Page 68 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 45

this effect is. It is in this fashion that nanoscale organic transistors
exhibit a remarkably different behavior in charge transport and
chemical sensing from the microscale counterparts.
According to the mobility study under low temperatures down to
77 K, the microscale transistors presented in the charge transport
68
and chemical sensing work exhibited a transport mechanism of ther-
mally activated hopping, which is mainly attributed to hopping at
grain boundaries, following the relation as μ ~ exp [−E /(kT)]. As
a
measured in an Arrhenius plot of temperature-dependent mobility
under different gate bias for a pentacene transistor with 2 μm channel
length, the activation energy E (energy barrier at a grain boundary)
68
a
is 130 meV under −5 V gate bias, and reduces to 52 meV under −30 V
gate bias. The mobility in the OTFTs presented in this work increases
with increasing gate bias, which is attributed to filling of the tail states
of the density of states (DOS). Also the gate dependence is stronger at
lower temperatures. These phenomena are well known for disor-
dered organic field-effect transistors where the band tail of localized
states has a much wider distribution at grain boundaries than within
each grain body, due to the increased disorder at grain boundaries. 27–28
It would be meaningful to investigate the influence of the sensing
event on the distribution of tail states through the combination of
sensing experiments and temperature-dependent charge transport
measurements. However, this requires much experimental care.
Most VOCs possess a high vapor pressure so that the analytes will
not remain at the sensing sites of the device for long enough time.
Therefore the comparison of temperature dependence of transistors
before and after sensing might not give an accurate view for what
happened during the sensing action. Furthermore, performing tem-
perature dependence experiments in the presence of analyte mole-
cules might be difficult due to the condensation effect of VOCs at
low temperatures.

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