Shenjia Hydraulics

Flexible thermoelectric energy conversion technology can transform ubiquitous temperature differences in the environment into electrical power output, holding great promise for applications in fields such as flexible electronics. However, current high-performance inorganic thermoelectric materials are all brittle and lack flexibility. While miniaturizing these materials and integrating them onto flexible substrates can provide a certain degree of bendability, they remain highly susceptible to fracture under significant bending or large deformations. On the other hand, although organic thermoelectric materials exhibit excellent flexibility and bendability, their charge carrier mobility is significantly lower than that of inorganic materials, making it difficult to achieve efficient energy conversion and electrical power output.

Recently, researchers including Researcher Shixun, Researcher Lidong Chen, Researcher Yiyang Sun, and Associate Researcher Pengfei Qiu from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, in collaboration with Professor Jian He from Clemson University in the United States, have developed a new type of high-performance inorganic flexible thermoelectric material and device based on Ag2S flexible semiconductors. This research has opened up a new direction in the study of inorganic flexible thermoelectric materials and has addressed the most fundamental and critical challenge in the development of all-flexible thermoelectric conversion technologies based on high-performance inorganic materials. The related research findings were published in Energy & Environmental Science under the title "Flexible thermoelectrics: from silver chalcogenides to full-inorganic devices" (2019, DOI: 10.1039/C9EE01777A).

New high-performance inorganic flexible thermoelectric materials must simultaneously balance good ductility and excellent thermoelectric performance. Earlier, the team reported the first-ever room-temperature inorganic flexible semiconductor material—Ag2S (Nature Materials, 2018, 17, 421–426)—which is shatterproof and freely bendable, exhibiting exceptionally outstanding flexibility and bending performance. However, Ag2S has a bandgap of about 1.0 eV, resulting in extremely low electrical conductivity and thermoelectric performance at room temperature, and its intrinsic defects—such as interstitial Ag atoms—show only very weak optimization effects. Therefore, the Ag2S matrix itself is not an ideal thermoelectric material. Doping and other modifications to Ag2S hold promise for significantly enhancing its thermoelectric performance; yet, whether the material can continue to maintain its excellent flexibility and ductility remains an unknown but critical question. In this study, the research team synthesized a series of Ag2S materials solid-soluted with Se or Te and found that solid-solution incorporation of Se and Te can significantly reduce the defect formation energy of interstitial Ag ions, leading to an increase in the concentration of interstitial Ag ions and thus markedly improving the material's charge transport properties. The power factor reached a maximum of approximately 5 μW·cm⁻¹·K⁻². Meanwhile, the solid-solution incorporation of Se/Te substantially narrowed the material's bandgap, shifting the peak of the figure of merit toward lower temperatures, with the highest thermoelectric figure of merit reaching 0.44 at room temperature.

Both Ag2Se and Ag2Te are brittle at room temperature and lack plasticity and flexibility. Therefore, solid-solution incorporation of Se or Te can significantly affect the mechanical properties of Ag2S. Mechanical test results, including compression tests and three-point bending tests, show that when the Se content is less than 60% or the Te content is less than 70%, the material maintains both plasticity and flexibility. Consequently, when the Se or Te content is in the range of 20% to 60%, the material exhibits both excellent plasticity and thermoelectric performance. The research team selected a bending radius of 3 mm for testing and found that after 1,000 repeated bending cycles, the electrical conductivity and Seebeck coefficient of the Ag2S0.5Se0.5 thin film remained virtually unchanged, indicating that the material's performance is minimally affected by stress and can meet the requirements for flexible wearable power supplies.

Building on the development of high-performance inorganic flexible thermoelectric materials, the research team fabricated an in-plane thermoelectric generator consisting of six pairs of n-type Ag2S0.5Se0.5 thermoelectric legs and p-type Pt-Rh wires. At a temperature difference of 20 K, the maximum normalized power density reached 0.08 W·m⁻², which is one to two orders of magnitude higher than that of the best currently available all-organic thermoelectric devices.

The new high-performance inorganic flexible thermoelectric materials and devices developed by the institute, based on Ag2S flexible semiconductors, can simultaneously deliver excellent flexibility and thermoelectric conversion performance. They also boast advantages such as being environmentally friendly, stable and reliable, and having a long service life. These materials are expected to find wide applications in next-generation smart micro- and nano-electronic systems characterized by distributed, wearable, and implantable technologies.

The research was funded and supported by the National Key R&D Program, the National Natural Science Foundation of China, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the Shanghai Young Science and Technology Star Program.

Link: https://doi.org/10.1039/C9EE01777A

a) The “plastic-zT” phase diagram of the Ag2S-Ag2Se-Ag2Te system. b) Excellent mechanical properties of Ag2S-based flexible thermoelectric materials. c) Schematic and actual photographs of Ag2S-based flexible thermoelectric devices. d) Comparison of power density between Ag2S-based flexible thermoelectric devices and other reported flexible thermoelectric devices.