Transistor Characteristics of Charge-Transfer Complexes Observed across a Neutral–Ionic Transition

For
basic understanding of transistor properties of doped organic
semiconductors, 3,3′,5,5′-tetramethylbenzidine charge-transfer
complexes are investigated, which change from neutral to ionic by
varying the acceptors. When going into the ionic state, the bulk conduction
increases more rapidly than the mobility, but sufficiently thin devices
exhibit transistor properties. The resulting ambipolar characteristics
are analyzed in the linear regions at small drain voltages in analogy
with graphene transistors. The model is further extended to include
partial overlap of electron and hole transport regions. In the temperature
dependence, the activation energy loses gate voltage dependence when
the neutral–ionic transition takes place by 0.1–0.2
eV above the equal acceptor and donor levels; the difference comes
from the typical trap (polaron) depth or the Madelung energy.