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4. The Solar Cells 23

vF ph
0DF ph ¼ dx ¼ aF ph dx (1.31)
vx
Because each absorbed photon generates an eeh pair, one can determine the pho-
togeneration rate g ph . By definition, g ph ¼ number of generated eeh pairs in certain
3
volume/volume (eeh pairs/cm /s). Therefore,
DF ph
g ph ¼ ¼ aF ph (1.32)
dx
The average photogeneration rate g pha
Z
x 1 þW eff h i
aF ph F ph ð0Þ
g pha ¼ dx ¼ e ax 1 e aðx 1 þW eff Þ (1.33)
W eff W eff
x 1
where x l is the dead layer in the n-region shown in Fig. 1.18.
Because the incident solar radiation has certain spectral distribution, we can
obtain the overall generation rate (g pht ) over the whole spectrum, considering that
a w 0 for E ph E g , by integrating Eq. (1.33) once again from E ph ¼ E g to N or
from l ¼ l c ¼ hc/E g to l ¼ 0, i.e.,
Z
l c
g pht ¼ g pha dl (1.34)
0
Substituting Eq. (1.32) into (1.33)
Z
F ph ð0Þe ax 1
l c
g pht ¼ 1 e aW eff dl (1.35)
0 W eff

FIGURE 1.18
The effective collection region of the photogenerated carriers.