoelectric performance at room temperature. These films address the growing demand for wearable thermoelectric generators that can conform to curved surfaces, such as human skin, while maintaining high thermoelectric efficiency, enabling effective energy harvesting from body heat during motion. We developed a novel fabrication method for freestanding silver selenide films with enhanced flexibility using an annealing treatment. The process begins with the synthesis of silver nanoparticles with diameters ranging from 4 to 14 nm. These nanoparticles are then converted into silver selenide particles measuring between 0.4 and 2.5 μm under hydrothermal conditions. The silver selenide particles are dispersed in chloroform and then drop-cast onto a polyimide substrate, yielding a uniform film. Following hot pressing at 230 °C and 5 MPa, a freestanding silver selenide film with a thickness of 27.1 ± 2.0 μm is fabricated.

During the annealing process, to prevent shape deformation, the freestanding film is sandwiched between aluminum foils and quartz plates, secured by stainless clips. This method allows for annealing at 300 °C in an N₂ atmosphere for 4 hours without surface damage, resulting in a flat 26.0 ± 3.1 μm-thick film. The annealed silver selenide films exhibit impressive thermoelectric properties. At room temperature, they achieve a Seebeck coefficient of -115 ± 1 μV K^-1, electrical conductivity of 858 ± 60 S cm^-1, and thermal conductivity of 0.888 ± 0.003 W m^-1 K^-1. These values yield a dimensionless figure of merit, represented as zT, of 0.381 ± 0.024 at room temperature. Although this value is slightly lower than that of the non-annealed films, which have a zT of 0.433 ± 0.041, it remains excellent for flexible thermoelectric materials. A notable feature is their flexibility. Non-annealed films bend to a radius of 5 mm, but annealed films bend elastically to 1 mm and plastically to less than 1 mm without losing thermoelectric performance. This flexibility is due to the development of low-angle grain boundaries during annealing, as evidenced by XRD and TEM analyses.

Taking advantage of the bendability of these films, we designed a stretchable and skin-conformal thermoelectric generator. Shaping the films into S-strips and embedding them in an elastomer body, the device stretches up to 70% strain without resistance changes. It conforms to curved surfaces like skin, maximizing temperature differences for energy harvesting. The generator produced a maximum power of 2.97 μW at a vertical temperature difference of 25.2 K. Attached to a human knee, it generated 0.0429 μW when straightened and 0.0640 μW when bent, utilizing about 63% of the temperature difference between skin and air. In conclusion, this research represents a significant step forward in flexible thermoelectric materials. The development of highly flexible and bendable silver selenide films with excellent thermoelectric performance brings new opportunities for wearable energy-harvesting devices. This work demonstrates the potential for powering small electronics directly from body heat, paving the way for future innovations in wearable technology.