Page 224 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 201

for very large area applications, where they offer photoactive surfaces
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of up to 5000 cm . On the negative side, PMTs require high operating
voltages of several thousand volts, have relatively narrow spectral
ranges, and are bulky, fragile, and expensive due to the need for vac-
uum housing. APDs are compact solid-state alternatives to PMTs,
although they exhibit markedly lower gains and bandwidths. They
are often used when light levels are too high for photomultiplier
tubes but too low for photodiodes. They too are very expensive. In
contrast to PMTs and APDs, photodiodes have no internal gain and,
for low light level measurements, usually require an external ampli-
fier to increase the signal to a manageable level. As we shall see later,
external amplification is a major cause of measurement noise, and
photodiodes are therefore best suited to detection in fairly high light
level conditions where amplifier noise is negligible. On the positive
side, photodiodes are more robust than APDs and PMTs, have very
low operating voltages and wide dynamic range, and are fairly cheap
due to their simple fabrication. Phototransistors offer an integrated
alternative to photodiode/amplifier combinations and are a good
choice for fairly high light level applications where simplicity of
design and low cost are important considerations, and high linearity
is not required. Photoresistors are ideal for ultralow cost applications
that do not require high levels of sensitivity, and for this reason tend
to be the solution of choice for the toy industry where economy of
fabrication is paramount. CCDs are the dominant technology for one-
and two-dimensional sensor arrays, although photodiode arrays are
gaining rapidly in importance. At present, there is little to distinguish
CCD and diode arrays in terms of performance and cost. CMOS
detectors have a slight edge in terms of responsivity and speed,
whereas CCDs have a marginal advantage in terms of dynamic range
and pixel uniformity.
The above discussion has highlighted the main types of photode-
tectors in common use today, and it is natural to ask at this point
whether there is really a need for a new detection technology at all.
OPDs are photodiodes with no internal gain and so––in their current
form at least––are unable to compete with PMTs and APDs in terms
of sensitivity. They are also high-capacitance devices based on low-
mobility materials and consequently exhibit slow response times
compared to PMTs, APDs, and conventional inorganic photodiodes.
On the other hand, their favorable processing characteristics may
open up new applications that are currently unviable, e.g., in large-
area light detection. The largest single-element photodetectors are
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20 in PMTs, and the largest imaging arrays are ~1600 cm amorphous
silicon panels. Using printing techniques, it should be possible to cre-
ate organic photovoltaic (OPV) devices of much larger area at greatly
reduced cost. In addition, by fabricating OPV devices on flexible sub-
strates, conformal photodetectors can be created that adapt to the
shape of the surface on which they are mounted. A critical area where