Silicon and the Photovoltaic Effect
Most basic description
Photons of light interact with the electrons in the silicon and impart
enough energy in some of these electrons to free them. Then a
“built-in” potential barrier in the cell acts on these electrons such that
the electrons can be used to drive a current through an external circuit.
Seek to understand more about how the cell works and how its
performance can be optimized. Probably have a few questions at this point:
1) How is the electron “freed” and where does the electron go?
2) What is the potential barrier that acts on the free electrons? How is it
formed?
3) When the potential barrier acts on the electrons, how do the electrons
then produce a current?
Understanding the silicon cell is a good groundwork for understanding many
types of PV cells.
The answers to the questions above will actually lead us to a fuller understanding
of the photovoltaic cell.
Lets answer question #2 first. (What is the potential barrier that acts on the
freed electrons, and how is it formed?)
To answer this, look at the atomic structure of silicon?
From periodic table, know that silicon has __________ electrons and a total of
__________ valence electrons (the outermost electrons).
Silicon makes what type of solid (molecular, ionic, network covalent, metallic)?
Thus silicon shares it four valence electrons one each with a neighboring
silicon atom.
(like carbon does in diamond). Each BASIC silicon unit in the solid then,
forms a tetrahedral arrangement, containing five atoms (the one silicon atom
plus
the four others it shares electrons with.
Each atom in the silicon solid is held in place at a fixed distance and angle
with
respect to each of the atoms with which it shares an electron to make a
bond. This
regular, fixed formation of a solid’s atoms is known as a __________
__________.
Solids often can form from several differently shaped crystal lattices. For
silicon,
the atoms are located so as to form the vertices of a cube with single atoms
centered
at each of the faces of the cubic pattern. And the cubic arrangement repeats
throughout the crystal. Figure
Light then can interact with the crystalline lattice.
Low energy infrared light can be absorbed by the material causing it to
vibrate or twist but the vibrating atoms do not break loose.
Light of higher frequency and higher energy can strike an electron on an
atom in the crystalline lattice and excite it to a higher energy level or if
the light is energetic enough, the bound electron can be torn from its
place in the crystal, leaving behind a silicon bond missing an electron.
(Figure
This electron is free to move about the crystal and is said to be in the
crystal’s conduction band, because free electrons are the means by
which electricity flows. There is also a bond missing an electron and this
is often called a hole.
The electrons and holes freed from their positions in the crystal in this
manner are said to be light generated electron-hole pairs. Both the freed
electron AND the hole are free to move about the crystal. The hole moves
in the following way:
An electron on a neighboring atom to the atomic position of the hole in the
lattice can jump into the hole, leaving a hole then on that neighboring
atom.
The same can happen again as an electron from a neighboring atom to the
original one can jump to the hole and so on …
Figure
This happens fast and frequently-electrons from nearby bonds trading
positions with holes, sending the holes in random and erratic patterns
through the crystalline solid.
What is the effect of the temperature of the material? The more agitated
the electrons and holes, the more they move.
The generation of electrons and holes by light is the central process in the overall
PV effect. Does this produce a current?
If there were no other mechanism involved in this type of solar cell, it would not be
a solar cell. The electrons would just meander about the crystal randomly for a
time and then lose their energy thermally as the return to valence positions.
MUST HAVE a built in Potential Barrier to exploit the electrons and holes produced
by the light.
The Potential Barrier
Set up such that there are opposite electric charges facing each other, on either
side of a dividing line. This potential barrier must be able to separate the electrons
and holes by sending the electrons to one side of the barrier and the holes to the
opposite side of the barrier where they are less likely to rejoin each other. Also
this separation creates a voltage difference between one side of the cell and the
other side of the cell.
Figure
What is the purpose of this voltage difference? It can drive an electric current in
an external circuit for the purpose of charging a battery, or running an electronic
gadget, etc
HOW can we make this separation occur?
This is the basis of all solar cells, and different types of solar cells accomplish this in
different ways.
For SILICON solar cells, the charge separation and potential barrier is accomplished
by Doping. In DOPING, impurities are introduced into a pure crystalline material
usually altering the electrical properties of the material.
There is a Negative-Carrier Dopant and a Positive Carrier Dopant.
The negative carrier dopant introduces an impurity into an otherwise pure silicon
crystal that has one more valence electron than a silicon atom! What atoms would
work for this?
Group V atoms such as N, P, As etc., all have 5 valence electrons.
The P atom (for instance) when introduced into the Si would occupy the same
position in the crystal as a normal Si atom, supplying an electron for each of Si’s four
bonds. BUT the phosphorus atom has one extra valence electron and thus:
There is one electron that does not have a bond to share, and this electron would be
relatively free.
