Page 25 - Organic Electronics in Sensors and Biotechnology

2 Chapter One

in most applications due to the low mobility values in organic materi-
als compared to those in single crystalline silicon. However, organic
devices are still of considerable interest to the industry because of the
low cost and mechanical flexibility of organic materials. To preserve
this advantage, inexpensive methods have to be developed to fabri-
cate organic devices. Organic field-effect transistors have been made
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with relatively high mobilities [of order 1 cm /(V⋅ s)] from small
organic molecules, such as pentacene, depo sited by vacuum-based
sublimation 9, 10 and soluble conjugated polymers, such as regio-regular
polythiophene, deposited by spin-coating, solution-casting, or ink-jet
printing processes. 11–13 Due to the relatively high cost in vacuum sys-
tems for subliming small molecules, conjugated polymers fabricated
onto devices by solution-based processes are preferred for industrial
applications. To obtain high performance of polymer electronic devices,
improvements in synthesis techniques are essential for high-quality
polymers with high purity, low residual doping, and air stability.
On the other hand, a solution-based precursor technique for small
molecules such as pentacene has also been developed to be used in
low-cost processes such as ink-jet or spin-coat, 14–16 which shed some
light on small molecules for their potential in industrial applications.
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We also note that 300 K mobilities of > 1 cm /(V⋅ s) have been reported
for a few single crystals and crystalline materials.
Figure 1.1 lists some of the small molecule and oligomeric organic
materials which have been commonly used in organic field-effect
transistors. Both α-4T and α-6T are oligomers of thiophene, tetracene
and pentacene are polyacenes, while CuPc (copper phthalocyanine)
has a coordinate structure. All the aforementioned are nominally p-
type semiconductors with holes as the primary charge carriers. In
these materials, the mobility of holes is generally much higher than
that of electrons, due to the presence of electron traps, particularly at
the interface with most dielectrics. We note that modifying the inter-
face with the gate insulator can reduce the electron trap concentra-
tion. n-type organic semiconducting materials have also been drawing
a lot of interest for applications such as OLED and Complementary
Metal Oxide Semiconductor (CMOS) circuits requiring both types of
charge carriers. C , copper hexadecafluorophthalocyanine (F CuPc),
60 16
and perylene derivatives are commonly used n-type organic semi-
conductor materials which have been used as channel semiconductor
materials in n-channel organic transistors. The semiconducting property
of these organic materials originates from the delocalized π-electron
(remaining p electron after sp hybridization) in conjugated bonds
2
z
(alternating single/double bond) between carbon atoms within one
molecule or one polymer chain. In semiconducting organic molecules
and polymers, there exist two type of coupling between neighboring
carbon atoms: single bond (σ bond only) or double bond (σ bond plus
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π bond). The σ bond is the coupling of sp hybridized orbitals from
neighboring carbon atoms, while the π bond is the coupling of the