Page 61 - Organic Electronics in Sensors and Biotechnology

38 Chapter One

interacting with the analyte in another non-chemical way, such as an
electrostatic interaction. There are examples of analytes which do
chemically dope the semiconductor, such as electrophilic gases like
NO , which remove electrons from CuPc and dope the material with
2
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holes. The fact that the responses of many of the microscale organic
semiconductor devices exposed to many of the organic vapors are
reversible and reproducible reductions in current (not increases)
would likely exclude any type of chemical reaction, including dop-
ing, as the predominant current modulation mechanism for most of
these combinations. The result of no change in refractive index and
no swelling or thickness change of the organic semiconductor that
were observed with an ellipsometer upon exposure to the analyte 104
also suggested that the interaction between the organic semiconduc-
tor and the organic vapor is not a product of a chemical reaction.
In a unified picture, the chemical sensing effects at grain bound-
aries and metal-organic semiconductor contacts both arise from the
dipole nature of analyte molecules. Due to its polaron nature, the charge
transport in organic semiconductors is fairly sensitive to the local
polar environment. Changes in the local crystal structure nearby
charge carriers and thus changes to the polarizability of the lattice
could drastically affect the local distribution of energy states. This
problem was further exacerbated in the grain boundaries due to a
large amount of disorder. Most of the analyte organic molecules used
in this study have one thing in common: they all have dipoles (hexane
does not, but it did not produce any appreciable response). The pre-
dominant mechanism that leads to a decrease in the magnitude of
the current is increased trapping of carriers in the grain boundaries
due to a modulation of the local electronic environment caused by the
presence of the polar organic vapors (an increase in the polarizability
of the semiconductor in the grain boundaries). An increased number
of traps in the grain boundary would lead to an increase in the activa-
tion energy for hopping through the grain boundaries which was
demonstrated by Sharma et al. using top-contact pentacene transis-
116
tors exposed to ethanol. The measured activation energy changed
from 77 to 92 meV when the analyte concentration was changed from
pure nitrogen to 100 ppm of ethanol. 102
It has been experimentally shown that the analyte molecules of
stronger dipole moments trigger stronger responses from the same
122
OTFT chemical sensor. Also results reported by Torsi et al. demon-
strated the importance of the analyte’s alkyl chain length in terms of
its interaction with the organic transistor. The longer the carbon
110
chain length, the greater the interaction of that analyte molecule with
the semiconductor and the higher the mass uptake. The same group
also showed that increased mass uptake occurred when the side chain
of the polythiophene derivative was made to be polar (by putting an
ester in the side chain). This enhancement of the mass uptake was
110
even more pronounced than the increase produced by longer analyte