Threshold-Voltage
Shifts in Organic Transistors Due to Self-Assembled Monolayers at
the Dielectric: Evidence for Electronic Coupling and Dipolar Effects
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Abstract
The mechanisms behind the threshold-voltage
shift in organic transistors due to functionalizing of the gate dielectric
with self-assembled monolayers (SAMs) are still under debate. We address
the mechanisms by which SAMs determine the threshold voltage, by analyzing
whether the threshold voltage depends on the gate-dielectric capacitance.
We have investigated transistors based on five oxide thicknesses and
two SAMs with rather diverse chemical properties, using the benchmark
organic semiconductor dinaphtho[2,3-b:2′,3′-<i>f</i>]thieno[3,2-<i>b</i>]thiophene. Unlike several
previous studies, we have found that the dependence of the threshold
voltage on the gate-dielectric capacitance is completely different
for the two SAMs. In transistors with an alkyl SAM, the threshold
voltage does not depend on the gate-dielectric capacitance and is
determined mainly by the dipolar character of the SAM, whereas in
transistors with a fluoroalkyl SAM the threshold voltages exhibit
a linear dependence on the inverse of the gate-dielectric capacitance.
Kelvin probe force microscopy measurements indicate this behavior
is attributed to an electronic coupling between the fluoroalkyl SAM
and the organic semiconductor