Solution-Processed Thermoelectric Materials: the Case of SnSe

The conversion of thermal energy to electricity and vice versa through solid-state thermoelectric devices is exceptionally appealing for many applications. Not only because thermal waste energy is generated in many of our most common industrial and domestic processes but also because thermoelectric devices can be used for temperature sensing, refrigeration, etc. However, their extended use has been seriously hampered by the relatively high production cost and low efficiency of thermoelectric materials. The problem is that thermoelectric materials require high electrical conductivity (s), high Seebeck coefficient (S), and low thermal conductivity (k); three strongly interrelated properties.
After decades of research, promising material candidates that are Earth-abundant and low cost have been identified; among them, SnSe. In fact, since the material was “re-discovered”  in 2014, SnSe has become the most studied thermoelectric material due to its extraordinary performance in its single crystal form with up to 2.8 values in the figure of merit (ZT = s S2 T k-1). However, the high cost and stagnant production of single crystals and their poor mechanical properties limit the large-scale production of SnSe-based thermoelectric devices. Therefore, a great deal of attention has been placed in replicating the single-crystal charge and thermal transport properties in its polycrystalline counterpart to produce cost-effective materials with enhanced mechanical stability. To date, this has been proven difficult due to the easy oxidation of small crystalline domains, the partial loss of anisotropy, and the difficulty to control the doping level finely. Herein, we present a scalable, low-cost, and environmentally friendly methodology to produce SnSe nanocomposites with outstanding performance. In particular, we synthesize SnSe nanoparticles in water and modify their surface before consolidation to finely tune its electronic structure and its charge and phonon scattering properties to achieve figures of merit up to 2.4. These results are of high promise to introduce waste heat recovery through thermoelectric devices into the market.
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