Page 236 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 213

to short circuit, whereas an applied bias of V is equivalent to open
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circuit (R = ∞); intermediate applied biases (0 < V < V ) are equiva-
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lent to intermediate load resistances (0 < R <∞). In Fig. 6.9b, we show
the photocurrent as a function of applied bias for a typical device
under a fixed illumination level. A general point (I′, V′) on the
photocurrent-voltage curve corresponds to the photocurrent I′ and
the photovoltage V′ that would be obtained if the photodiode were
connected to a load resistance R′ = V′/I′ (keeping the light intensity
the same). Also shown on the plot is the dark current I which
dark
increases quickly with voltage.
The photocurrent curve in Fig. 6.9b extends beyond the limited
range 0 < V < V that we considered in the discussion above. Inside
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this range, the power

P = I × V (6.13)
photo photo
is negative, which indicates that power is dissipated by the photo-
diode in the external circuit. This is the relevant regime for solar
cells, where we are required to dissipate power harnessed from
the sun in an external load resistance. In the case of photodiodes,
however, we can apply a bias of any desired size using an external
voltage source. If the photodiode is subjected to a reverse bias, we
obtain an internal field strength that is larger in magnitude than
the built-in field. This has three important benefits. First, from
Eq. (6.9), it enhances the value of I , giving rise to improved pho-
ph
tosensitivity. Second, it sweeps charges more rapidly from the
device, shortening response times. Third, it reduces the steady-
state charge density inside the device, reducing the amount of
electron-hole recombination and so extending the linear range to
higher intensities.
These benefits, however, come at an important cost since the
application of a reverse bias also generates a (negative) voltage-
dependent dark current. The dark current sets a baseline beneath
which it is difficult to measure a photocurrent since small fractional
drifts in the dark current (e.g., due to temperature changes) can
mask the smaller photocurrent; hence, if the dark current is in the
nanoampere range, it is difficult to measure photocurrents very
much smaller than this. In addition, as we discuss later, biasing the
device generates noise in the photodiode which degrades the signal-
to-noise ratio (see Sec. Shot Noise). The short-circuit quantum effi-
ciency of a well-optimized organic device can average 25% over
its full spectral range. Hence, at best a fourfold increase in quantum
efficiency can be obtained by applying a reverse bias. The dark
current, on the other hand, can increase by several orders of magni-
tude when a sizable bias is applied, meaning the slight increase