Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Fundamentals to the field of thermoelectric materials is the need to optimize a variety of conflicting properties. To maximize the thermoelectric figure of merit (ZT) of a material, large Seebeck coefficient (α), high electrical conductivity (σ), and low thermal conductivity (κ) are required. As these transport characteristics depend on interrelated material properties, a number of parameters need to be optimized to maximize ZT.
High ZT values are necessary for the high conversion efficiency (η) of the thermoelectric generator. The conversion efficiency is the ratio between the electric output power (P) and the heat input (Qh).
So far, various strategies have been suggested to increase ZT values of TE materials. However, despite recent improvements in the efficiency of TE materials, the device engineering of TE modules has been largely neglected by strictly limited processing methods to fabricate devices. As a solution of this challenge, we develop the inks of thermoelectric materials and processing to architecture TE materials in 2D and 3D.
Thin film fabrication of thermoelectric system offers new scope for ZT enhancement by tailoring microstructure, which are determined by interface and strain energy at the forming stage of the thin film. This offers a wide range of ways to control texturing and doping in thin films through the choice of substrates, the coating process and the manipulation of post-treatment conditions. Also, thermoelectric thin films are potentially useful for various applications such as micro-Peltier-coolers, micro-generators, and miniature sensors as well as flexible TE power supply. Furthermore, thermoelectric ink can be directly painted onto curved heat sources, minimizing heat loss which essentially increases the efficiency of thermoelectric module. Especially, 3D printing technology enable us to shape thermoelectric materials conformable to free-form heat sources, which will maximize the heat transfer to thermoelectric devices. Furthermore, this 3D printing process can not only minimize the abandoned materials, but also reduction of process steps, which are the issues of conventional manufacturing. The specific materials of interest for adopting the 3D printing technology are based on metal chalcogenide that have a wide temperature range from room temperature to high temperature operation with high thermoelectric efficiency.
We are designing novel all-inorganic inks comprising of molecular inorganic precursors or nanocrystals (NCs) capped with inorganic ligands for electronic or optoelectronic applications, including light-emitting diodes (LEDs), solar cells, field effect transistors (FETs) and photodetectors. In general, typical colloidal synthesis of NCs involves the use of organic ligands which are composed of long hydrocarbon chain. However, since long-chain organic ligands act as an insulator, easy charge transfer between the NCs is dramatically hindered. Therefore, our group introduces a surface modification engineering to NCs for effective charge transfer by ligand exchange from organic ligands to inorganic ones. Furthermore, these designed colloidal NCs can easily form NC arrays by solution-process. For instance, all-inorganic semiconductor NC arrays can act as not only active channel layers in FETs, but also photoconductive layers in photodetectors.
Based on this technology, we are expanding the versatility of our designed inorganic inks thorough combination of a broad range of NCs and functional inorganic precursors as well as a concept that NCs are embedded in a semiconductor thin film.
Catalysts are necessary parts for chemical reactions which control the overall reaction rates. As the further decrease of size of materials to nano-meter scale, their electrical and physical properties can be changed. One of the fascinating properties of nanomaterials is high surface to volume ratio which is optimized for catalytic reactions. Also, usage of noble metal (e.g. Pt, Pd) can be drastically reduced as the efficiency of catalyst is maintained. We are synthesizing platinum and other transition metals based catalytic nanoparticles for high catalytic efficiency and further designing catalyst precursors for improved durability and stability.
