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Thermoelectricsare famously challenging to optimize, because ofinverse coupling of the Seebeck coefficient and electrical conductivity,both of which control the thermoelectric power factor. Inorganic–organicinterfaces provide a promising route for realization of the strongelectrical and thermal asymmetries required for thermoelectrics. Inthis work, transport properties of inorganic–organic interfacesare probed and understood at the molecular scale using the STM-break junction measurement technique, theory, and a class of newly synthesizedmolecules. We synthesized a series of disubstituted thiophene derivativesvarying the length of alkylthio-linkers and the number of thiophenerings. These molecules allow the systematic tuning of electronic resonanceswithin the junction. We observed that these molecules have a decreasingSeebeck coefficient with increasing length of the alkyl chain, whileoligothiophene junctions show an increasing Seebeck coefficient withlength. We find that thiophene–Au junctions have significantlyhigher Seebeck coefficients, compared to benzenedithiol (in the rangeof 7–15 μV/K). A minimal tight-binding model, includinga gateway state associated with the S–Au bond, captures andexplains both trends. This work identifies S–Au gateway statesas being important and potentially tunable features of junction electronicstructure for enhancing the power factor of organic/inorganic interfaces. [ABSTRACT FROM AUTHOR]