Figure
At room temp., there should be sufficient thermal energy for this extra electron
to “enter the conduction band and “shake-loose” from the phosphorus atom,
even though it would leave behind a positively charged phosphorus impurity
atom.
This electron that has “shaken-loose” from the impurity atom has no hole
associated with it.
Overall the crystal is neutral, since there are just as many “shaken-loose”
electrons as there are positively charged phosphorus ions from which the
electrons have shaken loose.
BUT the crystals electrical properties would have been drastically altered.
HOW?
Band Theory Diagram
Dopants that have one extra electron are called donors because they donate an
electron to the crystal. Such a donor-doped crystal is known as an n-type
because it has free negative charges.
Is this enough by itself to create a barrier, the answer is no!
Need to also have the opposite charge as well. A Positive Carrier Dopant Material
What do you suppose that will be?
Atom with one fewer valence electron than silicon!
An atom with three valence electrons, boron, aluminum, etc. can form an
appropriately altered semiconductor material in silicon. When this sits in the crystal
lattice in place of a Si atom, then what happens?
One of the bonds to a Si atom would effectively be missing an electron, and there
would be a HOLE!
So holes move in the way mentioned previously and act like free positive charges
moving through the crystal lattice. As the hole moves, it leaves behind a negatively
charged center. Overall then the lattice is neutral, since there are just as many holes
moving as there are negatively charged boron ions from which the holes have
moved.
Dopants that have one less electron than Si are called ________ because they
accept an electron to fill a hole in the crystal
These acceptor-doped crystals are known as: p-type because it has freely moving
holes which act like positive charges.
In a p-type material, the positive charges are the so called majority (charge) carriers
as they are much more abundant than any free electrons. The electrons are the
minority carriers in a p-type material.
In an n-type material, the negative charges are the majority carriers, and the minority
carriers are the holes.
So now we have altered material with donor and acceptor dopant materials. By
joining these two materials together, we can create an overall material which has an
excess of freed electrons on one side and an excess of moving holes on the other
side.
The line dividing the n-type from the p-type silicon establishes the position of a
potential barrier essential to the operation of a solar cell. When the n and p
materials are in contact, free electrons in the n-type material are adjacent to the
many holes in the p-type material. The free electrons at the junction will jump into
the p-type material filling the holes, also valence band electrons from the n-type
material can jump into the holes in the p-type material.
So negative charges move from the n-type to the p-type side making the n-doped
side have a POSITIVE CHARGE.
And positive charges move from the p-type to the n-type side making the p-doped
side have a NEGATIVE CHARGE.
This sets up the POTENTIAL BARRIER at the interface between the n-doped and
p-doped materials as the carriers that form the junction lose their freedom of
movement as they are trapped by the holes or filled by extra electrons.
In fact the barrier potential opposes the crossing of majority charge carriers, but not
the minority carriers.
Free electrons on the p-type side of which there are few, (ie. minority carrier) are
driven by the potential junction to the opposite n-type side (which is positively
charged).
Thus although a charge imbalance exists at the junction there are very few free
electrons on the p-type silicon side to be pulled back to the n-type side, and very
few free holes on the n-type side that can be transferred back to the p-type
material.
SO, the charge imbalance is fixed in place.
How far away from the interface between the n-doped and p-doped material does
this charge imbalance exist?
The first charges that cross the junction essentially set up an electric force field that
opposes the further flow of free carriers; more carriers, more electric field, harder
for later charges to cross the barrier.
Eventually an equilibrium is established where no more electrons or holes switch
sides, leaving behind a fixed potential barrier, a junction barrier.
But under no illumination, there are so few minority carriers on their
respective sides that their movement is negligible, and in fact the little
movement that occurs is balanced by the few majority carriers that have
enough thermal energy to cross the barrier.
Potential Barrier Benefits
Suppose light strikes the solar cell and the light is absorbed by the Si and it is enough
energy to free an electron from a bond in the Si crystal.
Suppose an
electron-hole pair results from this absorption on the p-type side of the
cell. The electron generated has only a relatively short time of life because it is likely to
combine with one of the numerous holes on the p-type side.
BUT solar cells are designed so that in all probability, the electron will meander around
the crystal encountering the junction before is has the chance to combine with a hole.
If the free electron is makes it to the field of the junction potential, the electron is
accelerated across the barrier by the charge imbalance present at the junction.
(Remember the n-type silicon has a positive charge, the p-type a negative charge at the
barrier.) And if the electron makes it to the n-type side, there are few holes there to trap
-it and so it survives a longer time. Also it will not return to the p-type side because it
would have to overcome the repulsion of the junction’s field.
