Page 97 - Organic Electronics in Sensors and Biotechnology

74 Cha pte r T w o

the threshold voltage was observed. Besides, the reversible charge
transfer between analyte and sensitive active layer creates a charge
dipole that could influence the transport too. The responses of D3ANT
OTFT exposed to various NO concentrations in the 0.25 to 2 ppm are
2
shown in Fig. 2.8b. Three replicates for each concentration were gath-
ered and used to perform the linear regression analysis. Response
repeatability (expressed as relative standard deviation, RSD) was in
the 2 to 12% range. Similar results have been already reported for
dihexyl sexithiophene (DH-α6T) OTFTs used as alcohol sensors. 50, 51
Linear regression of the data (averaged over three replicates) has a
linearity coefficient R of 0.999. The slope of the calibration curve rep-
resenting the sensor sensibility was 3.21 (μA/ppm), and a detection
limit c of 100 ppb (referred to as a signal-to-noise ratio = 3, noise taken
d
as the standard error of the fit) was estimated from the equation

rS
c = dy x (2.3)
/
d
b
where c = detection limit
d
r = signal/noise ratio (3 for LOD)
d
S = standard deviation of fit
y/x
b = slope of regression curve
When an organic semiconductor is used as a sensitive membrane for
inorganic gas monitoring, the detection limits are generally below
1 ppm. However, cross sensitivities are not always fully investigated.
This was not the case in the study carried out for a poly(phenylene-
thienylene) bearing alkoxy groups (POPT) as side chains. POPT
143
chemiresistor sensor showed large responses upon exposure to NO ,
2
and concentrations as low as 50 ppb could be detected, and no
responses were seen for potential interfering gases such as carbon
monoxide, sulfur dioxide, and ammonia. However, a quite high work-
ing temperature (60 to 100°C) was required.
The amorphous bulk structure of the D3ANT active layer could
be also responsible for the good sensitivity and the fast recovery. It
has been already suggested that NO sensing with carbon nanotubes
2
and phthalocyanines proceeds via absorption and interaction occur-
ring preferentially at defect sites. 147, 148 Recently, a nitrogen dioxide
sensor based on amorphous poly(triarylamine) (PTAA) sensitive lay-
ers has been proposed as a room temperature OTFT sensor. The
149
lowest NO concentration detectable with PTAA FET devices inte-
2
grated in a pulsing oscillator circuit was 10 ppb. Unfortunately, no
data are reported on the linearity range.
As already demonstrated for DH-α6T based OTFTs and for differ-
ently substituted thiophene oligomers exposed to organic and inor-
ganic species, for the D3ANT OTFT a sensibility that increased with
the gate bias can also be seen. In Fig. 2.9, the slopes of the calibration