Page 59 - Applied Photovoltaics

Figure 3.5. Typical representation of an I-V curve, showing short-circuit current
(I sc and open-circuit voltage (V oc) points, as well as the maximum power point
(V mp, I mp).

The two limiting parameters used to characterise the output of solar cells for given
irradiance, operating temperature and area are (Shockley & Queisser, 1961):
1. Short circuit current (I sc )—the maximum current, at zero voltage. Ideally, if
V = 0, I sc = I L . Note that I sc is directly proportional to the available sunlight.
2. Open circuit voltage (V oc )—the maximum voltage, at zero current. The value
of V oc increases logarithmically with increased sunlight. This characteristic
makes solar cells ideally suited to battery charging.
Note that at I = 0,

nkT § I ·
V oc ln ¨ L ¸ 1 (3.3)
q ¨ © I 0 ¸ ¹

For each point on the I-V curve, the product of the current and voltage represents the
power output for that operating condition. A solar cell can also be characterised by its
maximum power point, when the product V mp × I mp is at its maximum value. The
maximum power output of a cell is graphically given by the largest rectangle that can
be fitted under the I-V curve. That is,
d IV
0
dV
giving

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