Sudan University of Science and Technology College of Graduate Studies Department of Electrical Engineering

Solar cells: 
Solar cells and photodetectors are devices that convert an optical input into 
current. A solar cell is an example of a photovoltaic device, i.e, a device that 
generates voltage when exposed to light. The photovoltaic e
ff
ect was discovered 
by Alexander-Edmond Becquerel in 1839, in a junction formed between an 
electrode (platinum) and an electrolyte (silver chloride). The first photovoltaic 
device was built, using a Si pn junction, by Russell Ohl in 1939. The functioning 
of a solar cell is similar to the photodiode (photodetector). It is a photodiode that 
is unbiased and connected to a load (impedance). There are three qualitative 
di
ff
erences between a solar cell and photodetector 1. A photodiode works on a 
narrow range of wavelength while solar cells need to work over a broad spectral 
range (solar spectrum). 
Solar cells are typically wide area devices to maximize exposure. 

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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 

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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

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