Environmental Engineering Reference
In-Depth Information
Figure 1.11
The log-linear and linear-linear graphs plotted for detailed
analysis of dark I-V characteristics of PV solar devices.
Aplotoflog
10
(
I
D
) versus the voltage applied (
V
)acrossthedevice
takes the form of Fig. 1.11 for a practical device. It should be noted
that both forward and reverse currents are plotted here in the
same quadrant by changing the sign of the reverse voltages for
convenience.
The rectification factor (RF) of the device, as defined by the
expression RF
=
forward current/reverse current at a given voltage
(say at
V
=
1.0 V) is a good indicator of the quality of rectifying
diode. For an e
cient solar cell, a large RF is desirable although an
RF with a value of approximately 10
3
is su
cient.
The straight-line portion of the graph provides a value for the
n
from its gradient:
e
Gradient
=
(1.8)
.
2
303
nkT
This is a useful parameter as it provides a general idea about the
current transport through the potential barrier [13]. For an ideal
diode, the current transport takes place only through thermionic
emission over the potential barrier and, hence, the
n
is equal to
unity (
n
=
1
.
00). If the depletion region and the interface are full of
recombination and generation (R&G) centres, the current transport
is dominated by the R&G process and the value of the
n
becomes
2.00. In practical devices, both these transport mechanisms take
place in parallel and, therefore,
n
takes values between 1.00 and
2.00. The situation becomes even more complicated when there is
a large series resistance (
R
s
) present in the device structure. The
