Electrolyte-gated transistors: a platform for exploring carrier transport at high charge densities in organic semiconductors

By virtue of their giant gate capacitance, electrolyte-gated transistors (EGTs) are a powerful platform for fundamental investigations of hole and electron transport in organic semiconductors as a function of continuously tunable charge densities up to 0.1-1 carrier per molecule or per monomer. 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). It will be shown that this holds for organic semiconductor crystals as well as polymer semiconductor films and that the cause is a marked collapse in carrier mobility at very large charge density. This mobility collapse, which corresponds to strong localization of carriers, will be discussed in terms of band-filling, carrier-carrier interactions, and ion-carrier interactions. Several specific organic semiconductors will be covered including C60 single crystals and polythiophene films in which variable length side chains are used to control ion-hole correlations and the maximum conductivities. For polythiophenes, maximum conductivities appear to occur universally near 0.15 hole/monomer.