Page 235 - Organic Electronics in Sensors and Biotechnology

212 Cha pte r S i x

As the load resistor is progressively increased in value from zero
to infinity, the photovoltage increases from its short-circuit value of
zero to a maximum value known as the open-circuit voltage V . The
OC
internal field strength therefore diminishes progressively from a
maximum (negative) value E to a minimum (negative) value E
BI OC
where
V − V
E = OC BI (6.12)
OC
d
The increasing photovoltage causes I (which is positive) to
Vphoto
increase steadily in value, and the decreasing field-strength causes I
ph
(which is negative) to decrease steadily in magnitude. The observed
photocurrent I photo therefore decreases steadily in magnitude from a
peak value of I at short circuit (where the photovoltage is zero) to a
SC
value of zero at open circuit (where the infinite load resistance pres-
ents an unsurpassable obstacle to current flow).
The effects of a load resistance can be replicated using a simple
voltage source (Fig. 6.9a). An applied bias of zero (R = 0) is equivalent

I

OPV
device
I dark

A
V
(0,V OC )
[R = ∞]
V I photo
(I′, V′)
(a)
[R = V′/I′]
(I SC ,0)
[R = 0]
(b)

FIGURE 6.9 (a) Schematic of an organic photodiode connected to a variable voltage
source. (b) Model current-voltage curve for a device in the dark and under
illumination. The short-circuit current I occurs at V = 0 (i.e., when the load
SC
resistance is zero), and the open-circuit voltage V occurs at I = 0 (i.e., when the
OC
load resistance is infi nite). A general point (I ′, V ′ ) on the current-voltage curve
corresponds to the current that would be obtained if the device were connected
to a load resistance R ′= V ′/I ′.