Page 63 - Organic Electronics in Sensors and Biotechnology

40 Chapter One

Therefore on contacting each other, no injection barrier is expected
between Au and pentacene, as shown in Fig. 1.22b. However, due to
the accumulation of holes at the pentacene side of the contact, these
positive charges and their image charges (negative) at Au side of con-
tact form a type of interface dipole, and the interface dipoles shift the
vacuum level at the Au side of the contact by an amount Δ. 123, 124 This
shift in the vacuum level changes the band bending at the contact and
forms a barrier to hole injection from Au into pentacene, as shown in
Fig. 1.22c.

1.2.5 Sensor Response to Different Analytes
and the Function of Receptors
It would be interesting and meaningful to investigate the scaling
behavior of chemical sensing with organic semiconductor-analyte
combinations other than pentacene and 1-pentanol. P3HT is a soluble
conjugated polymer with relatively high mobility and thus has a great
potential to be manufactured into chemical sensors with low-cost
techniques such as ink-jet printing. 127–129 Similar to pentacene transis-
tors, for micron scale or larger channel lengths, the drain currents of
P3HT transistors decreased in response to the analyte 1-pentanol,
whereas an increase in current was observed for nanoscale channel
130
lengths. Various types of analytes have been applied to investigate
the chemical sensing responses of OTFTs. Among these analytes,
vanillin is widely used in pharmaceuticals, perfumes, and flavors.
P3HT transistors with a series of channel lengths in the submicron
range were employed to examine the sensing responses of the conju-
gated polymer upon exposure to another type of analyte—vanillin.
As shown in Fig. 1.23a for submicron channels, the 215 nm or larger
channels exhibited a decreasing response in drain current upon delivery
of vanillin, whereas the 125 nm or smaller channels behaved in the
opposite direction, i.e., an increase in current as the sensing response.
The crossover of response behavior exists in the interval of channel
length 125 to 215 nm. Figure 1.23b shows the SEM image of a 75 nm
channel taken before depositing P3HT. Based on the results for the
sensing measurements of various channel lengths to analyte 1-pentanol
and vanillin, there are two mechanisms influencing sensor behavior:
one causes a decrease in current (dominant in large L devices) and the
other causes an increase (dominant in small L devices).
By incorporating small receptor molecules, it is possible to enhance
the sensitivity and selectivity of chemical sensing without the neces-
sity of chemically editing the molecule of the organic semiconductor.
The small receptor molecules can be incorporated by drop-casting onto
the organic semiconductor layer from a solution of receptor molecules
in chloroform after the fabrication of an OTFT, or directly mixing the
solution with the solution containing the polymer chains and then
depositing the mixture onto the prefabricated electrode pattern and