Page 67 - Advances in Renewable Energies and Power Technologies

40 CHAPTER 1 Solar Cells and Arrays: Principles, Analysis, and Design

FIGURE 1.33
IeV characteristics showing the soft IeV in reverse.

the surrounding regions in the diode. Fig. 1.33 illustrates the reverse characteristics
of solar cells. They are soft. It means that the leakage current is large and increases
continuously with the reverse voltage. Soft breakdown occurs for shallow pen junc-
tions as solar cells have points of weak breakdown. Therefore, the breakdown cur-
rent flows in small spots. Because the current is high and the voltage is high, a
relatively large power is consumed in this small area, which becomes hotter as in
Fig. 1.34. This effect can lead to thermal breakdown of the cell [31].
To avoid destruction of solar cells, we connect across them a bypass diode back
to back as shown in Fig. 1.35. Solar cells can be cracked or disconnected because of
unreliable assembly and the effect of environment. This effect is exactly like the
shadowing except it is permanent. Protection diodes have an additional vital

FIGURE 1.34
Hot spot formed in a weak pen junction region.