Electric field effect in 'metallic' polymers

The charge transport properties of the "metallic" polymer, poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS), with a conductivity around 30 S/cm are studied in this thesis. The PEDOT:PSS is incorporated into a field effect transistor (FET) structure as an active component. Considering the screening effect of metals, it is unexpected to observe a dramatic conductance change in PEDOT:PSS under the application of a gate electric field. The conventional FET model is used to further investigate this phenomenon. Though the current-voltage (I-V) characteristics of PEDOT:PSS devices are similar to the conventional field effect transistors (FETs), the extracted field effect mobility (mu FET) from I-V curves is two orders of magnitude larger than that estimated from the conductivity. Further investigating the I-V curves, a hysteresis behavior is observed and varies with drain voltage sweeping rate. This hysteresis phenomenon suggests ion motion is involved in the PEDOT:PSS conductance suppression. Since the structure of the metallic polymers is viewed as metallic ordered regions embedded in poorly conducting disordered media, charge carriers conduct electricity by hopping over or resonant tunneling through the localized states in the disordered regions. Therefore, several experiments are performed to understand the origin of the electric field penetration inside the metallic polymer.;Using the transient current measurements, the relationship between inserted ion charges and PEDOT:PSS conductance variation is examined. Around 2% replacement of hole charges on the PEDOT:PSS backbone with inserted ionic charges enables the modulation of the conductance of PEDOT:PSS by three orders of magnitude. This small fraction of charge compensation of counterions by inserted ion charges suggests a percolation phenomenon for PEDOT:PSS conduction suppression.;The role of inserted ions is further investigated by measurements of the temperature dependence of this FET structure. In the presence of inserted ions, the in situ conductivity measurements indicate that the degree of disorder in the poorly conducting regions of PEDOT:PSS increases and the observed field effect is a bulk effect, which supports the percolation phenomenon; the in situ ESR measurements done simultaneously show no apparent change in the density of states at the Fermi level ( N(EF)) in the ordered regions of PEDOT:PSS.;Combining the results in the disordered regions, the ordered regions and the percolation phenomenon lead to the conclusion that the inserted ions interacting with the counterions initially present in the disordered regions increase the hole hopping distance resulting in a conductor-nonconductor transition.