Session:

Electrochemical transistors are a powerful platform for fundamental investigations of hole and electron transport in organic semiconductors as a function of continuously tunable charge up to very large densities of order 0.1-1 carrier per molecule. There is increasing evidence that ion-charge carrier interactions are profoundly important in these devices. This talk will describe ongoing experiments to examine transport in electrochemical transistors based on single crystals and thin films of organic semiconductors. In one study, electron transport on the surface of electrolyte-gated C60 single crystals is measured as a function of cation radius in the gate electrolyte. For small cations, surface conductivity versus electron density exhibits a peak near 0.5 electron/C60. Conductivity is completely shut down at 1 electron/C60. As cation size increases, this peaked conductance behavior becomes less prominent and average electron mobility increases, indicating that cation-electron interactions at the surface of the C60 crystal play an important role. For devices gated with the largest cations, temperature dependent transport shows the gate-induced electrons are significantly delocalized. In a second experiment, transport is examined in a series of polythiophenes as a function of side-chain length. Again, a strong peak in conductance versus carrier density is observed, with the peak occurring near 0.15 hole/monomer for all of the polymers. Further, there is a marked dependence of the conductance maximum on side-chain length, with longer side chains giving higher conductances. The results are interpreted in terms of ion-carrier Coulomb interactions and the implications will be discussed.

Daniel Frisbie

University of Minnesota

Appearances