In recent years, with the advancement and development of some new effects and mechanisms of thermoelectric transport, many new high-performance thermoelectric materials have been discovered. Among them, the diamond-like compounds are derived from the diamond structure. Because of the different atomic radius and chemical valence states of the elements, the lattice of the materials is distorted and the cubic structure of the diamond is transformed into non-cubic structure. The intrinsic low thermal conductivity and controllable electrical properties of diamond-like compounds make them promising thermoelectric materials. Since the thermoelectric properties of quaternary compounds Cu2CdSnSe4 and Cu2ZnSnSe4 were first reported by the thermoelectric research team of Shanghai Silicate Research Institute of Chinese Academy of Sciences in 2009, diamond-like compounds have attracted wide attention in the thermoelectric field. So far, more than 20 kinds of diamond-structured compounds have been reported. Thermoelectric properties of many p-type materials are higher than 1, which is comparable to traditional thermoelectric materials. However, the pyroelectric value of n-type diamond-like compounds is generally low, which limits the development of high-efficiency diamond-like compounds thermoelectric devices.

Recently, Qiu Pengfei, an associate researcher of Shanghai Silicate Institute, and Shi Xun, Chen Lidong, in collaboration with Yang Jing, a professor of Shanghai University, discovered a high performance n-type diamond-like compound AgInSe2 with intrinsic very low lattice thermal conductivity and electrical properties. At 900K, the highest thermoelectric optimal value of AgInSe2-based compounds reaches 1.1, which is comparable to the best p-type diamond-like compounds such as CuGaTe2 and CuInTe2 reported so far. On this basis, the research team for the first time prepared diamond-like thermal components with good application prospects.

The band gap of AgInSe2 is about 1.2eV. Previous research on AgInSe2 mainly focused on the application of photoelectric field. It was found that AgInSe2 has much lower lattice thermal conductivity than other diamond-like compounds. At room temperature, the lattice thermal conductivity of AgInSe2 is only 0.99W m-1K-1, which is equivalent to that of amorphous glass. First-principles calculations show that there are a large number of low-frequency optical branches in the phonon spectrum of AgInSe2. Strong scattering of lattice phonons with similar frequencies is the fundamental reason for the low thermal conductivity of AgInSe2. Further studies show that these low-frequency optical branches come from the cooperative vibration of the Ag-Se cluster. In AgInSe2 crystal structure, Ag and Se combine with strong chemical bonds, while in and the above two atoms have weak chemical bonds. Therefore, Ag and Se can form "Ag-Se cluster" with larger overall mass, and its binding force is weak, so it shows low phonon vibration frequency. On the other hand, by introducing Se vacancies in AgInSe2 or doping Cd elements in Ag sites, the electrical conductivity of materials can be increased in order of magnitude. Preliminary studies show that the thermoelectric value of AgInSe2 compounds with a small amount of Se vacancies reaches 1.1 at 900K.

Based on the high-performance n-type AgInSe2 compounds and the p-type CuInTe2 compounds (J. Mater. Chem. A, 2016, 4, 1277) reported by the research team earlier, the diamond-like thermoelectric components with two pairs of thermocouples were prepared for the first time. Using electroplating and brazing technology, Ni and Mo-Cu electrodes were successfully connected at the cold and hot ends of thermocouples. The preliminary test results show that the maximum output power of the device is 0.06W at 520K temperature difference. If the contact resistance and thermal resistance at the interface of the device can be further optimized, its performance will be further improved.

Relevant research results are published in Advanced Science. The research was supported by the National Natural Science Foundation of China, the Key Deployment Project of the Chinese Academy of Sciences, the Youth Innovation Promotion Association and the Shanghai Outstanding Discipline Leader Program.