25
In photodiodes,
the metric is quantum e
ffi
ciency, which defines the signal to
noise ratio, while for solar cells, it is the power conversion e
ffi
ciency, which is
the power delivered per incident solar energy. Usually, solar cells and the
external load they are connected to are designed
to maximize the delivered
power.
Figure 3.4 Spectral irradiance vs wavelength
Working principle:
A simple solar cell is a
pn
junction diode. The schematic of the device is
shown in figure 3.5. The n region is heavily doped and thin so that the light can
penetrate through it easily. The p region is lightly doped so that most of the
depletion region lies in the p side. The penetration depends on the wavelength
and
the absorption coe
ffi
cient increases as the wavelength decreases. Electron
hole pairs (EHPs) are mainly created in the depletion region and due to the built-
in potential and electric field, electrons move to the n region and the holes to the
p region. When an external load is applied, the excess electrons travel through
the load to recombine with the excess holes. Electrons and holes are also
generated with the p and n regions, as seen from figure 3.5.
The shorter
wavelengths (higher absorption coe
ffi
cient) are absorbed in the n region and the
longer wavelengths are absorbed in the bulk of the p region. Some of the EHPs
generated in these regions can also contribute to the current. Typically, these are
EHPs that are generated within the minority carrier di
ff
usion length, Le for
26
electrons in the p side and
L
h
for holes in the n side. Carriers produced in this
region can also di
ff
use into the depletion region and contribute to the current.
Thus, the total width of the region that contributes to the solar
cell current is wd
+
L
e
+
L
h
, where
w
d
is the depletion width. This is shown in figure 3.6.The carriers
are extracted by metal electrodes on either side. A finger electrode is used on the
top to make the electrical contact, so that there is su
ffi
cient surface for the light
to penetrate. The arrangement of the top electrode is shown in figure 3.7.
Consider a solar cell made of Si. The band gap e.g. is 1.1 eV so that wavelength
above 1.1 µm is not absorbed since the energy is lower than the band gap. Thus,
any λ greater than 1.1 µm has negligible absorption. For λ much smaller than 1.1
µm the absorption coe
ffi
cient is very high and the EHPs are generated near the
surface and can get trapped near the surface defects. Therefore, there is an
optimum range of wavelengths where EHPs
can contribute to photocurrent,
shown in figure 3.6.
Figure 3.5 Principle of operation of solar cell