Page 83 - Organic Electronics in Sensors and Biotechnology

60 Cha pte r T w o

semiconducting properties with an average mobility in top-contact
2
–2
configuration of 1.2 × 10 cm /(V · s) with on/off ratios higher than
2
10 , while the highest mobility obtained was 0.055 cm /(V · s). Gas
4
sensing measurements performed on D3ANT-based OTFT showed
high selectivity toward the NO gases with detection limit at sub-
x
ppm levels.
2.3.1 Introduction
Until now, polycyclic aromatic hydrocarbons (PAHs) have been
among the most studied organic materials for OTFT applications.
Particular attention has been paid to linear PAHs composed of later-
75
ally fused aromatic rings and called simply acenes. Among all the
studied acenes, with no doubt pentacene is known to have the highest
2
hole mobility [>1.0 cm /(V · s)] for high vacuum deposited thin
films, 76–78 and it is used as reference standard for all newly synthe-
sized p-type organic semiconductors. Thermally evaporated penta-
cene OTFTs have been used in sensor applications as well. Pentacene
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OTFTs were tested as humidity sensors by Zhu et al. reporting a
saturation current reduction up to 80% after a relative humidity (RH)
change from 0 to 30%. The gas sensing response was attributed to a
decrease of the hole mobility due to the water molecule. Moreover,
the sensitivity depended on the thin-film thickness. Other experi-
mental work was conducted to shed light on the sensing mechanisms
of pentacene. 80, 81 The reduction of both hole mobility and source-
drain current has been related to the interaction at grain boundary of
the charge carriers with the polar water molecules.
The gas sensing properties of pentacene OTFT sensors have been
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explored also toward 1-pentanol by employing devices with chan-
nel lengths ranging from 36 μm to 20 nm. The source-drain current
was found to decrease upon analyte exposure for the larger-scale
devices whereas the opposite happened surprisingly for the smaller
ones. These results were explained by considering the effects of two
competing mechanisms: a reduction in mobility due to charge carrier
trapping at grain boundaries and an increase in charge density asso-
ciated with the interactions between the analyte molecules and pen-
tacene grains. However, the role of the contact resistance was not
ruled out. Moreover, it was observed that channel length correspond-
ing to gas sensing crossover point was dependent on the pentacene
grain size.
To fully realize the advantages of organic materials for sensor
devices, all the components of organic circuits and in particular the
semiconductor layer should be constructed by solution or printing
methods at ambient temperature and pressure. Unfortunately, even
though pentacene has shown the highest charge mobility, its limited
solubility prevents the use of solution deposition techniques. For this
reason many research groups have set out to synthesize a processable