Biomedical Engineering Reference
In-Depth Information
9.2.2.4 OrganicElectronics
Recently, there has been considerable interest in the i eld of organic elec-
tronics because of their immense applications in several devices such as
organic light-emitting diodes, photovoltaic solar cells, thin i lm transistors
and i eld ef ect transistors. Among them, LCs have drawn special attention
because of their long range orientational order and adaptive structure that
generate defects into dif erent kinds [2, 63-65]. Particularly, chromonic
mesophases formed by noncovalent π-stacking are better candidates for
these sorts of applications, as chromonic mesophases with dense packing
is preserved during transfer of aggregates from the LC solution into dried
thin i lms [50, 66, 67]. h ese chromonic LC i lms usually orient in either
an homogeneous or homeotropic manner. Whereas, planar/tilted orienta-
tion is a necessary requirement for i eld ef ect transistors to ensure charge
migration between the source and the drain, the homeotropic one is essen-
tial for a better performance in photovoltaic and LED devices.
Tolkki and his coworkers [66] studied the self-assembling properties of
a perylene-3,4,9,10-tetracarboxylic acid diimide (PDI12) that produced
well-organized aggregates due to strong π-π interactions in both solution
and in dry i lms. h e aggregates were coni rmed through the existence
of long l uorescence lifetime in both solutions and in i lms. h is particu-
lar compound exhibited more pronounced self-ordering properties than
those of other PDI compounds and the molecules stack into columns. It
was demonstrated that spin-coating aligns the column axis parallel to the
substrate, whereas in i lm formed by Langmuir-Schaef er technique the
alignment became perpendicular to the substrate. Double-layer prepara-
tion between PDI12 columns and C
60
fullerenes shows promising pho-
toelectric properties as chromonic dye acts as an electron acceptor and
C
60
acts as an electron donor (Figure 9.9). Fluorescence quenching and
photoinduced voltage were present as evidence of interaction between the
chromonic dye and C
60
. Nazarenko
et al.
[50] reported chromonic semi-
conductors for water-solution processable organic electronics, and their
electric properties were strongly inl uenced by their aggregate structure
and the orientation of aggregates onto the substrate. h ey determined dif-
ferent charge carrier mobilities parallel and perpendicular between the two
gold electrodes spaced 15 μm apart.
In conclusion, there has been an increasing interest in lyotropic chro-
monic LCs, not only because chromonic molecules represent a unique set
of properties dif erent from conventional ones, but because they also have
huge potential for commercial exploitation. Chromonic dye polarizing
i lms have shown promise to act as both polarizers and orienting layers
