Page 266 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 243

ITO/PEDOT:PSS/C60/bathocuproine/Al device where the multipli-
cation factor M = ΔI/eℜ is plotted against wavelength for a variety of
61
applied biases. At first glance, these data seem to suggest that at a
reverse bias of -4 V, as many as 40,000 electrons are generated in the
external circuit for each absorbed photon. Needless to say, this is not
the case, and in reality the effect is photoconductive rather than pho-
tovoltaic. The behavior is encountered when there is strong trapping
of one or both of the charge carriers. We consider here the situation in
which both charge carriers are trapped. As previously described, the
current I under illumination can be divided into two parts: a pho-
photo
tovoltaic part I due to the continuous conversion of absorbed pho-
ph
tons into electron-hole pairs and a photoconductive part I due to
Vphoto
the applied bias.
I + I + I (6.69)
photo ph V photo

In the dark state under a reverse bias V , the current is entirely
rev
photoconductive and is very low due to repeated trapping and (slow)
detrapping of the electrons and holes. Under illumination, some of
(or all) the trap sites are filled by the photogenerated charges, leading
to an increase in the mobilities of the remaining untrapped charges.
This in turn causes a reduction in the resistance R of the device and
changes the photoconductive current by an amount

V V
ΔI = rev − rev (6.70)
V photo
(
(
RV , )0 RV , ℜ)
rev rev
where the resistance R depends on both the bias V and the rate ℜ
rev
at which photons strike the device: if ΔI >ℜ/e, the illusion of
V photo
gain will be given, although in reality the effect is entirely photocon-
ductive in origin.
Although photoconductive OPV devices can undergo large
changes in current in response to very small light intensities, they
have two main drawbacks: First, they tend to exhibit poor linearity
since the effective charge mobilities need not increase linearly with
the light intensity. Second, since by definition they have massively
reduced impedance under illumination, they suffer from considera-
ble thermal noise. They do not therefore exhibit the virtually noise-
free cascade amplification processes found in PMTs and APDs and
hence will never offer comparable levels of sensitivity. It may yet be
possible to develop organic devices that exhibit genuine internal gain,
but this is likely to require the development of new OPV materials
that exhibit high carrier mobilities and are able to tolerate the high
fields required for impact ionization.