Page 334 - Organic Electronics in Sensors and Biotechnology

Organic Electronics in Memories and Sensing Applications 311

when crosslinked with a crosslinker and also meet requirements for fab-
rication of organic circuits. 64, 65 Polymer electrolytes are also used as high-
capacitance dielectric layers to boost OFET currents and to enable low
operating voltages. 66, 67 Very few polymers such as PVA support ambipo-
lar transport which resulted in complimentary-like inverters. 68, 69
Poly(vinylidene fluoride-trifluoroethylene) (P(Vdf-TrFE) is a ferroelec-
tric polymer used in the fabrication of ferroelectric field-effect transistors
(FeFETs) and memory elements. In its nonpolar form it facilitates the
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fabrication of hysteresis-free OFETs. Poly(m-xylylene adipamide)
(MXD6) is most likely a glassy dipolar polymer which has been used in
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nonvolatile memory elements, but not a ferroelectric polymer. Poly(α-
methylstyrene) (PαMS) has also been demonstrated to be useful as elec-
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tret in a nonvolatile memory element. However, criteria for memory
elements are extremely tight (in terms of access times, retention time,
and endurance to mention only a few), and so far none of the demon-
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strated polymer-based memories fulfill all these requirements. The
aforementioned applications suggest that polymeric dielectrics have to
be very robust. For example, it should be possible to print them as large-
area thin films without pinholes; they should adhere firmly to a variety
of conducting substrates, compatible with p- and n-type organic semi-
conductors; and they should display low-leakage currents and high
thermal stability. There is also debate on choosing high or low dielec-
tric constant materials. Insulator layers with a high dielectric constant
can negatively affect the mobility because they usually have randomly
oriented dipole moments near the interface which increase the ener-
getic disorder inside the semiconductors. 75, 76 On the other hand, high
dielectric constant materials are employed for reduced operating voltage
OFETs.

8.2 Single-Element Devices

8.2.1 Memory Elements
Nonvolatile flash memory, which uses silicon and its oxides, revolu-
tionized consumer electronics: it is used to store information in mobile
phones, pictures taken with digital cameras, data in memory sticks,
and even as hard-disk replacement in cheap laptop computers. Bista-
bility in such flash memories is achieved by introducing a second
“floating gate” to a silicon transistor between the normal control gate
(which regulates the flow of current through the transistor) and the
semiconducting substrate, in order to define the spatial position of
trapped charges (see Fig. 8.7a). However, the existing technology
based on Si has not been currently employed in organic circuits.
Hence, there are tremendous research efforts ongoing to develop an
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electrically accessible nonvolatile organic memory technology. At
present only two approaches to these challenges have been reported: