### 2.9:  The solar cell's output parameters

The output power P of a solar cell is given by:

P = IV,

which means current times voltage. This is similar to the power a mill wheel can deliver: it is proportional to the amount of water splashing onto the wheel (the current), times the height (potential) from which the water splashes onto the wheel.

At a given IV point, P can be visualized by drawing a square (as shown in the figure): its area is equal to the output power at that point.

If the cell under illumination is not contacted, we say that it is in "open-circuit" condition. In this case, it does not deliver a current but the open-circuit voltage VOC (see figure). Hence, the output power is zero. If the cell under illumination is short-circuited, it cannot deliver a voltage but the short-circuit current JSC. In this case, the output power is again zero. However, the cell delivers an output power at any other point on the IV curve in the figure.

Going along the illuminated IV curve in the figure, the output power (given by the various squares in the figure) is maximal at some point, called the maximum power point (MPP).

An important parameter is the cell efficiency eff, which is defined at MPP and is: We will discuss in "The solar spectrum" chapter that the standard spectrum has a radiation power density of 100 mW/cm2. Hence, the efficiency is given by: where JMPP is the current at MPP and VMPP is the voltage at MPP. Notice that, in this case, the units must be either mA/cm2 and V, or A/cm2 and mV.

An other useful output parameter is the fill factor FF: The fill factor is a measure for how close the IV curve is to a square-shape: if the IV curve had a square shape, the value for the fill factor would be 1. However, because the IV curve has an exponential shape, the fill factor is always smaller than one.

You will see in subsequent lessons that these output parameters (JSC, VOC, eff, FF, JMPP, and VMPP) are very useful for characterizing solar cells. Figure: A solar cell's current–voltage curve under illumination.