a) TE figure of merit zT and b) power factor PF for Ag₂(S, Se), Ag₂(S, Te), and Ag₂(S, Se, Te) at 300 K. c) Upper panel: a schematic of the Ag₂S_0.5Se_0.5/Pt-Rh in-plane device with Ag₂S_0.5Se_0.5 as n-type legs and Pt-Ru wire as p-type legs. Bottom panel: optical image of a six-couple flexible Ag₂S_0.5Se_0.5/Pt-Rh TE device. d) Comparison of normalized maximum power density (PmaxL/A) among the Ag₂(S, Se)-based inorganic TE device, inorganic-organic hybrid flexible TE devices, and organic flexible TE devices [Image: Prof. Shi Xun’s group]

The internet era is accompanied by an increasing demand for alternative power sources in the W-to-mW range to drive distributed, wearable, and implantable microelectronics. It is hard for traditional electrochemical batteries to keep up with the fast-paced progress of microelectronics in miniaturization, packing density, mechanical flexibility, bio-safety, and reliability. Nonetheless, a thermoelectric battery excels where an electrochemical battery falls short. Thermoelectrics is the simplest technique to directly convert heat, ubiquitous in the environment, into electricity, a versatile form of energy. Thermoelectric devices are all solid-state, free of greenhouse emissions or moving parts, friendly for miniaturization and low-maintenance in the long term. However, they generally lack mechanical flexibility, a key ingredient for wearable electronics.

Based upon the early discovery of the flexible inorganic semiconductor Ag₂S (Nat. Mater., 2018, 17, 421-426), the research team led by Prof. Shi Xun and Prof. Chen Lidong at the Shanghai Institute of Ceramics, in collaboration with Prof. He Jian from Clemson University, successfully fabricated the world’s first flexible full-inorganic thermoelectric power generation module based on silver calcogenides. Since Ag₂S has poor thermoelectric performance despite its flexibility, and Ag₂Se and Ag₂Te exhibit the opposite, it took serious materials research efforts via doping Se and Te on the S-site and controlling native defects to attain a delicate balance between the material’s thermoelectric performance (state-of-the-art figures of merit zTs up to 0.44 at 300 K and 0.63 at 450K) and flexibility.

The material’s mechanical, electrical and thermal properties survived bending tests, meeting the requirements of wearable electronics. The team has also solved several of the device’s architecture design problems. The as-fabricated 6-leg device exhibits a normalized maximum power density up to 0.08 W·m-1 near room temperature under a temperature difference of 20 K, orders of magnitude higher than organic devices and organic-inorganic hybrid devices. These results constitute a key initial step towards the new paradigm of flexible thermoelectrics, and will inspire more follow-up research efforts, e.g., developing p-type legs in place of Pt/Rh wires.

The study was published in Energy and Environmental Science with the title “Flexible thermoelectrics: from silver chalcogenides to full-inorganic devices”. This work is primarily supported by the National Key Research and Development Program, the National Science Foundation of China, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the Shanghai Rising-star program.

For more information, please contact:

Prof. Shi Xun

E-mail: Xshi@mail.sic.ac.cn

Source: Shanghai Institute of Ceramics, Chinese Academy of Sciences