(Invited) Thermoelectric Thin Films and Nanocomposites Synthesized Via Solution-Phase Chemistry

Tuesday, 3 October 2017: 14:00
Chesapeake F (Gaylord National Resort and Convention Center)
R. Wang (Arizona State University)
Solution-phase processing routes to thermoelectric materials have the potential to decrease costs and enable novel device architectures. The best thermoelectric materials are crystalline inorganic semiconductors, which makes finding solution-phase routes to these materials of high interest. Unfortunately, inorganic semiconductors are generally insoluble due to their strong covalent bonds. One way around this hurdle is to create soluble semiconductor precursors that can be transformed into crystalline semiconductors after deposition. Furthermore, these precursor chemistries can be combined with colloidal nanocrystals to create nanostructured thin films with controlled composition and microstructure.

We first present the synthesis of CuSeS and Ag-doped CuSeS thin films via soluble precursors and then report their thermoelectric properties. Specifically, we study the effect of Cu vacancies, Se:S ratio, and Ag-doping. We find that Ag-doping leads to appreciable improvements in thermoelectric performance by increasing Seebeck coefficient and decreasing thermal conductivity, while having little to no change in electrical conductivity. Overall, we find that the room temperature thermoelectric properties of these solution-processed materials are comparable to measurements on CuSeS alloys made via conventional thermoelectric material processing methods. Achieving parity between solution-phase processing and conventional processing is an important milestone and demonstrates the promise of this approach to making thermoelectric materials.

Next, we present new chemical syntheses for soluble PbSe, PbTe, SnSe, and SnTe precursors. These precursors are especially relevant because PbSe and PbTe are the best-performing conventional thermoelectric materials in the moderate-to-high temperature regime. This chemical discovery is also important because soluble precursor routes to PbSe and PbTe thin films have not been reported in the literature. In addition, SnSe has the highest thermoelectric figure of merit of any known material, which makes our SnSe synthesis of interest as well.

Lastly, we combine these precursors with colloidal nanocrystals to create nanostructured thin films with controlled composition and microstructure. Colloidal nanocrystals are made via solution-phase chemistry and consist of an inorganic core with short ligand molecules bound to its surface. These nanocrystals can be made with excellent control over size, shape, and composition. We demonstrate that our precursors can replace the typical organic ligands used in colloidal nanocrystal synthesis. This creates new pathways to inorganic nanostructured thermoelectric thin films with controlled composition and microstructure, and we report on the thermoelectric properties of said films.