Page 232 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 209

d
Vacuum level
= f – f
LUMO of V BI a c
acceptor
f a f

OPV c
device

A
Anode Cathode
R

V HOMO
of donor
(a) V BI
E = – d
BI
(b)
FIGURE 6.8 (a) OPV device driving a load resistance R; under illumination,
the photodiode generates a photovoltage V and a photocurrent I .
photo photo
(b) Energy level diagram for a single-layer solar cell; the electric fi eld exerts a
force on the charge carriers that tends to drive the electrons and holes to
the cathode and anode, respectively. In a simple device, the electric fi eld
strength is determined by the work function difference between the
electrodes and the thickness of the device.

the short-circuit photocurrent J , and a photovoltage V of zero
SC photo
(since the two electrodes are shorted together). The aligned Fermi
levels of the two electrodes create a negative internal field E =−V /d,
BI BI
known as the built-in field, 44, 45 that helps drive electrons toward the
cathode and holes toward the anode (Fig. 6.8b), resulting in a nega-
tive photocurrent.
Under weak illumination, the size of the photocurrent is directly
proportional to the intensity of the incident light––a property that is
essential for most light-sensing applications. The linearity comes
about because, at steady state, the rate of free carrier generation by
photoexcitation must be exactly balanced by the rate of free carrier
loss by extraction into the external circuit. Hence, if the incident light
level doubles, so too must the photocurrent to compensate. This
equality holds true only in the low-intensity regime where internal
losses due to electron-hole recombination in the bulk are negligible
(and the charge density is too low to affect the electric field distribu-
tion inside the device). In well-optimized devices, recombination
effects are appreciable only at high illumination levels exceeding 46