Increasing light energy
2.0 eV
2.5 eV
3.1 eV
c
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u
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v
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1.7 eV
LIGHT
Solar Cell A
Bandgap
1.0 eV
2.0 eV
c
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Solar Cell B
Bandgap
ENGRI 1110
Homework #3
Solutions
1)
Solar cell
A
uses a semiconductor with a band gap of 1 eV.
Solar cell
B
uses a
semiconductor with a band gap of 2 eV.
Either cell is thick enough to absorb all the light
above its band gap.
Both cells can work simultaneously with one on the top of the other
to operate more efficiently in a small space.
a)
Which cell has to be on top (first to receive the sunlight) for both to work
simultaneously, and
b)
why?
There are, of course, two configurations for this problem: 1)
Cell A on top and
cell B on bottom 2)
Cell B on top and cell A on bottom.
Since either cell is thick enough
to absorb all energies above it’s band gap, in order for both cells to work simultaneously,
the second configuration must be used.
Let’s look at configuration 1 and see what would happen.
Since the bandgap of
cell A’s semiconductor is 1eV, it would absorb all energies (1.7-3.1eV) in the visible
spectrum.
No light would be transmitted through cell A and cell B would not generate
any electricity.
Configuration 2 has the larger bandgap material of cell B on top.
Cell B would
absorb all energies above 2eV and allow the lower energy light to filter down to cell A.
This configuration would allow both cells to work and be a more efficient use of space.
1.7 eV
<2.0 eV
>2.0 eV
2.5 eV
3.1 eV
LIGHT
Increasing light energy
2.0 eV
1.0 eV
c
o
n
d
u
c
t
i
o
n
b
a
n
d
(
e
m
p
t
y
)
c
o
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d
u
c
t
i
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n
b
a
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(
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)
v
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e
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(
f
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)
v
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c
e
b
a
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d
(
f
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d
)
Solar Cell A
Solar Cell B
Bandgap
Bandgap

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