Page 67 - Organic Electronics in Sensors and Biotechnology

44 Chapter One

Grain boundary
Grain body

Ti/Au + + Ti/Au
+ +

SiO 2 Injection through Schottky Charge carriers trapped
barrier or interface dipole in grain boundaries

Si

FIGURE 1.25 A graphic summary of the origins of the sensing response. The
response arises because of trapping in the grain boundaries and/or the
mediation of carrier injection at the contacts.

different parts of a device and in combination modulate the overall
conductance of the device which gives rise to chemical sensing effect.
Figure 1.25 outlined the sites in an OTFT device playing important roles
in charge transport and thus chemical sensing, which become domi-
nant at different scales. In a large-scale device (channel length much
larger than the grain size of active semiconductor layer), the charge
transport is dominated by the localized charges in tail states at grain
boundaries due to high disorder there, and therefore the sensing
mechanism is analyte dipole-induced charge trapping at grain bound-
aries, which generally leads to a decrease in device current upon
exposure to a polar analyte. In some cases, the analyte dipoles result
in a transient increase in channel conductivity, presumable because of
more charges being induced in the channel. For smaller channel
lengths, the number of grain boundaries within a channel decreases
so that the weight of influence of grain boundaries on electrical trans-
port and chemical sensing gives its place to organic semiconductor-
metal interfaces. At smaller channel dimensions, especially when the
channel length is comparable to or smaller than the grain size of poly-
crystalline organic molecules or conjugated polymers, the charge
transport is dominated by the injection through the Schottky barrier
and interface dipole at the metal-organic semiconductor contact as
well as influenced by the local coverage of organic semiconductor
material on the nanoscale channel, both of which overwhelm the
intrinsic transport within the body of grains. Correspondingly dur-
ing chemical sensing events in nanoscale OTFTs, the analyte mole-
cules diffuse near the source/drain contacts through the porous
semiconductor layer and modulate the charge carrier injection at the
source and drain contacts. The smaller the channel length, the stronger