This equation confirms that the diode I-V characteristic is shifted down into the fourth quadrant by an amount equal to the light – generated current IL This is show in Figure 2.10
Most people are unfamiliar with curves in the fourth quadrant so for convenience the I-V characteristics of a solar cell are normally flipped over to the first quadrant This is equivalent to plotting V against –I past(b) of the figure illustrates a family of such curves for a typical crystalline silicon cell rated by the manufacturer at 2 Wp Each curve represents a different strength of sunlight and hence a different value of IL You will recall that PV cells and modules are normally rated in peak watts (Wp) indicating the maximum power they can deliver under standard conditions (insolation 1000W/m2 cell temperature 25 c AM 1.5 solar spectrum) Therefore we should first consider how the rated power of 2Wp relates to the 1000W/m2 I-V curve
In general the cell power output equals the product of its voltage and current No power is produced on open circuit (maximum voltage zero current) or short circuit (maximum current zero voltage) The full rated power is obtained by operating the cell slightly below maximum voltage and current at its maximum power point (MPP) shown as P1 against the 1000W/m2 curve and corresponding to about 4A at 0.5 V or 2 W We can only obtain the promised output power by operating the cell at its MPP Three other curves are shown for lower insolation values of 750 500 and 250W/m2 each has its own MPP (P2 P3 P4) indicating the maximum power available from the cell at that particular strength of sunlight