Page 50 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 27

a high negative voltage for n-channel) to remove the trapped charges
which result from the semiconductor/analyte interaction. Addition-
ally, compared to inorganic semiconductor sensors, organic/polymeric
transistors possess the advantages of being able to add specific func-
tional groups on the semiconductor molecules/backbones able to
selectively bind analytes, and the compatibility to incorporate small
receptor molecules for better sensitivity and selectivity. Organic and
polymeric field-effect transistors employing different active layers are
able to detect a variety of analyte molecules with good stability and
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significant sensitivity. Chemical detection is possible through direct
semiconductor-analyte interactions, specific receptor molecules per-
colated in the semiconductor layer for selective analytes, varying the
end/side groups of the semiconductor material, and controlling the
thin-film morphology of the semiconductor layer. These advantages
of OTFTs offer a basis to construct combinatorial arrays of sensors
with different responses to the components of an odor mixture. Fur-
thermore, gas-sensing complementary circuits and logic gates with
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OTFTs have been demonstrated. These advances, in addition to
circuitry for pattern recognition, could lead to an electronic nose.

1.2.2 Vapor Sensing in Micron-Sized Organic Transistors
and Trapping at Grain Boundaries
The vapor-sensing behavior of an organic transistor depends on the
morphological structure and interface properties of the device
because analyte molecules are able to act at different interaction sites
of the device and correspondingly modulate the overall conductance
of the device. In previous work on micro-sized OTFTs, fabricated
with a variety of active semiconductor layers, 102, 104 Crone et al. and
Torsi et al. investigated the relation of vapor sensing to thin-film
morphology upon exposure to different analyte molecules. A corre-
lation of the vapor response characteristics to the length of end
groups (flexibility at the molecular level) and grain size (porosity at
the morphological level) of the semiconductor was demonstrated by
observing the transient source-drain current under vapor flow and
performing transmission electron microscopy (TEM) for morpho-
logical characterization.
These experiments have demonstrated that the sensing response
of an OTFT channel to analytes of moderate dipole moments (e.g.,
alcohols) is enhanced with decreased grain size and looser molecular
packing of the organic semiconductor layer. Smaller grains yield
more grain boundaries which provide more interaction sites for sens-
ing events. The response also becomes stronger with increasing film
thickness, again due to the increased number of grain boundaries
(the surface morphology becomes more structured as the films grow
thicker from the flat and featureless ultrathin film). Analytes binding
to the disordered and thinner grain boundaries are closer to the