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258 Cha pte r S i x

other potential applications of organic devices in chemical and bio-
logical analysis, many of which are described in other chapters of this
book.

6.8 Conclusions
This chapter has focused on the emerging field of organic photodi-
odes. OPDs have received far less attention than the other major
organic devices, but it is fast becoming clear that they hold very
great promise as photodetectors, combining excellent optoelectronic
performance with simple scalable processing and thin, lightweight,
flexible form factors. There are already a few obvious application
areas where these benefits have been recognized, and OPV devices
are being used to significant advantage (albeit at a precommercial
level at present), and many more are bound to emerge in the coming
years. In writing this chapter it is our hope that more researchers
will be encouraged to turn their attention to OPD development.
The vast majority of organic photodetector research to date has
been carried out as a curiosity-driven offshoot of standard solar
cell research, rather than a discipline in its own right. This is unfor-
tunate as the technical challenges involved in designing high-
performance OPDs are quite distinct from those involved in
designing efficient solar cells. New materials systems, device
architectures, and fabrication techniques are urgently needed if
OPD technology is to progress sufficiently to challenge the incum-
bent competition. These advances, however, are only likely to
come about as the result of a dedicated research effort focused
squarely on OPD optimization. The organic photodetector field offers
some fascinating scientific challenges and tremendous technological
opportunities, and we wholeheartedly encourage the interested
researcher to get involved.

Appendix: Noise Analysis

Determining the Thermal Noise of a Resistor
In an RC network, the resistor is the only circuit element that contrib-
51
utes thermal noise. Hence, to determine the noise characteristics of
a resistor, we can consider a simple RC circuit, knowing that all noise
in the circuit is directly attributable to the resistor. We model the resis-
tor as a (mythical) noise-free resistor R in series with a noise source of
unknown amplitude σ per square root of frequency. The situation
V
is depicted in Fig. 6.36 where it is apparent that the resistor and
capacitor act as a potential divider for the noise source.
In the case of a white noise source, a noise source of frequency f
and bandwidth Δf will generate an amount σ f
V Δ of noise. Treating