28
I
sc
. By definition of current, this is opposite to the photo current and is related to
the intensity of
the incident radiation, I
op
, by
I
sc
=
−I
ph
=
−kI
op
……………….…………. 3.1
Where k is a constant and depends on the particular device. K is equivalent to an
e
ffi
ciency metric that measures the conversion of light into EHPs. Consider the
case when there is an external load R, as shown in figure 3.8. The equivalent
circuit for this case is shown in figure 3.9. There is a voltage across the external
load, given by V = IR. This voltage opposes the built in potential and reduces the
barrier for carrier injection across the junction. This is similar to a
pn
junction in
forward bias, where the external bias causes injection of minority carriers and
increased current. This forward bias current opposes the photocurrent generated
within the device due to the solar radiation. This is because
I
ph
is generated due
to electrons going to the n side and holes to the p side due to the electric field
within the device i.e. drift current while the forward
bias current is due to
di
ff
usion current caused by the injection of minority carriers. Thus,
the net
current can be
written as
I
= −
I
ph
+
I
d
……………………………….…………. 3.2
I
d
=
I
s0
[exp(
𝑒𝑣
𝐾
𝐵
𝑇
) − 1]
………………………….……. 3.3
I
= −
I
ph
+
I
s0
[exp(
𝑒𝑣
𝐾
𝐵
𝑇
) − 1]
……………….…………. 3.4
Where Id is the forward bias current and can be written in terms of the reverse
saturation current, Is0 and external voltage, V. The overall I-V characteristics is
plotted in figure 3.10. In the absence of light, the dark characteristics is similar
to a
pn
junction I-V curve. The presence of light (
I
ph
) has the e
ff
ect of shifting
the I-V curve down. From figure 3.10, it is possible to define
a photo current
I
ph,
29
which is the current when the external voltage is zero and an open circuit voltage,
V
oc
, which is the voltage when the net current in the circuit is zero. Using
equation 2,
V
oc
can be calculated as
I
ph
=
I
s0
[exp
(
𝑒𝑣
𝑜𝑐
𝐾
𝐵
𝑇
) − 1]
……………………………...……. 3.5
V
oc
≈
𝐾
𝐵
𝑇
𝑒
ln [
𝐼
𝑝ℎ
𝐼
𝑠0
]
……………………………….…………. 3.6
Higher the photon flux, higher is the value of
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