Controlling the Thermoelectric Properties of Thiophene-Derived Single-Molecule Junctions

Abstract

Thermoelectrics are famously challenging to optimize, because of inverse coupling of the Seebeck coefficient and electrical conductivity, both of which control the thermoelectric power factor. Inorganic–organic interfaces provide a promising route for realization of the strong electrical and thermal asymmetries required for thermoelectrics. In this work, transport properties of inorganic–organic interfaces are probed and understood at the molecular scale using the STM-break junction measurement technique, theory, and a class of newly synthesized molecules. We synthesized a series of disubstituted thiophene derivatives varying the length of alkylthio-linkers and the number of thiophene rings. These molecules allow the systematic tuning of electronic resonances within the junction. We observed that these molecules have a decreasing Seebeck coefficient with increasing length of the alkyl chain, while oligothiophene junctions show an increasing Seebeck coefficient with length. We find that thiophene–Au junctions have significantly higher Seebeck coefficients, compared to benzenedithiol (in the range of 7–15 μV/K). A minimal tight-binding model, including a gateway state associated with the S–Au bond, captures and explains both trends. This work identifies S–Au gateway states as being important and potentially tunable features of junction electronic structure for enhancing the power factor of organic/inorganic interfaces.