Flexible thermoelectric foil for wearable energy harvesting

Flexible thermoelectric (TE) devices have been of rapidly growing interest for long-lasting and maintenance-free wearable power source that makes use of the temperature difference between human skin and ambient environment. Despite the high TE performance, conventional inorganic TE semiconductors, such as Bi2Te3, skutterudites, are restricted for this application due to their non-flexibility structure and non-scalable manufacturing techniques. In this paper, we report large-area free-standing TE foil with several centimeters in size through a scalable, cost-effective and solution-based approach for flexible thermoelectric devices. The foil is made by self-assembling two-dimensional hybrid superlattices of TiS2 layers and hexylamine molecules. Through a Lewis base-acid reaction, electrons are transferred from the hexylamine molecules into the TiS2 layers, making the material n-type and generating a large power factor of 0.23 mW/m K2, comparable to the best p-type conducting polymers. The flexible foil shows superior bending deformation tolerance, even better than the conducting polymers. A power output of 32 µW cm−2 for a 15-µm-thick foil were obtained under a temperature difference of 20 K at ambient temperature, high enough to drive the wearable nanowatt-to-microwatt level circuit chips or devices.