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The Maximum Possible Efficiency of a Solar Cell (Solar Energy Course 2020 Part 10 of 12)

February 26, 2020

Is there a theoretical limit for the efficiency
of a solar cell? What is it? How are scientists overcoming it? Find out in this video. This video is part of iPolytek’s online course
on solar energy. iPolytek, Professional Development Courses
for Engineers. What is the maximum possible efficiency for
a solar cell that has just one PN junction? In 1961, William Shockley and Hans Queisser
calculated this limit based on the second law of thermodynamics which states that the
maximum efficiency of a “heat engine” is a function of the temperature of the hot reservoir
(sun) and of the cold reservoir (the cell) and the following assumptions Number 1: The cell does not make use of the entire solar spectrum – meaning that part of the radiation that falls on the solar cell is
systematically transformed into heat. Number 2: The sunlight is not concentrated by mirrors
or lenses. The figure on the right summarises their findings.
It shows how the energy of a photon is distributed as a function of the bandgap energy of the
semiconductor. The majority of the energy that falls on the
solar cell is lost, yielding a Shockley-Queisser maximum efficiency limit of about 30% for
a cell having just one PN junction. Here we see the Shockley-Queisser Efficiency
Limits for Various Semiconductors with 1 PN junction. At one sun, meaning without the use of concentration
by mirrors or lenses, a maximum efficiency of about 30% can be obtained from solar cells
made of Copper Indium Selenium, crystalline silicon, Indium phosphide, Gallium arsenide,
and cadmium telluride. Amorphous silicon has a slightly lower maximum
efficiency of 29%. Researchers are working on several solutions
to overcome the Shockley-Queisser limit. Here are three examples. Multi-junction (MJ) solar cells have multiple
p-n junctions made of different semiconductor materials that produce an electric current
in response to different wavelengths of light. The theoretical SQ efficiency limit of MJ
solar cells is on the order of 80%. Here’s why. On the left, we see the energy absorbance
spectrum of a crystalline silicon solar cell having only one junction. The area shaded in gray is the
AM1.5 spectrum supplied. The energy actually absorbed by the cell is shaded in red. As
you can see, much of the energy falling upon the solar cell is wasted. On the right, we see the performance of a
multijunction solar cell. As you can see, this solar cell is able to absorb a wider
range of wavelengths and wastes much less energy. This is thanks to the use of different types
of semiconductors, each having a bandgap energy chosen to absorb a specific part of the solar spectrum. When combined together into one solar cell, they allow more energy to be absorbed from sunlight yielding
a much higher efficiency. Up to 40% efficiency has been obtained in
the lab so far. A second approach to increasing solar cell
efficiency is to use lenses to focus sunlight onto multijunction solar cells.
Lenses placed above the multijunction solar cell concentrate the sunlight by a factor
of 100 to 1000. An average of 300 “suns” is used. The active solar cell surface area is significantly
reduced using this technique. To date, efficiencies exceeding 45% have been
measured in the lab. Here is a closer look at a typical set-up. On the left, we see that sunlight hits a cell
and lense assembly. These lenses are known as Fresnel lenses. Several of these assemblies
are combined to make up a solar module. Several of these modules may be mounted on
a tracker which follows or tracks the sun throughout the day. On the right, underneath the fresnel lense,
we see an example of a multijunction solar cell. Note that in the diagram, the solar
cell is rainbow colored indicating that it can
absorb a wide range of wavelegths. Another method being developed to increase
solar cell efficincy is the use of quantum dots. Quantum dots are nanocrystals that can be
incorporated into photovoltaic cells to absorb different parts of the solar spectrum and
create electron-hole pairs. By adjusting the size of these nanoparticles, researchers can
target specific parts of the solar spectrum. Theoretically, this method could be used to
make solar cells with efficiencies of up to 60%. So far, 10% efficiency has been achieved in
the lab. Here is a picture of colloidal quantum dot mixture
being irradiated with UV light. The quantum dots absorb the UV lights and convert it to
visible light. Quantum dots of different sizes emit different colors of light as seen in
this picture. Today, these solutions are already bearing
fruit in the laboratory, with efficiencies as high as 25% to 46%. The efficiency of silicon solar cells with
concentration is 25% and that of multijunction solar cells with
concentration exceeds 40%. In this video, we’ve seen that the efficiencies
of solar cells are on the rise. But how expensive is this technology? What is the cost of solar power in comparison
to other types of power plants? Find out in our next video! Thanks for watching and see you soon.

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