Page 316 - Organic Electronics in Sensors and Biotechnology

Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 293

photocurrent of a P3HT:PCBM [poly(3-hexylthiophene-2,5-diyl blended
with the fullerene derivative [6,6]-phenyl C -butyric acid methyl ester]
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photodiode after excitation with a nanosecond laser pulse. With a
reverse bias voltage of −5 V, a quantum efficiency comparable to the
best Si photodiodes and a response time of about 11 ns are observed.
Organic photodetectors not only exhibit functionalities and spec-
ifications comparable to their inorganic counterparts, but also offer a
range of other features which hardly can be realized with conven-
tional devices. One example is the possibility to fabricate devices
with customized spectral sensitivity. This can be realized by either
choosing special organic materials with a certain spectral sensitivity
or with the help of microcavity photodetectors. 105
Organic photodiodes could also be integrated into organic laser-
based sensor systems. The spectral response of such devices can ideally
be tuned to the emission spectra of the many organic lasers and fluores-
cence markers. The temporal response is sufficient to detect even short
laser pulses at high repetition rates. Furthermore, organic detectors can
be fabricated on a wide range of substrates, 106, 107 thus offering the possi-
bility to integrate them on the same substrate with the laser source.
The structuring options for organic photodetectors are another
important issue. Depending on the sensor system layout, the detec-
tors can be structured to have either a very large sensitive area
(> 1 cm²) or a specific and small active area. The structuring can be
realized by simple photolithographic methods which define the
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active area of the photodetector. In such a way the photodetectors
could also be combined with waveguide structures.

7.5 Conclusions
In this chapter we discussed the opportunities given by organic lasers
for biosensing applications. Due to their spectral tunability and the
ease of processing such devices bear a huge potential for integrated
analysis systems. We showed the possibilities for integrating such
devices with optical waveguides for harvesting the laser radiation in
microfluidic structures based on cost-effective replication techniques.
In addition, we pointed out that the performance of organic photodi-
odes is already comparable to that of their inorganic counterparts. In
combination with the techniques for integration, such devices might
pave the way for future fully organic lab-on-a-chip structures.

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