Environmental Engineering Reference
In-Depth Information
value of
R
s
can be approximately evaluated using the potential drop
at the high forward end of the I-V curve using Eq. 1.9:
R
s
(1.9)
Large values of
R
s
tend to reduce the gradient of the log-linear
curve at the high forward-bias region and, hence, increase the
n
.
Tunnelling through the device can also tend to increase the low
forward bias current, decreasing the gradient of the log-linear I-V
curve and, hence, increasing the
n
. When both effects are present
in one device, the log-linear curve deviates accordingly, making
the analysis more complicated. It should be noted that the largest
gradient of this curve should be used to evaluate the smallest value
of
n
andother parameters for accurate analysis of the device.
Theinterceptofthestraightlinewiththehighestgradient
provides a more accurate value for
I
0
, and hence the
φ
b
present in
the device can beevaluated from:
I
0
=
SA
∗
T
2
V
=
I
·
·
exp
−
e
φ
b
kT
(1.10)
This is a key parameter for a rectifying diode, and the production of
an interface with a large
φ
b
is desirable for making an e
cient PV
cell. In addition, the depletion region and the interface should have
minimumamountsofR&Gcentrestoproduceahigh-qualityPVcell,
minimisingthe detrimental leakage process through the barrier.
By plotting I-V data measured under dark conditions, in a linear-
linear graph (as in Fig. 1.11), series resistance (
R
s
) and shunt
resistance(
R
sh
)canalsobeestimatedfromtheforwardandreverse
currentportionsrespectively.Therefore,thedarkI-Vmeasurements
provide the most important parameters (
n
,
φ
b
,
R
s
,and
R
sh
)forthe
device structure and provide the quality of the depletion region and
the information on current transport mechanisms in the device via
the parameter
n
.
These parameters then help in establishing an equivalent circuit
for the solar cell device, as shown in Fig. 1.12.
1.5.2
I-V Characteristics of a Solar Cell Under Illuminated
Conditions
The directions of electron flow under dark and illuminated
conditions are shown in the band diagrams in Fig. 1.13. This shows
