Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 239

sidered by the capacitor C located in parallel to the diode. Series resistance R S
consists of the resistance of contacts and cables as well as of the resistance of the
semiconductor material itself. To minimise losses, cables should be provided with
a maximum cross-section.
Parallel or shunt resistance R P includes the "leakage currents" at the photo-
voltaic cell edges at which the ideal shunt reaction of the p-n-junction may be
reduced. However, for good mono-crystalline solar cells shunt resistance usually
is within the kΩ region and thus has almost no effect on the current-voltage char-
acteristic.
Fig. 6.7 shows the typical shape of a current-voltage curve for various operat-
ing modes (i.e. changing irradiation and temperature). At the intercept points of
the curve and the axes the short-circuit current I SC (which is in 1 st order approxi-
mation equal to I Ph ) is supplied at U = 0 and the open-circuit voltage U OC at I = 0.
Starting with the short-circuit current, the cell current is at first only slightly re-
duced and decreases over-proportionally shortly before reaching open-circuit
voltage when increasing the cell voltage continuously. These effects result in the
characteristic shape (see /6-2/, /6-5/).

1,000 W/m²
273 K

1.0

1,000 W/m²
293 K

800 W/m²
273 K
600 W/m²
273 K

0.8

1,000 W/m²
313 K Maximum Power Point (MPP)

0.6

1,000 W/m²
323 K

400 W/m²
273 K
200 W/m²
273 K

0.4

0.2

0.0

0.0 0.25 0,5 0.75 1.0

Fig. 6.7 Influence of radiation and temperature on the characteristic current-voltage curve
assuming standard test conditions (typical curve shape for a silicon solar cell according to
e.g. /6-11/)

Voltage relating to open-circuit voltage

Electric power is defined as the product of voltage and current. Thus at a cer-
tain point of the characteristic curve the maximum power of the solar cell is
reached. This operating point is referred to as MPP (Maximum Power Point). The
characteristic curve, and thus also the MPP, are a function of solar radiation and
the temperature of the photovoltaic cell.
− Photo-current or short-circuit current increases almost linearly with increasing
irradiance of the photovoltaic cell. Also, open-circuit voltage is increased ac-
cording to Equation (6.1); however, the increase is logarithmic. The current-
voltage curve moves thus parallel to the vertical axis with increasing solar ra-