The Science of
Solar Cells
May 15, 2008
Announcements
Review of the Lab Report
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Handouts
Excel demo
Sunlight to Electricity
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So we are somehow converting light, which has a certain energy, to a
flow of electrons (current)
So the question is: How does this conversion process take place?
This was the difficulty people had with trying to make a solar cell…until
around 1954 in Bell Labs
And it turns out our old friend, the PN junction was the missing link that
when discovered, made the first Si solar cell possible back in 1954
solar radiation
DC Electric
Current out of PV
device
e- e- e-
Some Application
Silicon Material
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Solid Si forms as a regular
crystal lattice of material,
forming covalent bonds
between the Si atoms
Si Si Si Si Si
Si Si Si Si Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si Si
Si
Si
Si
Si
Si
Si
Si Si Si
Si
Si
Si Si Si Si
Si
Si
Si
Si Si Si Si Si
Si
Si
Si
Si
solar radiation
Silicon Material
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Solid Si forms as a regular
crystal lattice of material,
forming covalent bonds
between the Si atoms
If light with enough energy hits
Silicon, these bonds can be
broken momentarily, freeing
one of the electrons that made
up the bond and leaving behind
a hole
So, in this case, the electron
does not get very far before it is
attracted back to the hole and
recombines with the hole to
form the bond again
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Remember the electron has a
negative charge, the hole has a
positive charge
So current does not flow
Si Si Si Si Si
Si Si Si Si Si
Si Si
Si
Si Si Si
Si Si Si Si
Si Si Si Si Si
Detour: Forming an Electric
Field
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An electric field forms in the vicinity of any electric charge
A simple way to form an electric field is like this:
Now let’s take a look at two situations
and see what effect the electric field has:
1.
2.
A negatively charged object in the electric
field
A positively charged object in the electric
field
This simple concept turns out to be key to
the operation of a Si solar cell, the key
that alluded researchers for many
years…
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In many cases the simplest ideas are the
Nobel Prize winning solutions!
-
+
Silicon Material
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Now consider the same situation
we were looking at before, but
now let there be an electric field
acting inside the Si material
Now, when the same light hits
the material and again
momentarily breaks the bond—
what will happen?
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material
So an electric field would be
useful to separate the electron
from the hole after the light
breaks the bond!
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Solar radiation again breaks the
bond
Now though, electron is free to
flow or conduct through the
It would allow current to flow!
How can we generate an electric
field inside the Si? (hint: it will
involve the PN junction)
solar radiation
-
+
Si Si Si Si Si
Si Si Si Si Si
Si Si
Si
Si Si Si
Si Si Si Si
Si Si Si Si Si
Silicon
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Quick chemistry crash-course! (déjà-vu, I know)
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This time we’ll be looking specifically at Silicon, the common
material used for solar cells today. But the basic concept
extends to other types of solar devices and materials
Silicon (Si): Group 4  4 electrons in outer shell
Si
Silicon Material
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Solid Si forms as a regular crystal lattice of material,
with the Si atoms sharing 8 electrons between them
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Let’s see how we can
form a PN junction in Si
material
Si Si Si Si Si
Si Si Si Si Si
Si Si
Si
Si Si Si
Si Si Si Si
Si Si Si Si Si
The Silicon PN Junction:
N Doping
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What if a Si atom is replaced with a Phosphorus (P) atom?
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Atomic number of 15: Meaning it has 15 total protons (positive charges)
and 15 total electrons (negative) giving a net zero charge
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Remember, P is in Group 5
 So P has 5 electrons in its outer shell
P
5 outer
(valence
electrons)
The Silicon PN Junction:
N Doping
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What if a Si atom is replaced with a Phosphorus (P) atom?
Remember, P is in Group 5
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So P has 5 electrons in its outer shell
Si Si Si Si Si
Si Si Si Si Si
Si Si
Si
Si Si Si
Si Si Si Si
Si Si Si Si Si
The Silicon PN Junction:
N Doping
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What if a Si atom is replaced with a Phosphorus (P) atom?
Remember, P is in Group 5
N-type Si
 So P has 5 electrons in its outer shell
Electrons are the mobile
charge carriers
Si ‘doped’ with P is known
as N-type since the
carriers of charge are
electrons--which have a
Negative charge
Si Si Si Si Si
Si P Si P Si
Si Si
Si
Si Si Si
P Si
P Si
Si Si Si Si Si
The Silicon PN Junction:
P Doping
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What if a Si atom is replaced with a Boron (B) atom?
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Atomic number of 5: Meaning it has 5 total protons (positive charges) and 5
total electrons (negative charges) giving the atom a net zero charge
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And remember, B is in Group 3
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So Mg has 3 electrons in its outer shell
B
3 outer
(valence
electrons)
The Silicon PN Junction:
P Doping


