Environmental Engineering Reference
In-Depth Information
R
I
I C
Light
D
S
E
Consumer resistance
I
R
U
DC
P
Ph
Radiation-induced
current
Light
Grid contact of
-
front side contact
n-layer
+
Radiation-induced
Electric field
voltage
Rear side contact
Anti-reflecting coating
p-layer
Diffusion current from
charge carriers
Fig. 6.6 Structure of a typical solar cell and an equivalent circuit diagram (top left) (see
also /6-1/, /6-10/; for symbols see text)
eU
kT
0
I
=
I
I
e 1
Ph
0
(6.1)
UT ln 1
e
I
I
=⋅ ⋅
Ph
I
0
0
I stands for the current flowing through the terminals, I Ph for the photocurrent
and I 0 for the saturation current of the diode, whereas e 0 represents the elementary
charge (1.6021 10 -19 As), U the cell voltage and k the Boltzmann constant
(1.3806 10 -23 J/K), and θ stands for the temperature. However in Equation (6.1)
the sign for current I have been inverted compared to the conventional notation.
This is why the characteristic curves (Fig. 6.7) are not located in the forth but in
the first quadrant. However, this kind of representation has become common prac-
tice.
Under realistic conditions, the performance of a solar cell can be described as
shown in the equivalent circuit diagram illustrated in Fig. 6.6, left top. Without
irradiation, the solar cell is equal to an ordinary semiconductor diode whose effect
is also maintained at the incidence of light. This is why diode D has been con-
nected in parallel to the photovoltaic cell in the equivalent circuit diagram. Each
p-n-junction also has a certain depletion layer capacitance, which is, however,
typically neglected for modelling of solar cells. At increased inverse voltage the
depletion layer becomes wider so that the capacitance is reduced similar to
stretching the electrodes of a plate capacitor. Thus, solar cells represent variable
capacitances whose magnitude depends on the present voltage. This effect is con-
 
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