Page 238 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 215

0.0006 0.6

0.0004 0.4
I SC (A) V OC (V)

0.0002 0.2

0 0

0 0.2 0.4 0.6 0.8 1 1.2 1.4
Intensity (mW)
FIGURE 6.11 The short-circuit photocurrent and open-circuit voltage as
a function of light intensity for the ITO/PEDOT:PSS/P3HT:PCBM/Al bulk
heterojunction device shown in Fig. 6.10.

increased by an order of magnitude, giving rise to an approximate
10-fold increase in I , then V need change by only a very small
ph OC
amount to generate a compensating increase in I . The sublinear
Vphoto
intensity dependence of V is far less useful than the linear response
OC
of I . Moreover, V inherits from I a strong temperature depen-
SC OC Vphoto
dence that makes it far more susceptible to drifts in temperature than
I . Consequently in light-sensing applications, I is virtually always
SC SC
the preferred sense parameter.
In the interests of simplicity, the discussion above has focused on
simple single-layer bulk heterojunction devices. The situation is
slightly different in the case of discrete heterojunction devices since
asymmetry in the generation profiles of the electrons and holes gen-
erates sizable concentration gradients at the heterojunction that tend
to drive the charges to their respective parent electrodes even in the
absence of an electric field. Nonetheless similar arguments to those
47
provided above apply, and the resultant photocurrent-voltage curves
are qualitatively similar in appearance. Importantly, our conclusion
that I is the preferred sense parameter due to its linear response and
SC
superior temperature stability also applies to discrete heterojunction
devices. This same message also extends to more complex multilayer
architectures.

6.3.4 The Equivalent Circuit
A photodiode—organic or otherwise—may be represented conceptu-
ally by an equivalent circuit comprising an (infinite impedance) cur-
rent source I in parallel with a diode D, a shunt resistor R and a
S sh