What if a Si atom is replaced with a Boron (B) atom?
Remember, B is in Group 3

So B has 3 electrons in its outer shell
Si Si Si Si Si
Si Si Si Si Si
Si Si
Si
Si Si Si
Si Si Si Si
Si Si Si Si Si
The Silicon PN Junction:
P Doping
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What if a Si atom is replaced with a Boron (B) atom?
Remember, B is in Group 3
P-type Si
 So B has 3 electrons in its outer shell
Do you think this material
will allow current to flow?
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Yes, now there is a free
electron that is free to
move and carry charge
SiN ‘doped’ with Mg is
known as P-type since the
carriers of charge are
‘holes’--which have an
effectively Positive charge
Si Si Si Si Si
Si B Si B Si
Si Si
Si
Si Si Si
B Si
B Si
Si Si Si Si Si
The PN Junction Revisited
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N-type material has negatively charged free electrons able
to carry charge
P-type material has positively charged free ‘holes’ able to
carry charge
When a p-material is brought into contact with a n-material,
the resulting device is called a PN junction
Let’s look in further detail at what happens when this PN
junction forms…
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+
+
+
P-type
+
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+
- N-type
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The PN Junction Revisited
(In Further Depth)
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What will happen when the positively charged holes meet up with the negatively charged
electrons at the PN junction? (when the PN junction is created)
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Hint: What happens when you spray cologne on one of a room
Diffusion: Movement of particles from a region of high concentration to one of low
concentration
Electrons diffuse to P side take the spot of the holes
Holes diffuse to the N side to cancel out electrons
+
+
+
+
+
+
+
P-type
+ +
+
+
+
+
+
+
+
-
+
+
-
-
-
-
-
-
-
+
+
-
+
+
+
-
+
+
+
+
+
-
-
N-type
-
-
-
-
The PN Junction Revisited
(In Further Depth)
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What will happen when the positively charged holes meet up with the negatively charged
electrons at the PN junction? (when the PN junction is created)
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Hint: What happens when you spray cologne on one of a room
Diffusion: Movement of particles from a region of high concentration to one of low
concentration
Electrons diffuse to P side take the spot of the holes
Holes diffuse to the N side to cancel out electrons
A region is left surrounding the PN junction that is depleted of free electrons and holes—
called the ‘Depletion Region’
+
+
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+
+
+
+
+
P-type
+ +
+
-
+
-
-
+
+
-
+
+
-
+
+
-
-
+
+
-
-
N-type
-
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-
-
The PN Junction Revisited
(In Further Depth)
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But what’s left behind when the electrons leave the n side and the holes leave
the p side?
An electric field between the positively charged P atoms and the negatively
charged B atoms forms automatically when the PN junction is made!
This electric field prevents electrons from recombining with holes when light from
the sun breaks a bond
+ -
Before
electron
diffusion- B
atom: zero
charge
+
+
+
+
+
After electron
diffusion- B atom
with extra
electron: net
negative
charge
B
+
+
+
B
+
P-type
+ +
+
+
+
+
+
+
+
+
+
BB-
P+
-
-
-
P+
-
-
B-
P+
B-
P+
B-
P+
Before electron
diffusion- P
atom: zero
charge
-
-
-
N-type
-
-
-
P
After electron
diffusion: P
atom missing
an electron:
net positive
charge
P+
The PN Junction Revisited
(In Further Depth)
solar radiation
breaks bonds
Before
electron
diffusion- B
atom: zero
charge
Creates electrons and holes: Electric
field sweeps electrons to the right
and holes to the left
+
+
+
+
+
After electron
diffusion- B atom
with extra
electron: net
negative
charge
B
+
+
+
B
+
P-type
+ +
+
+
+
+
+
+
+
+
+
+ -
-
B-
P+ -
B-
P+
B-
P+
B-
P+
B-
P+
-
Before electron
diffusion- P
atom: zero
charge
-
-
-
-
-
N-type
-
-
-
P
After electron
diffusion: P
atom missing
an electron:
net positive
charge
P+
Silicon Material


Now consider the same situation
we were looking at before, but
now let there be an electric field
acting inside the Si material
Now, when the same light hits
the material and again
momentarily breaks the bond—
what will happen?



material
So an electric field would be
useful to separate the electron
from the hole after the light
breaks the bond!


Solar radiation again breaks the
bond
Now though, electron is free to
flow or conduct through the
It would allow current to flow!
How can we generate an electric
field inside the Si? (hint: it will
involve the PN junction)
solar radiation
-
+
Si Si Si Si Si
Si Si Si Si Si
Si Si
Si
Si Si Si
Si Si Si Si
Si Si Si Si Si
Power of the Sun Video
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Time allowing (10 min)
Lessons From the Lab
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Does what you saw in the lab make sense
with what you’ve learned today?
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Voltage stays constant—dependent on the solar
material
Current changes with light intensity--more
electrons from more light
Summary
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Separation of charges is key to the operation
of a solar device
In Silicon solar cells, the electrons and holes
are separated using a PN junction
Another Way to Think About
it: Energy Band Diagram
solar radiation
with the right
energy
This is why different
materials absorb
different parts of the
sun’s energy!
Increasing
Energy
Conduction Band:
The next available energy level of
electrons above the valance band
where they are broken free of the bond
and can conduct through the material
Bandgap Energy: (EGAP)
The Approximate energy
needed to break a Si bond
e-
e-
e-
e-
e-
e-
e-
Energy level diagram
for Silicon
Valence Band: Energy level of
outer (valance) electrons when
they are being used to form a bond