High Thermoelectric Performance in n-Doped Silicon-Based Chalcogenide Si₂Te₃

Achieving large thermoelectric figure of merit in a low-cost material, having an appreciable degree of compatibility with the modern technology is required to convert waste heat into electrical energy efficiently. Using first-principles density functional theory and semiclassical Boltzmann transport theory, we report high thermoelectric performance of a silicon-based chalcogenide Si₂Te₃. Previously unknown ground state structure of Si₂Te₃ was obtained by finding out the 8 most energetically favorable sites for Si in a unit cell of 12 Te and 8 Si atoms. Out of total C(28,8) combinations of structures, the search was narrowed down to 15 by using Wyckoff positions of space group P3̅1c. The minimum energy configuration having layered structure exhibits combination of desirable electronic and transport properties for an efficient thermoelectric material, including confinement of heavy and light bands near the band edges, large number of charge carrier pockets and low conductivity effective mass for n-type carriers. These features result into high thermopower and electrical conductivity leading to high power factor for n-type carriers. Furthermore, Si₂Te₃ possesses low frequency flat acoustical modes, which leads to low phonon group velocities and large negative Grüneisen parameters. These factors give rise to low lattice thermal conductivity below 2 W/mK at 1000 K. The combination of these excellent inherent electronic, transport and phononic properties renders an unprecedented ZT of 1.86 at 1000 K in n-doped Si₂Te₃, which is comparable to some of the best state-of-the-art thermoelectric materials. Our work presents an important advance in a long-standing search for the silicon-based thermoelectrics having exceptionally good energy conversion efficiency, and which could be integrated to the existing electronic devices.