Optical Biochips - Biophotonics - page 229

the performance of conventional light sources. The highest luminous e cacy
of a white oLED reported in mid 2006 by the Japanese company Konica
Minolta is 64 lm W − 1 , which compares very favorably with the luminous e -
cacy of conventional light bulbs (15 lm W − 1 ), halogen lamps (25 lm W − 1 ), or
compact fluorescent lamps (60 lm W − 1 ).
Because of the simplicity and the e ciency with which oLEDs - also of
different color - can be produced on large surfaces, they are increasingly used
for the fabrication of displays in cameras, for PCs, and for TV screens.
12.8.3 Organic Lasers
The first optically pumped organic (polymer) laser has been demonstrated
already in 1996 [23]. However, because of the high electrical resistivity of
organic semiconductors, the electrically pumped organic CW laser working at
room temperature remains still elusive.
The fact that no intense, narrow-band or even monochromatic light sources
can currently be fabricated with organic semiconductors is one of the major
shortcomings for the realization of monolithic optical biochips and labs-on-a-
chip using solely organic materials.
12.8.4 Organic Photodetectors and Image Sensors
Once the problem of dissociation of photogenerated, tightly bound charge
pairs (Frenkel excitons) had been solved using finely dispersed polymer blend
heterojunctions, the quantum e ciency of photodetectors fabricated with or-
ganic semiconductors approached that of inorganic semiconductors [24]. Total
external quantum e ciencies exceeding 50% are routinely obtained today in
photosensors realized with organic semiconductors. By using thin multilayer
structures, it is also possible to produce high-speed photodetectors despite the
low mobility of charge carriers in organic semiconductors: Maximum detection
frequencies of about 500 MHz have already been demonstrated.
A particular advantage of photodetectors fabricated with organic semicon-
ductors is the high number of sensors (pixels) that can be realized simultane-
ously on a very large substrate. As an example, the cost-effective fabrication
of page-sized image sensors has already been demonstrated.
Since different organic semiconductors with various spectral responses can
be easily integrated on the same substrate, the fabrication of color photo-
sensors with good colorimetric performance has been achieved already early
in [24].
12.8.5 Organic Photovoltaic Cells
One of the major problems of a practical, disposable optical biochip is electri-
cal power supply. If it were possible to replace the conventional electrochemical