Publication date: 21st July 2025
By virtue of their giant gate capacitance, organic electrochemical transistors (OECTs) are a powerful platform for fundamental investigations of hole and electron transport in organic semiconductors as a function of continuously tunable charge up to 0.1-1 carrier per molecule. At these charge densities essentially all organic semiconductors examined so far exhibit a strong peak in conductivity versus charge density (or drain current vs. gate voltage). We have observed similar peak behavior in OECTs based on p- and n-type single crystals (rubrene and C60)1,2 and p- and n-type polymers, including polythiophenes3,4 and BBL. The conductivity peak appears to be due to filling of a sub-band (or even a full band) in the density of states. In some cases, the shape or height of the peak, and the reverse scan hysteresis, depend on the size of the ions in the electrolyte, suggesting that ion-carrier interactions can be important. The high carrier densities obtained offer exciting opportunities to examine insulator-metal transitions and carrier correlation effects in organic systems. In poly(3-hexylthiophene) (P3HT) we approach the insulator-metal transition as gate voltage is increased toward the conductivity maximum, but do not quite reach it.3 However, we observe a large Coulomb gap, which decreases with carrier density as the carriers become more delocalized. Recent work on polymer EGTs by Sirringhaus, et al. also demonstrates a Coulomb gap and striking band filling effects in polythiophenes.5 In C60 devices, we have good evidence for Mott-Hubbard band splitting at carrier densities of 1 e/molecule and above, Figure 2.2 The Mott-Hubbard picture is modified by ion-size dependence of the transport behavior. Carrier-carrier as well as ion-carrier correlations are abundant in organic conductors with ample opportunity for molecular design to manipulate these interactions and profoundly affect the transport.
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