The hole partner of the electron-hole pair, however, remains on the p-type side because
it is repelled by the junction potential AND furthermore is it in danger of recombining?
No because there are already many holes on the p-type side.
Does a similar situation exist when the light is generated on the n-type side of the
junction?
Here the freed electrons remain on the n-type side because they are repelled by the
barrier, and the holes encounter the junction before they recombine and cross to the
p-type side. Here the holes move around unhindered, and there are very few free
electrons available to fill them.
Thus, the illumination and charge separation cause the presence of uncombined
excess negative charges on the n-type side, and excess holes on the p-type side
and a charge imbalance exists in the cell.
If the n-type side is connected to the p-type side of the cell by means of an external
electric circuit, current flows through the circuit, why?
Reduces the charge imbalance that exists in the cell.
Figure
Negative charges flow out of the electrode on the n-type side, through a load (light
bulb, radio, etc.) and perform useful work on the load.
The light energy originally absorbed by the electrons is used up while the electrons
power the external circuit.
An EQUILIBRIUM can be maintained: the incident light continually
creates more electron-hole pairs and more charge imbalance, and the
imbalance is relieved by the current which gives up energy in
performing work.
The work performed could also be the charging of a battery.
The amount of light incident on the cell creates a nearly proportional
amount of current.
The amount of energy it takes to raise an electron to the conduction
band is the amount of energy the light imparts to the electron. It is thus
the Maximum that can be retrieved fro the electron in the external
circuit.
Summary:
Conditions necessary for solar cell to produce electricity!
1) incident light to free the charge carriers
2) a barrier to accelerate the carriers across the junction and keep them at
opposite ends of the cell
3) a charge imbalance to drive a current. through the circuit.
Solar Cell Efficiency
Most of the energy that reaches a cell in the form of light is lost before it can
be converted. Again typical efficiencies are in the range of 10-15%
Major phenomena limiting cell efficiency are:
1. Reflection from the cells surface
2. Light that is not energetic enough
3. Light having extra energy beyond needed to put the electron in the conduction
band.
4. Light-generated electrons and holes (empty bonds) that randomly encounter
each other and recombine before they can contribute to the cell performance
5. Light generated electrons and holes that are brought together by surface and
material defects in the cells.
6. Resistance to current flow.
7. Self shading resulting from top-surface electrical contacts.
8. Performance degradation at non-optimal (high or low) operating temps.
Reflection
Normal untreated silicon reflects 36% of the incident sunlight. With thin film
coatings of silicon nitride or texturing this can be cut to 5%.
Light with too Little Energy
For silicon, the energy associated with the band gap is 1.1eV. This energy is
specific to silicon. If light does not have this threshold of energy, then it will likely
go right through the solar cell or it could be absorbed and cause vibrations in the
atoms.
Light with too Much Energy
Here the light will free the electrons, but excess energy will cause atomic vibrations
in the lattice and then heating occurs.
Overall these inefficiencies result in about 55% of the energy from the original
sunlight being wasted. How much more efficient would it be if the sun’s spectrum
consisted of just one wavelength and that wavelength was matched exactly to the
energy required to generate the electron-hole pairs.
Recombination of Electron-Hole Pairs
This is inadvertent recombination of electrons and holes before they can contribute to
an electric current. Usually small losses because solar cells are designed to minimize
this.
a) Direct Recombination – happens when an electron and a hole randomly meet.
makes heat It is a problem only before the carriers are separated by light.
b) Indirect Recombination – occurs in many ways don’t just run into each other
much more of a problem than direct recombination.
dangling bonds, impurities, of fractures (defects)
Another mechanism is when a free charge carrier has a collision reducing its energy
allowing it to fall into a bond. Related to intrinsic resistance, more resistance – more
recombination, less current and voltage
The junction can also be a site of recombination. Fractures crossing the junction can
exist giving charge carriers a path that effectively avoids the junction’s field.
On average it has been shown that electrons and holes are lost via recombination in
about a hundredth of a second. Calculations show that the direct recombination of an
electron and hole would take place in about 1 second. Thus indirect recombination is
dominant, causing a hundred fold increase in the recombination rate.
Surfaces
Surfaces are also prone to dangling bonds, and allow collisions and are regions
where electrons and holes have an opportunity to combine.
Resistance
Resistance to flow of electrons occurs in any circuit. Ohms law R = V/I is not true
for solar cells because recombination losses are not proportional to resistance.
Resistance is ever-present where the flow of current is accompanied by collisions
between charge carriers and the material the charges are flowing through.
