Page 324 - Organic Electronics in Sensors and Biotechnology

Organic Electronics in Memories and Sensing Applications 301

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material challenges and applications, n-type organic semiconduc-
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tors and oligomers, and interface effects. Depending on the require-
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ments of device functionality, we put organic semiconductors into
three categories: p-type semiconductors, n-type semiconductors, and
ambipolar semiconductors.
p-Type Semiconductors
If holes can be easily injected into the valence band (HOMO level) of
an organic semiconductor, i.e., can sustain stable radical cations, and
these cations (positive polarons) can move throughout the solid phase,
than we will call this material p type. To achieve high-performance,
solution-processable organic semiconductors with high charge car-
rier mobilities, ordered structures are needed at the tertiary nano-
structure of the organic thin films. Designing the material to exhibit
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microcrystallinity or liquid crystallinity or self-organization or mak-
ing use of specific interactions with a templating substrate is sug-
gested for this route. The other approach aims to produce a com-
pletely amorphous micro/nanostructure to provide a uniform path
for charge transport, with a minimum degree of site energy fluctua-
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tions. Among the traditional molecular semiconductors, pentacene,
thiophene oligomers, and metallophthalocyanines are well known as
p type (see Fig. 8.1). Among polymers, polythiophenes, polyfluorenes,
polyarylamines, and poly(benzo-bis-imidazobenzophenanthroline)
are promising. Currently semiconducting polymers with consider-
able air stability and high charge carrier mobility are the subject of
intense research interests. Among the polymeric semiconductors,
significant efforts continue to be focused on derivatives of poly(3-
hexylthiophene) (P3HT). One of the reasons for using P3HT thin
films is the presence of microcrystalline and lamellar π stacking,
which results in large charge carrier mobilities. The ionizaion poten-
tial (typically around 4.9 to 5.0 eV) is best suited to form ohmic
contacts with many air-stable electrodes such as Au or conducting
polymers such as polyethylenedioxy-thiopehene doped with poly-
styrene sulfonic acid (PEDOT/PSS). However, P3HT tends to exhibit
large positive threshold voltage shifts V upon exposure to air, pre-
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sumably due to slight doping of the polymer with oxygen. 10, 11 This can
be improved by increasing the ionization potential of the polythiophene
backbone either by adopting a fully planar conformation through the
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side chain substitution pattern or by incorporating partially conju-
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gated co-monomers into the main chain. Field effect mobilities
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exceeding 0.15 cm /(V . s) have been reported from such materials
in air. Similarly, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]
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thiophene (PBTTT) exhibits mobilities of 0.7 to 1 cm /(V . s). 14
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An alternative route to solution-processable polymeric materials is
to use small-molecular semiconductors either processed from solution
or evaporated (sublimated) from a heated source onto a target sub-
strate. Such an example is pentacene, an aromatic compound with five