The
Basics of a Photovoltaic Solar Cell:
The
common solar cell that many of us have seen is called a photovoltaic solar
cell. These are the type of cells
found in solar-powered calculator and on satellites to generate electrical
energy. These cells convert light energy directly into
electrical energy.
These
cells are made up of silicon and are made up of semiconductors. An atom of silicon has 14 electrons arranged
in three general electron shells. The
first two shells closest to the nucleus are full. The outer shell has four electrons and is only half-full. However, silicon will and can share electrons
with its neighbors. In the case of
crystalline silicon, there are other silicon atoms that will provide electrons
for sharing.
However,
pure crystalline silicon is a poor conductor as there are no extra electrons
free to move about. Because of this
a solar cell is constructed with silicon mixed with impurities; this is called
doping the silicon. For example, phosphorus
atoms could be mixed with the silicon. Phosphorus has five electrons in its outer shell. It still bonds with the silicon atoms but now
there is an extra electron.
When
energy is added to the crystal, this extra electron can break free of bond
and this leaves a hole. Throughout
the lattice, we have a series of extra electrons moving freely of their bonds;
these electrons are called free carriers.
When
silicon is doped with phosphorus, it is called an N-type crystal as it has
extra electrons (N for negative). Part
of a common photovoltaic cell is created as an N-type crystal.
However,
it is also possible to make P-type (positive) silicon but doping it with a
substance having three electrons instead of five. Instead of using phosphorus, born can be used. Boron has three valence or outer electrons.
It, consequently, has more holes available for the free electrons created
when the N-type silicon is energized. A
hole can be thought of as an absence of an electron or a positive charge. The holes can appear to move just like free electrons. Both electrons and holes can be called charge
carriers.
The
common photovoltaic cell has a N-type silicon put together with a P-type. When placed atop each other the free electrons
in the N-type move into the holes available in the P-type. This occurs immediately when the two types
of silicon are first brought together, but only close to the contact surface
of each type.
The
N-type silicon becomes positively charged at its surface and the P-type silicon
becomes negatively charged at its surface due to this migration of electrons.
This creates an electric field when both types are in contact.
When the two types of silicon are sandwiched, there is a creation of
a ‘p-n junction’, the name of the interface between both types of silicon. This p-n junction acts as a diode permitting
electric current to flow in one direction only. At this junction, there are virtually no mobile charge carriers.
However, the electric field pulls electrons and holes in opposing directions. Electrons are forced to flow from the P-type silicon to the N-type.
Sunlight
is composed of packets of light energy called photons. When a photon of the correct energy strikes
the solar cell, it can knock out lattice electrons from outer valence electron
orbitals (or bands) and energize these electrons into the conduction band. This leaves a hole in the valence band.
Excess
heat is often produced and this represents energy unavailable to be converted
into electricity. If these electron-hole
pairs are created in the correct location in the solar cell (near the p-n
junction), the electrons will be attracted to the positive n-type silicon
and the holes will be attracted to the negative p-type silicon.
From what I can tell, the base (boron-doped,
P-type silicon) makes up a thicker part of the solar cell than the emitter
(phosphorus-doped, N-type silicon). The emitter is around 1 micron or less in thickness while the total
cell thickness can be up to 500 microns.
To construct the crystalline silicon cell, the boron-doped, P-type
silicon is layered to a conductive back contact. The back contact is aluminum or some alloy. An antireflection coating similar to those
used on cameras and other optical equipment is used to limit the reflection
of sunlight on the top of the emitter.
The
busbar and fingers represent the top electrode. It is constructed to allow ample sunlight into the emitter and base
and still allow a pathway for electrical flow of current.
As
photons hit the silicon layers, some of these photons create electron-hole
pairs. These freed electrons and hole
drift and some recombine and are useless for electricity generation. Some do reach the p-n junction; electrons are
attracted to the emitter (N-type, phosphorus-doped) region and holes are attracted
to the base. The electrons attracted
to the emitter can reach the fingers and from there to the bus. If the circuit is complete (external load or
complete pathway), these energized electrons can give up part of its extra
energy as electrical power and return to recombine with the extra holes via
the rear contact.
Each
cell produces about 0.5 Volts with a power output of approximately .05
Watts. These are pictures of me
checking the voltage and current of a silicon solar cell using a multimeter. On the right side, I am checking the maximum
voltage—I get 0.488 Volts. On the left
side, I get a maximum current of about 100 milliAmperes (mA)
Solar cells are added together into a unit called a module.
Modules are linked together in units called arrays.
Arrays put onto roofs of house may produce up to 6000 Watts which would
be enough to provide for the needs of the occupants.
These
basic silicon-based crystalline solar cells have an efficiency of about 15%.
