Civil Engineering Reference
In-Depth Information
where I
= the saturation current of the diode
Q = electron charge = 1.6 · 10
D
Coulombs
-19
A = curve fitting constant
K = Boltzmann constant = 1.38 · 10
Joule/°K
T = temperature on absolute scale °K
-23
The load current is therefore given by the expression:
QV
AKT
Voc
R
oc
II Ie
=−
−
1
−
(8-3)
L
D
sh
The last term, the ground-leakage current, in practical cells is small com-
pared to I
, and can be ignored. The diode-saturation current can,
therefore, be determined experimentally by applying voltage V
and I
L
D
in the dark
and measuring the current going into the cell. This current is often called
the dark current or the reverse diode-saturation current.
oc
8.4
Open Circuit Voltage and Short Circuit Current
The two most important parameters widely used for describing the cell
electrical performance is the open-circuit voltage V
and the short-circuit
oc
current I
. The short-circuit current is measured by shorting the output
terminals, and measuring the terminal current under full illumination. Ignor-
ing the small diode and the ground-leakage currents under zero-terminal
voltage, the short-circuit current under this condition is the photocurrent I
sc
.
The maximum photovoltage is produced under the open-circuit voltage.
Again, by ignoring the ground-leakage current, Equation 8-3 with I = 0 gives
the open-circuit voltage as the following:
L
I
I
AKT
Q
V
=
Log
L
+
1
(8-4)
oc
n
D
The constant KT/Q is the absolute temperature expressed in voltage
(300°K = 0.026 volt). In practical photocells, the photocurrent is several
orders of magnitude greater than the reverse saturation current. Therefore,
the open-circuit voltage is many times the KT/Q value. Under condition of
constant illumination, I
is a sufficiently strong function of the cell tem-
perature, and the solar cell ordinarily shows a negative temperature coeffi-
cient of the open-circuit voltage.
/I
L
D
