Page 94 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 71

Indeed, solvent-induced aggregation studies point out that the forma-
tion of ordered structures should be an intrinsic property of D3ANT.
Generally for amorphous thin films, field-effect mobilities in the
−6
138
2
−4
range of 10 to 10 cm /(V · s) have been observed. In the case of
D3ANT the quite high mobilities of ~10 cm /(V · s) obtained for spin
2
−2
coating suggest that a totally amorphous organization of the thin
films is unlikely. This observation is further supported by the STM
studies as well as by the solvent-induced aggregation studies. Finally,
we believe that during the spin coating process few ordered mono-
layers are formed on the dielectric surface wherein charge transport
takes place while the subsequent layer nucleates in a disordered man-
ner, affording the observed GIXRD spectra.
2.3.4 Gas Sensing Measurements
Gas sensing measurements were performed by using bottom-contact
OTFT with channel width and length of 11,230 and 30 μm, respec-
tively. D3ANT was deposited as thin film by spin coating as previ-
ously discussed. The devices, after an annealing treatment, were
tested in a clean-room environment exhibiting good field-effect prop-
2
−3
erties with a mobility of 1.2 × 10 cm /(V · s) and an I /I ratio of
on off
4 × 10 similar to the performance observed in a nitrogen atmosphere.
4
Nitrogen oxides, carbon monoxide, and hydrogen sulfide were
supplied by certificated cylinders (5 ppm NO, CO; 2 ppm H S and
2
NO ) connected to a gas delivery station by means of four different
2
fed systems to prevent cross contamination. Nitrogen was used as
carrier gas and to dilute the test gases. The gas mixing was carried
out with computerized flowmeters. All the sensor testing was per-
formed at room temperature and at a pressure of 760 Torr by using a
constant flow rate of 200 sccm. The ambient humidity condition was
also kept constant at about 40% RH. The thin films were exposed to
each gas at different concentration levels through a nozzle located
about 3 mm above the device.
The I transient curves, at fixed source-drain and gate voltage
ds
(V =−40 V and V =−40 V) were measured while exposing the
g ds
D3ANT OTFT to each gas for 60 s. Afterward the device was exposed
for 120 s of pure N . The results are reported in Fig. 2.7a. It is apparent
2
that the I current systematically increases at each NO exposure cycle
ds 2
(dotted line), and the responses in the working range (0.2 to 2 ppm) are
also linearly dependent on the analyte concentration. A similar behav-
ior can be seen (solid line in Fig. 2.7a) for the interaction with NO
molecules. The interaction of strong electron-accepter molecules,
such as NO , with a p-type organic semiconductor, such as D3ANT,
2
leads to a change of the transport properties, ascribable to a doping
process, as already reported in previous studies. Indeed, similar cur-
rent increases in metal and metal-free phthalocyanines, 139–141 porphy-
142
143
rine, and tailored phenylene-thienylene copolymer thin film