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. Some photons do not carry enough energy to create the electron-hole pair.  Only about two-thirds of the photons from the sun cannot be utilized by a silicon solar cell.

 

 

 

 

 

 

 

 

 

 

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.

Photons absorb into electron-hole pairs, which diffuse to contacts of solar cell

 

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)

 

 

I have created several videos of testing the solar cell. See my youtube channel (http://www.youtube.com/view_all_playlists)

 

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:

 

 
See my youtube channel (http://www.youtube.com/view_all_playlists)

Several New Directions in Photovoltaics

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.


 

 

Grätzel cells (dye-sensitized solar cell): Electrochemical liquid cells using nano-titanium dioxide as a substrate. A type of a dye is added to the substrate and an electrolyte is used to provide electrons. The sun or light source energizes the electrons to provide electrical power. These cells were invented by Michael Grätzel and Brian O'Regan in 1991 in Switzerland and are promising because they can be constructed with low-cost materials with relatively little elaborate manufacturing apparatus. At this point, they are less efficient than other types of photovoltaics but their relatively inexpensive production cost should allow them to compete with fossil fuels.

Carbon nanotubes in photovoltaics: Researchers are now starting to integrate carbon-based nano-tubes into different types of photovoltaics. These nanotubes have been used to more efficiently channel electrons away from the electron-hole pairs and as electrodes. They have been used in traditional silicon-based photovoltaics, as well as with thin films and dye-sensitized solar cells. It is the goal of researches to make more efficient photovoltaics with the carbon nanotubes.

 

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

Lawrence High School Science Department

back to Mr. Gleue's home page


Created: June, 2008.
Modified: July, 2008.