Biomedical Engineering Reference
In-Depth Information
number of additional uses at home, in industry, in safety/security, and for
environmental applications. Current approaches employ many different mate-
rials to perform the various required functions: Optical, mechanical, fluidic,
optoelectronic (light generation and detection), signal processing, information
display, etc. The resulting solutions are hybrid systems, consisting of many
different components, whose integration into a final product is not very cost-
effective.
There is a promising class of materials, however, that might make it pos-
sible to realize a monolithic lab-on-a-chip for low-cost optical biosensors: or-
ganic semiconductors, in particular, polymer semiconductors. In the following,
a few salient points of this material class and its potential for the fabrication
of highly functional yet cost-effective optical biosensors are summarized.
12.8.1 Basics of Organic Semiconductors
The scientific field of organic semiconductors is barely 30 years old, and it
started with the development of organic light-emitting diodes (LED) with
useful conversion e
ciency by the American company Kodak. Since then,
this field has progressed very rapidly, due to the promise of a new class of
materials offering all the optoelectronic functionality required for the realiza-
tion of generic photonic microsystems: light generation, light detection, analog
and digital electronic signal processing, as well as photovoltaic power gener-
ation [21]. In addition, e
cient and fast production methods are available,
which are often modified printing techniques such as inkjet, silk-screen, or
gravure printing. Therefore, it is anticipated that the fabrication costs will
be about 100 times lower than that for silicon-based semiconductor circuits
in the near future (
0.1 cm
−
2
10 cm
−
2
compared to
for standard CMOS
¤
¤
chips).
However, the performance of organic semiconductors can be significantly
lower in several respects than the performance of their inorganic counterparts.
The charge carrier mobility is about 1,000 times lower (about 1 cm
2
V
−
1
s
−
1
compared to 10
3
cm
2
V
−
1
s
−
1
), the diffusion length of good material is several
orders of magnitude smaller (typically 10 nm compared to several 100
m), the
electrical resistivity is significantly higher (since it is much more di
cult to
dope organic semiconductors), and organic semiconductors deteriorate much
more rapidly if exposed to ultraviolet light, to oxygen or to water vapor.
For these reasons, organic semiconductors are not a cheaper and simpler
replacement of inorganic semiconductors, and it is rather necessary to evaluate
the performance of each device individually for the intended application.
µ
12.8.2 Organic LEDs
Since a large variety of organic semiconductors are available, organic LEDs
(oLED) of many different colors can be fabricated [22]. Their external quan-
tum e
ciency can exceed 10%, and their power e
ciency already surpasses
