Page 85 - Organic Electronics in Sensors and Biotechnology

62 Cha pte r T w o

herringbone pattern, but stacks in two-dimensional columnar arrays
with significant overlap of the pentacene rings (the interplanar spac-
ing of the aromatic rings is 3.47 Å for TIPS pentacene, compared with
6.27 Å for pentacene). Recently better results were obtained with
2
drop-casted films. Hole mobilities as high as 1.8 cm /(V · s) and on/
8
off current ratios of 10 have been achieved by drop-casting a 1%
w
toluene solution of TIPS pentacene on top of a bottom-contact FET
94
device substrate. The approach proposed by Anthony has also been
95
extended to other acene derivatives such as pentacene ethers, acen-
97
edithiophenes, and longer hexa- and heptacenes. Remarkably,
96
drop-casted films of a bis-triethylsilylethynyl anthradithiophene
2
yielded devices with mobilities as high as 1.0 cm /(V · s) and on/off
7 92
current ratios of 10 .
Although acenes result in appealing materials for OTFT applica-
tion, only few sensing application examples are found in literature. In
addition to pentacene thin film employed in alcohols and humidity
OTFT sensors, other experimental works deal with the use of naph-
talene tetracarboxylic derivates film as sensitive materials for water
58
vapor, 56, 98 ketones, thiols, and nitriles. Differently from acenes, oligo-
and polythiophene have been widely investigated for the sensing of
alcohols, 58, 72, 99 esters, thiols, ammonia, chloroform, lactid acid,
100
58
58
101
102
and glucose. Moreover, in the case of the regio-regular polythio-
phenes the fundamental role of the alkyl- and alkoxy-side chain in
72
conferring selectivity to an OTFT has been fully demonstrated,
pointing out the importance of the chemical functionalization for the
control of the sensing element selectivity.
In conclusion, the ability to functionalize organic semiconductors
in order to control their chemical-physical properties, the type of
interaction occurring in solid state, and the gas sensing properties of
the thin film are a critical aspect in the perspective to develop and
commercialize a new sensing platform with high performance and
low power consumption.
2.3.2 New Materials for OTFT Sensing Applications
As seen, more stable and solution processable pentacene deriva-
tives have been synthesized, some of which showed high field-effect
mobilities comparable to or even greater than the parent pentacene
molecule. An alternative approach to obtain semiconductive mate-
rials consists of the construction of suitable oligomeric structures
with smaller polycyclic benzenoid systems. 103–107
The adoption of this strategy is justified by the following reasons.

• Smaller acenes are less conjugated and have a greater oxida-
tive stability; this characteristic is fundamental in applica-
tions where durability and reproducibility are the deciding
factor.