Electrolyte gated single-crystal organic transistors to examine transport in the high carrier density regime
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uction Free charge-carrier density is one of the key parameters controlling the electronic properties and phase behavior of insulators, semiconductors, and superconductors.1 One clear effect of mobile charge density is the giant span of electrical conductivity from metals (∼107 S/m) to insulators (∼10–22 S/m). In organic semiconductors, the role of free-carrier density on electronic properties is not as well understood as it is in more conventional electronic materials.2 In particular, significant charge-carrier correlations can be anticipated in these materials because they are characterized by low dielectric constants (low charge screening) and relatively narrow bandwidths. Figure 1 summarizes the near zero-temperature electronic properties of several prototype organic materials that have been investigated as a function of their free-carrier concentrations. A noteworthy example is C60, which exhibits a superconducting phase when each fullerene molecule is doped by three alkali metal atoms but converts back to a semiconducting/insulating state at higher or lower doping levels.3 As in other electronically correlated materials, it is clear that systematic control of charge density over a wide range is of great importance to
obtain exciting and desirable electronic properties in organic semiconductors. The electrostatic field-effect approach serves as an ideal technique for controlled and reversible tuning of charge densities without introducing disorder into the active material. In this framework, an external electric field is applied to a semiconductor material across a gate insulator using a gate electrode. This creates a thin accumulation or depletion layer near the surface whose electrical conductivity between source and drain electrodes can be modified systematically by the gate voltage. Current flows when a drain bias is applied, and its magnitude is determined by the standard field-effect transistor (FET) equations as
ID =
W μV C (V − VTh ) (if VD < VG − VTh ) L D i G
ID =
W 2L
μC i (VG − VTh ) 2 (if VD ≥ VG − VTh ),
(1) (2)
in which ID is the drain current (A), VD is the drain voltage (V), W and L are channel width and length (μm), respectively, μ is the carrier mobility (cm2 V–1 s–1), Ci is the specific capacitance
Wei Xie, Department of Chemical Engineering and Materials Science, University of Minnesota; [email protected] C. Daniel Frisbie, Department of Chemical Engineering and Materials Science, University of Minnesota; [email protected] DOI: 10.1557/mrs.2012.310
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • JANUARY 2013 • www.mrs.org/bulletin
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ELECTROLYTE GATED SINGLE-CRYSTAL ORGANIC TRANSISTORS: HIGH CARRIER DENSITY REGIME TRANSPORT
efforts in understanding the device physics in electrolyte-gated organic semiconductors and particularly in organic crystals. In the following, we summarize recent research highlights in fundamental aspects of electrolyte-gated organic crystals. We present the general techniques of how to apply electrolytes as a high-capacitance insulator, how to determi
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