Page 90 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 67

at 607 nm is developed along with decrease of the absorption band at
504 nm. The wavelength of this new absorption band is comparable to
that of thin-film D3ANT (603 nm), indicating formation of molecular
aggregation. Fluorescence of D3ANT in the solvent/nonsolvent mix-
ture reveals a similar trend (Fig. 2.4b). As the methanol concentration
increases, the fluorescence intensity around 568 nm tended to decrease.
Simultaneously, the relative band intensity around 635 nm increased.
Moreover, the fluorescence intensity of both bands dramatically
decreases with increasing the methanol concentration.
The appearance of the red-shifted fluorescence band and the
strong decrease of the fluorescence intensity can be interpreted by the
intermolecular aggregation as in the film state. The formation of
strong intermolecular interaction in the solid state is one of the essen-
tial requisites for efficient charge transport. The observation of aggre-
gate formation for the synthesized oligomers is a good clue for the
application of the oligomers as channel materials in OTFTs.
The electrochemical behavior of D3ANT was studied in CH Cl 2
2
+
using Ag/Ag as reference electrode and employing ferrocene as inter-
nal standard for the potential calibration. The cyclic voltammogram
showed two quasi-reversible oxidation peaks (1.05 and 1.20 V) and an
irreversible oxidation peak at 1.49 V. This behavior is likely due to the
sequential abstraction of three electrons from the three anthracene moi-
eties. Two irreversible reduction peaks at −1.26 and −1.53 V were also
observed. Moreover, taking −4.8 eV as the HOMO level of the ferrocene
125
system, the HOMO and LUMO energy levels were estimated from
the onset of the first oxidation and reduction as −5.23 and −3.15 eV,
respectively, resulting in an energy gap of 2.1 eV. The obtained electro-
chemical HOMO/LUMO gap is in reasonable agreement with the
optical gap in CHCl (2.2 eV). A HOMO level of 5.23 eV aligns well
3
with the work function of gold (4.8 to 5.1 eV) and in principle should
lead to a good injection of charge carriers from the Au electrodes.

2.3.3 Device Performance
OTFT devices were fabricated by using both bottom- and top-contact
geometry. In the bottom-contact configuration, drain and source elec-
trodes (Ti 10 nm/Au 50 nm) at various channel widths and lengths
were fabricated on the Si/SiO surface by photolithography. The sili-
2
con substrate serves as gate electrode. The substrates were cleaned as
follows: the wafers were first immersed in a piranha solution for 8 min
to remove traces of adsorbed organic materials, rinsed with deionized
water (DI), and eventually dried with a nitrogen flow. They were then
washed according to the standard cleaning (SC) procedures. 126, 127 Thin
films of D3ANT were deposited onto the SiO surface by spin coating
2
from 0.05 % (w/w) solutions in anhydrous CHCl at 2000 rpm for 30 s.
3
Some of the films were subjected to an annealing procedure consisting
of a treatment at 100°C for 30 min under a nitrogen atmosphere.