Page 51 - Organic Electronics in Sensors and Biotechnology

28 Chapter One

channel than those on the top surfaces of the grains and thus exert
greater influence on the charge transport in channel. It was also found
that the degree of sensing response increased as the length of the
organic semiconductor’s hydrocarbon end group increased. This is
due to the elongated lamellar morphology and looser molecular
packing which allow greater access of analyte vapor and increased
surface area, as well as change the electronic or spatial barriers
between grains. Therefore the alkyl chains facilitate the adsorption of
the analyte molecules by the sensing film. This adsorption mecha-
nism could be a combination of hydrophobic interactions, intercala-
tion to fill defect vacancies, and simple surface binding. All these pro-
cesses are favored at grain boundaries.
The interaction between the alcohol and the organic semiconduc-
tor film in OTFT (dHα6T) does not involve the bulk of the crystalline
grains of the film since no change in refractive index and no swelling
or thickness change of the film were observed with a single wave-
length ellipsometer during exposure of the film to the analyte vapor. 104
It was suggested that the sensing interaction is a surface-type interac-
tion involving grain boundaries. This result also helped to rule out a
chemical reaction in the organic semiconductor upon exposure to the
105
analyte. This is believed to be similar for CuPc and pentacene. It is
now beneficial to determine where physically on the device the cur-
rent modulation occurs. The study performed by Torsi et al. also
showed that there is very little penetration of the organic analytes
into the highly ordered crystalline grains evidenced by the fact that
104
no appreciable swelling was measured, most likely due to the tight
crystalline packing of the molecules in the grains (in the case of
105
conducting polymers, swelling is a known result of exposing them to
organic vapors ). This would indicate that there is very likely little
104
interaction, chemical or otherwise, between the molecules in the
ordered grains and the organic vapors. In the same study, dHα6T
was deposited on a quartz crystal microbalance, and a change in the
mass of the semiconductor was measured when exposed to varying
104
concentrations of 1-pentanol. This demonstrated that although the
1-pentanol did not penetrate into the film, it nonetheless added to its
mass. This means that the analyte adsorbed to the surface of the
104
organic layer. This leads to a conclusion that a decrease or increase
in source-to-drain current must then be attributed to something
that occurs outside of the crystalline grain. Another study by Someya
et al. exploring the interaction between the semiconductor α,ω-
dihexylquarterthiophene and 1-pentanol demonstrated that the mag-
nitude of the sensing response was directly related to the number of
grain boundaries in the channel when the interaction resulted in a
106
decrease in I . Given that almost all the charge within the channel of
ds
an organic semiconductor lies close to the semiconductor/dielectric
interface, it must be the case that the analytes percolate down to this
dielectric interface through the grain boundaries.