Page 264 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 241

In solar energy applications, there is an optimal band gap that, for
a given illumination source, will yield the highest achievable power
conversion efficiency: if the band gap is too high, few of the incident
photons are absorbed; and if it is too low, much of the incident energy
is wasted through internal conversion. The optimal band gap is vari-
ously estimated to lie between 1.0 and 1.5 eV, depending on the spe-
cific assumptions made (e.g., concerning the absorption spectrum,
exciton binding energy, and light source). APFO-Green1 falls roughly
in the middle of this range with a band gap of about 1.2 eV. In the case
of photodiodes, no such optimum exists: the smaller the band gap
ΔE, the lower the photocurrent onset energy and the wider the spec-
tral range (assuming, of course, that a functioning device can be
made). At the time of writing, we are aware of no published work
that has focused on ultralow band gap materials (ΔE << 1 eV). While
such materials would be of little interest for solar energy applications
(since they would yield extremely low power conversion efficiencies),
they would be of great interest for photodiodes: for instance, a single
device that could continuously harvest photons from < 300 nm up to
3 μm would offer something genuinely new that conventional photo-
diodes are unable to deliver. The inorganic photodiodes with the
widest spectral range are based on InGaAs which, when suitably
†
optimized, are sensitive from 250 to 1700 nm. These are expensive
high-end photodetectors used for ultrafast applications; a low-cost
organic alternative that offered comparable or wider spectral range
would find many applications. A new generation of “black” organic
semiconductors is now under development, which can absorb con-
tinuously from 300 to > 2500 nm. Also of interest are carbon nano-
tubes, which can absorb continuously up to 2000 nm. These materials
60
have not yet been implemented in OPV devices—due to some
processing issues—but could potentially offer much wider spectral
ranges than could be achieved with other technologies.

6.6.4 Gain
As discussed in Sec. 6.1, PMTs and APDs are the most sensitive detectors
available due to their internal gain processes. In the case of PMTs, the
noise introduced by the gain process is virtually zero, and in well-
optimized APDs it is little more than a factor of two. In both cases the
resultant noise is far lower than would be incurred using an external
amplifier of equivalent gain, and so it is possible to detect extremely low
light levels right down to the single photon level. In OPV devices, under
normal operating conditions, each incident photon can generate at most

† Tandem photodetectors (comprising a UV/Vis sensitive photodiode on top of
an IR-sensitive photodiode) have also been reported, but do not appear to be in
widespread use perhaps due to manufacturing complexity or cost.