Over 50% of the photons that hit the silicon solar cell cannot be utilized
for energy because they either don’t have enough energy to create electron-hole
pairs or are too energetic and create energy along with electron-hole pairs.
Even if electron-holes are created, they often recombine before they
can be split into useable current. Some
of the sunlight is reflected from the front and because of this isn’t absorbed
into the silicon. Additionally, there
are resistive effects in the silicon and the circuit that can lead to efficiencies.
One
way to analyze a solar cell is with a current-voltage graph. A resistance is placed in a simple circuit
and the voltage and current is measured.
The resistance is then changed and new voltage and current readings
are obtained. Here is my current-voltage
graph for the solar cell I was using. I
obtained this graph at about 1:45 pm on a sunny day using resistances varying
from 0 Ohms to 2200 Ohms.
It
is electrical power that is important for a solar cell. Power is the product of voltage and current. Additionally because the photons are striking
an area, we can calculate the power per area of solar cell. This quantity is called the power density. The solar cell is 8 cm long and 5.5 cm wide.
Consequently, it has an area of 44 cm^2. The
following is my graph of the power density of the solar cell.
On
this graph, we get a peak power density.
This point represents the maximum power point. We can determine the electrical energy conversion efficiency at
this point for the solar cell. The
insolation value is the incoming solar energy that strikes the ground at a
particular city. For Kansas City,
I found that our insolation level was 25.7 mW/cm^2.
Peak
power density = 0.251 mW/cm^2
Electrical
energy conversion efficiency = 0.251 mW/cm^2 / 25.7 mW/cm^2
= 0.0098. Multiplying this value by
100% gives me an electrical energy conversion efficiency of around 1% for
this solar cell.
Even with this low efficiency, I was able to power a small motor:
Solar
cell powering motor (quicktime, 1.12 MB)
Solar
cell powering motor (windows media, 1.39 MB)
Surface Structure: In this case, the silicon is structured in pyramid shapes.
This allows more effective light absorption with less reflective lose.
Tandem, multi-layered, or stacked cells: As mentioned earlier, only photons above around 1 eV can create electron-hole pairs in typical phosphorus and boron-doped silicon. By using other elements as doping agents, other photon energies can be utilized. Silicon doped with these other elements can be stacked together to form a multilayer cell that can reach efficiencies of around 40%.
| Layer | Band Gap (eV) | Light Absorbed |
| Gallium indium phosphide | 1.8 | UV, visible |
| Gallium arsenide | 1.4 | near infrared |
| Germanium | 0.7 | far infrared |
Concentrator cells: A higher light intensity will be focused on the solar cells by the use of mirror and lens systems. This system tracks the sun, always using direct radiation. For example, in the picture below, lenses concentrate solar energy on the array to produce an equivalent of 250 suns.
Thin Film Photovoltaics:
Technologies are being researched now to create silicon photovoltaics using
non-crystalline silicon. This non-crystalline form of silicon is called amorphous
silicon and it can be doped in similar ways to the crystalline form. Additionally,
it can be constructed in thin films which reduce the costs and the amounts
of silicon needed. However, these photovoltaics are less efficient at converting
solar energy to electrical energy. However, they do have the advantage of
being flexible and light.
Resources:
http://www.youtube.com/watch?v=u0hckM8TKY0 (video on the physics of the silicon solar cell)
http://www.soton.ac.uk/~solar/intro/tech6.htm (pictures and information)
http://www.udel.edu/igert/pvcdrom/ (animation, pictures, and information)
http://www.solarserver.de/wissen/photovoltaik-e.html (information)
http://www.doe.gov/energysources/solar.htm (U.S. Department of Energy)
http://projectsol.aps.com/ (information and pictures)
http://www.pbs.org/wgbh/nova/solar/ (pbs nova episode about solar energy)
http://www.tamilmovies.net/modules.php?name=Tamil_Movie_Wikipedia&title=Photoelectric_cell (information)
http://www.allaboutcircuits.com/vol_3/chpt_3/12.html (information about diodes)
http://www.solarnavigator.net/solar_cells.htm (information and pictures about solar cells)
http://www.apricus.com/html/insolation_levels_usa.htm (insolation values for cities in the US)
http://www.uni-solar.com/ (thin film photovoltaic technology)
http://www.g24i.com/ (Grätzel cell & thin film flexible photovoltaic technology)
http://www.nanowerk.com/spotlight/spotid=1500.php (carbon nanotubes in photovoltaics)
http://en.wikipedia.org/wiki/Carbon_nanotubes_in_photovoltaics (carbon nanotubes in photovoltaics)
http://spie.org/x18499.xml (using nanotubes in dye-sensitized solar cells)
Alan D. Gleue 
Lawrence High School Science Department
Created: June, 2008.
Modified: July, 2008.