This is a photograph of the TE ink printed in various shapes with curves and straight lines. Source: UNISTA new advanced energy harvesting system has been developed by the Ulsan National Institute of Science and Technology (UNIST) that is capable of generating electricity by attaching to clothes, windows and outer walls of a building.

The device is based on a temperature difference between the hot and cold sides. That difference can be increased as high as 20.9 degrees Celsius, much higher than the typical temperature differences of wearable thermoelectric generators driven by body heat. The development could improve the efficiency of wearable solar generators by raising the temperature difference.

A thermoelectric generator (TEG) refers to a device that converts waste heat energy, such as solar energy, geothermal energy and body heat into additional electrical power. Recently, the study of wearable thermoelectric generators was undertaken using the temperature difference between the body heat and surrounding environment. However, one of the drawbacks is that the temperature difference is typically between 1.5 degrees and 4.1 degrees Celsius.

UNIST was able to solve this low temperature difference by introducing a local solar absorber on a PI substrate. This absorber is a five period Ti/MgF2 superlattice where each structure and thickness was designed for optimal absorption of sunlight. This allowed researchers to increase the temperature difference as high as 20.9 degrees Celsius, the highest value of all wearable TEGs to date.

"Through this study, we have secured a temperature difference with the ten-fold increase from the conventional wearable solar thermoelectric generators," says Yeon Soo Jun, a graduate student at UNIST. "Since the output of a TE generator is proportional to the square root of the temperature difference, one can significantly increase the output with the help of this technology."

Researchers designed the wearable solar thermoelectric generator by integrating flexible BiTe-based TE legs and sub-micron thick solar absorbers on a polyimide (PI) substrate. The TE legs were prepared by dispenser printing with an ink consisting of mechanically alloyed BiTe-based powders and an Sb2Te3-based sintering additive dispersed in glycerol. Researchers say a W-STEG comprising 10 pairs of p-n legs has an open circuit voltage of 55.15 mV and an output power of 4.44 μW when exposed to sunlight.

"Our new wearable STEG is expected to be useful in various applications, such as in self-powered wearable electronic devices," says Kyoung Jin Choi, professor in the School of Materials Science and Engineering at UNIST. "It will also serve as a catalyst to further improve the future wearable electronic technology market."

The full research can be found in the journal Nano Energy.