Development of high-efficient tin selenide-based thermoelectric materials

Abstract

Thermoelectric materials can realize direct energy conversion from heat to electricity based on thermoelectric effects, thus have been considered as a green and sustainable solution to the global energy dilemma by harvesting electricity from waste heat or sunlight. The conversion efficiency can be expressed as ZT=S2σT/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. To date, two major strategies for achieving high ZT are optimizing the power factor S2σ and reducing lattice thermal conductivity κl by band and structural engineering, respectively. However, the current commercial thermoelectric materials such as bismuth telluride (Bi2Te3) have their ZT values limited to <1.5, which have been the bottleneck challenge for their wider practical applications. In this regard, finding next generation thermoelectric systems with ZT ≥ 1.5 is urgently needed.Stannous mono-selenide (SnSe) has attracted much attention because of its great potential in realizing high-performance, low-toxic and low-cost thermoelectric devices. To achieve high performance in polycrystalline SnSe, three major strategies including doping, multi-phase alloying, and micro/nanoscale texturing, have been employed. The peak ZTs have been improved from ~0.5 to ~1.0, which are still much lower than their single crystal counterparts. To further improve the ZT of SnSe-based thermoelectric materials, in this thesis, multiple strategies including nanostructuring and band engineering guided by theoretical modelling and/or simulations, were constructively employed to simultaneously minimize κl and maximize the S2σ, leading high ZTs in both p-type and n-type polycrystalline SnSe. These works are summarized as follow:1. For p-type SnSe, we realize a high peak ZT of 1.36 in polycrystalline Sn0.98Se macro-sized plates, fabricated via an advanced solvothermal method. The obtained exceptional thermoelectric performance comes from their high S2σ of 6.95 μW cm-1 K-2 and ultra-low κ of 0.42 W m-1 K-1 at 823 K. Through our Hall measurements, we found the high hole carrier concentration p of 1.5×1019 cm-3 derived from the self-doping, which contributes to a high σ and a moderate S. Moreover, detailed structural characterizations reveal a strong preferred orientation in our sintered Sn0.98Se pellets. The phonon scattering sources such as grain boundaries synergistically coupled with the anharmonicity bonding of Sn0.98Se crystals with a high density of 98.5 %, result in an intrinsic ultra-low κ. (Energy Storage Materials 2018, 10: 130-138).2. We achieve a high Cu solubility of 11.8 % in single crystal SnSe microbelts. The pellets sintered from these microbelts show a high S2σ of 5.57 μW cm-1 K-2 and low κ of 0.32 W m-1 K-1 at 823 K, contributing to a high peak ZT of ~1.41. The high S2σ comes from the obtained Cu+ doped state, and the intensive crystal imperfections such as dislocations, lattice distortions, and strains, play key roles in keeping low κ. (Chemical Science, 2018, 9: 7376-7389).3. We realize a high ZT of ~1.7 at 823 K in p-type nanoporous polycrystalline SnSe. We successfully induce indium selenides (InSey) nanoprecipitates in the as-synthesized SnSe matrix of single crystal microplates, and the nanopores are achieved via the decompositions of these nanoprecipitates during the sintering process. Through detailed structural and chemical characterizations, it is found that the extra-low κ of 0.24 W m-1 K-1 is caused by the effective phonon blocking and scattering at induced nanopores, interfaces, and grain boundaries, contributing to high ZTs. (ACS Nano, 2018, 12: 11417-11425).4. We achieve a ZT of ~1.7 at 823 K in p-type polycrystalline Cd-doped SnSe by combining cation vacancies and localized-lattice engineering. It is observed that the introduction of Cd atoms in SnSe lattice induce Sn vacancies, which act as p-type dopants. The Sn vacancy can be boosted to a high level of ~2.9 %, which results in an optimum p of ~2.6×1019 cm-3 and an improved S2σ of ~6.9 μW cm-1 K-2. Simultaneously, a low κ of ~0.33 W m-1 K-1 is achieved by effective phonon scattering at localized crystal imperfections, as observed by detailed structural characterizations. Density-functional-theory calculations reveal that the role of Cd atoms in the SnSe lattice is to reduce the formation energy of Sn vacancies, which in turn lower the Fermi level down into the valence bands, generating holes. (Advanced Energy Materials, 2019: 1803242).5. For n-type SnSe, we realize a record high ZT of ~1.1 at 773 K in n-type highly-distorted Sb-doped microplates via an advanced solvothermal method. The pellets sintered from the Sb-doped SnSe microplates show a high S2σ of ~2.4 μW cm-1 K-2 and an ultralow κ of ~0.17 W m-1 K-1 at 773 K, leading a record high ZT. Such a high S2σ is attributed to a high electron concentration n of 3.94×1019 cm-3 via Sb-enabled electron doping, and the ultralow κ derives from the enhanced phonon scattering at intensive crystal defects, including severe lattice distortions, dislocations, and lattice bent, observed by detailed structural characterizations. (Advanced Energy Materials, 2018: 1800775)