Page 332 - Organic Electronics in Sensors and Biotechnology

Organic Electronics in Memories and Sensing Applications 309

thin films with a high molecular weight of 8,000,000 Da. These self-
assembled DNA-surfactant complex materials, with good processa-
bility, may have applications in molecular optoelectronics. It has been
proposed that such a self-organized structure arises because the alkyl
chains are oriented perpendicular to the film plane, and chiral DNA
helices were oriented in the direction parallel to the film plane. 46

8.1.3 Electroactive Polymers
The role played by the polymeric gate dielectric is as important as the
role of the organic semiconductor in organic electronic devices. In
OFET sensors, the gate dielectric plays an even more important role
than the organic semiconductor.

1. Gate dielectrics permit the creation of the gate field in order
to establish a two-dimensional channel charge sheet.
2. The accumulated charge carriers transit from the source to the
drain electrode in an area close to the dielectric/semiconductor
interface. Hence, the chemical nature of the semiconductor/
dielectric interface greatly affects how the accumulated charges
move in the semiconductor.
3. Gate dielectrics are responsible for device stability.
4. They determine the operating voltage of devices according to
the dielectric constant and thickness of the gate dielectric.
5. Gate dielectrics may display quasi-permanent charge storage
or polarization used in nonvolatile memory elements.
6. They are essential in flexible displays and sensors since poly-
mers are usually easily bendable.
7. When transparent, gate dielectrics enable the fabrication of
photosensitive transistors. Hence the development of poly-
meric gate dielectric materials is of fundamental importance to
the progress of organic electronic devices.
Solution-processable polymeric dielectric materials are
attractive, partly because films with excellent characteristics
can often be formed by spin coating, casting, or printing at low
process temperatures under ambient conditions. Moreover,
the capability of easy film formation has practical advantages
when coupled with low-cost patterning techniques for poly-
meric dielectrics as well as other materials needed in the fabri-
cation of OFETs. From this point of view, polymeric dielectrics
offer large potential as compared to their inorganic counter-
parts. Due to the limitations of space, no attempt is made to
review the device physics of gate dielectrics in depth, nor will
we give an overview of single crystal OFETs. For these impor-
tant subjects we refer to other excellent recent reports and
review articles. 54–56