Resistance losses occur in:
a) the bulk of the base material
b) the narrow top surface
c) interface between the cell and the electrical contacts with the outside
Usually it is better to highly dope silicon to reduce resistance, but lattice damage
and eventually there can be so many free carriers that the junction no longer exists.
Self Shading
The electrical contacts should not be placed far away from the charge carriers.
The result is a grid of conductive material contacting the silicon, spread over the
top surface of the cell, and this grid shades the surface. Typical shading loss can
be as high as 8%, but some cells have losss as high as 20% and as low as 3%.
Performance at non-optimal Temps.
Silicon is a good material for ambient terrestrial temperatures.
At high temps., there is a significant drop off in efficiency.
High Temp. Losses
Physical effects related to temp are complex.
Figure
Two predominant causes for efficiency losses at high temp.
1) Lattice vibrations interfere with the free passage of charge carriers
2) Junction begins to lose its ability to separate charges.
1) Occurs even at room temp.
2) Does not occur until about 300oC.
At High Temps., huge numbers of electrons are jostled about and greatly
outnumber the free electrons donated by the n-type dopant.
Simultaneously, holes are formed on the n-type side, left behind by the jostled
free electrons.
The number of _______ _______ and holes becomes the same, and the n-doped
silicon loses it n-type character.
A similar process happens on the p side and it loses its p character.
Then
1) Thermally agitated charge carriers have so much energy, they cross over the
junction in both directions, as if the barrier were not there.
2) Ultimately, the junction itself disappears, no longer an n and p-type side.
Efficiency diminishes to zero.
Remember that light energy beyond the bandgap causes heating, so it is
imperative to either
Low Temp Losses
Two effects play a role,
1) As temp falls, thermal energy is less able to free charge carriers from
either dopant atoms or intrinsic silicon
Light generated charge carrier mobility drops.
2) At very low temp, so little energy that even dopants behave as if they
were normal silicon atoms.
n-type retains it extra electron, p-type its extra holes so junction
dissapears.
Schottky Barrier Cells
This is a solar cell whose junction is induced whn a metal contact is
applied to the surface of silicon that has been suitably doped.
Due to the electronic properties of the Si and the metal, the charge
carriers are distributed in side the surface similar to that of a p-n
junction.
Figure
Light striking the Schottky barrier cell, made with n-type silicon,
causes electron-hole pairs to be generated in the silicon.
Holes then migrate into the metal contact and electrons from the
metal migrate into the silicon. This causes a current to flow.
Figure
Advantage: Eliminates Top Surface doped collector layer.
Problems: The semitransparent metal on top has higher resistance than is
optimal since it so thin and thus additional contacts, a contact grid, must be
applied . This causes shading effects.
Also, high temp. is often used to apply the contact grid, but this damages the
thin metal of the Schottky barrier.
Additionally, slight fluctuations in the quality of the metal-silicon interface lead
to much lower electricity-producing efficiencies.
They have poor long term reliability, and efficiencies are consistently lower
than pn junction cells.
Two new variants of Schottky cell.
MIS (metal insulator semiconductor) cell
Silicon oxide (an insulator) was found to help, rather the efficiency of Schottky.
a metal, silicon oxide, silicon structure with the insulator thickness less than 20
angstroms which allows the passage of current during illumination
SIS (semiconductor-insulator-semiconductor) cell
Places the thin transparent later of metal on the surface of the cell with a
thicker layer of heavily doped, conductive transparent –semiconductor like
indium tin oxide.
Top layer acts like the metal inducing the junction, but does not need a metal
grid since its resistance is low enough.
MIS and SIS cells have similar problems to pure Schottky cells, with
degradation.
Results have been more favorable on polycrystalline and amorphous silicon
materials.
Inversion Layer Cells
A layer of SiO deposited on SiO2 coated with p-type silicon induces a junction near
the top of the p-type silicon.
Fig
Vapor SiO loses electrons as it solidifies, so SiO has a positive charge.
The positive charge pulls the few free electrons in the p-type silicon to the interface
between the Si and SiO2, causing that region to behave as if it were n-type Si.
The value of the insulating SiO2 layer is to prevent electrons from entering and
neutralizing the SiO.
Layer just below the SiO2 becomes n-type and the bulk silicon is p-type, a pn
junction is induced in the Si.
When illuminated, the junction separates electrons and holes just like
regular pn junction.
The light generated electrons have enough energy to tunnel through the
SiO2 and enter the metal contacts to the outside.
Advantage:
Simple to handle SiO and SiO2, induced junction is fault free, less doping
means less lattice defects
The efficiencies reported are as high as 17%