Page 234 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 211

field, the more effective it is in driving the photogenerated charges
toward their parent electrodes, and hence the closer in value are α
e
and α to unity. In addition, in some material systems, an electric field
h
8
is needed to promote exciton dissociation, causing γ to increase
monotonically with increasing field strength. The short-circuit quan-
tum efficiency is therefore optimized by maximizing the built-in
field, which requires the use of electrodes with widely differing work
functions. 44, 45
If a load resistance R is now connected between the terminals of
the photodiode, a positive voltage

V = R × I (6.6)
photo photo
will appear between the anode and the cathode, where I is the
photo
observed photocurrent. The appearance of the photovoltage reduces
the electric field strength inside the solar cell to a value

V − V
E = photo BI (6.7)
d

I is always smaller (less negative) than the short-circuit photocur-
photo
rent I , which can be understood by dividing I into two parts:
SC photo
I = I + I (6.8)
photo ph V photo
The first part I arises from the continuous generation and extraction
ph
of free carriers into the external circuit. By direct extension of the
argument presented above, I is given by
ph
E
I ()
ph =−γ E ()[α E () + α E () − 1 ]ℜ (6.9)
e e h
The second part, I , is due to the photovoltage V between
Vphoto photo
the two electrodes. The photovoltage is indistinguishable in its effects
from an externally applied positive bias, and so causes electrons and
holes to be injected from their parent electrodes into the bulk of the
device. This gives rise to a positive current that opposes the negative
current I . In most devices, under normal operating conditions, I
ph Vphoto
is identical to the dark current I .
46
dark
I E () = I E () (6.10)
V photo dark
which implies
I = I + I (6.11)
photo ph dark