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A fundamental understanding of charge transport in polymeric semiconductors requires knowledge of how the electrical conductivity varies with carrier density. The thermopower of semiconducting polymers is also a complex function of carrier density making it difficult to assess structure-property relationships for the thermoelectric power factor. We examined the thermoelectric properties of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT-C14) by measurements of an electrochemical transistor using a polymeric ionic liquid (PIL) gate dielectric that can modulate the carrier concentration from 4 × 1018 to 3 × 1020 cm-3. As carrier density increases, so does the concentration of associated counterions, leading to a greater degree of energetic disorder within the semiconductor. Using thermopower measurements, we show experimentally that the electronic density-of-states broadens with increasing carrier density in the semiconducting polymer. The origin of a commonly observed power law relationship between thermopower and electrical conductivity is discussed and related to the changes in the electronic density-of-states upon doping. [ABSTRACT FROM AUTHOR]

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]

The heterogeneous microstructure of semicrystalline polymers complicates the relationship between their electrical conductivity and carrier concentration. Charge transport models typically describe conductivity with an assumption of uniform doping throughout the material. Here, the evolution in morphology and optoelectronic properties of poly(3‐hexylthiophene) (P3HT) is reported as a function of carrier concentration in an organic electrochemical transistor using a polymeric ionic liquid (PIL) as the gate insulator. Operando grazing incidence X‐ray scattering reveals that negatively charged ions from the dielectric first infiltrate the amorphous regions of the semiconductor, and then penetrate the crystalline regions at a critical carrier density of 4 × 1020 cm−3. Upon infiltration, the crystallites expand by 12% in the alkyl stacking direction and compress by 4% in the π–π stacking direction. The change in crystal structure of P3HT correlates with a sharply increasing effective carrier mobility. UV–visible spectroscopy reveals that holes induced in P3HT first reside in the crystalline regions of the polymer, which verifies that a charge carrier need not be in the same physical domain as its associated counterion. The dopant‐induced morphological changes of P3HT rationalize the dependence of mobility on carrier concentration, suggesting a phase transition of crystalline regions at high carrier concentration. Operando grazing incidence X‐ray scattering reveals structural changes during electrochemical gating of poly(3‐hexylthiophene) transistors using a polymeric ionic liquid gate dielectric. [ABSTRACT FROM AUTHOR]