(Invited) Nano-Length-Scale Inorganic/Organic Hybridization for Thermoelectric Materials

Thermoelectric materials have been mostly inorganics based on metals, alloys, and inorganic compounds for a long time, but recently we have witnessed a rapid growth of organics for near room-temperature applications. High TE performance of organic materials appear to be mainly due to their low thermal conductivity, and the guiding principle to get high ZT is to enhance the power factor of low-thermal-conductivity organics. This is completely opposite to that for exploiting inorganic TE materials which usually possess high power factor but high thermal conductivity. Accordingly, nano-length-scale hybridization (not a mere composite) of inorganic and organic compounds would be promising to realize high-power-factor and low-thermal-conductivity simultaneously to get high ZT.

We have recently found that TiS₂-based inorganic/organic hybrid superlattices show a reasonable power factor and ultralow thermal conductivity; ZT=0.21~0.28 at 300~373 K in an ambient atmosphere¹⁾. It was also found that the polar molecules coexisting with organic cations in the van der Waals gap between TiS₂ monolayers have dielectric screening effects to suppress the electrostatic attractive force between organic cations present in the gap and carrier electrons present in the TiS₂ monolayers, and hence carrier mobility is enhanced while lattice thermal conductivity is lowered by increasing dielectric constant of polar molecules, which leads to enhanced ZT²⁾. Such hybrid superlattice materials are mechanically flexible, which would be beneficial for a variety of energy-harvesting applications.

Inorganic/organic hybridization should be a promising concept for the future TE materials design. There are an infinite number of combinations of inorganic and organic species. Nano-length-scale hybridization of the known compounds composed of non-rare and non-toxic elements must be cultivated to discover high-ZT materials.

1. C. L. Wan, K. Koumoto et al., Nature Mater. (2015). [DOI:10.1038/nmat4251]

2. C. L. Wan, K. Koumoto et al., submitted.