Solar Photovoltaic Physics

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This topicwill cover the fundamental semiconductor physics that makes solar photovoltaics possible and reviewthe materials needed to

 

make solar cells.For a basic understanding of solar photovoltaic physics see

 

“Solar Electricity”, 2ndEdition by Tomas Markvart (Editor), 2000.

 

For a more advanceddiscussion of solar photovoltaic physics see “The Physics of Solar Cells” byJenny Nelson, Imperial College

 

Press, 2003.

 

For a detaileddiscussion of the future of solar photovoltaic physics see “Third Generation Photovoltaics : Advanced

 

SolarEnergy Conversion” by Martin Green, Springer, 2003

 

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Photovoltaic (PV) systems convert light energy directly into electricity. They are commonly known as “solar cells.”

 

The simplest systems power the small calculators we use every day. More complicated systems will provide a large portion of the

 

electricity in the near future. PV represents one of the most promising means of maintaining our energy intensive (with appropriate

 

and technically viable efficiency gains) standard of living while not contributing to global warming and pollution. Solar Photovoltaics is

 

the future of energy

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Solar cell technology is actually pretty old. Thephotovoltaic effect was discovered in the late 1830’s. An explanation of thephotoelectric

 

effect won Einstein the Nobel prize. There was a big break throughin 1954 because the first crystalline silicon solar cell was developed.

 

Fouryears later it was used on a space satellite. The good news is that it worked,the bad news is that the solar cells kept working past

 

when NASA expected sothen the satellite kept sending data to earth when it was no longer needed andtook up E and M bandwidth

 

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In the mid 1970’speople were getting worried because of the oil crisis and OPEC. Because peoplewere worried, more thought and money was given to find

 

new and cheaper ways ofmaking solar cells. One development that came out of this research push was thefirst amorphous silicon cell developed by Chris

 

Wronsk iand Dave Carlson. The discovery of the amorphous silicon solar cells caused alot of excitement because it was a fundamentally cheaper material

 

than standardcrystalline silicon. This material is also used for the thin film transistorsthat drive modern flat panel displays. They have been mass produced real

 

cheap,and over the years the price for a flat panel has gone down significantly. Thesame is happening for solar cell prices

In the 1980’s thesolar photovoltaic community made steady progress towards higher efficiency andmany new types of solar cells were introduced so that by

 

the 1990’s there werelarge scale production of solar cells more than 10% efficient with the followingmaterials: Ga-As and otherIII-V’s, CuInSe2 and

 

CdTe,TiO2Dye-sensitized, and finally the largest producers Crystalline, Polycrystalline,and Amorphous Silicon solar cells.

 

Today prices continue to drop and new “3rdgeneration” solar cells are researched.

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Thisgraph show the record efficiency of different types of solar cells as afunction of time. What is important to see here is that all the

 

different typesof solar cells follow the same trend – as more research is undertaken theefficiencies go up which will drive down the

 

cost of solar electricity relativeto fossil fuel fired electricity

 

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Solarcells are made from semiconductors - themost important being silicon. Semiconductors have special electronic properties

 

which allow them to be insulating or conducting depending on their composition.In photovoltaic materials you are dealing with the

 

semiconductors (yellow).Most of the doping comes from boron and phosphorous for silcion solar cellsate. There are differenttypes

 

of solar cells such as cadmium telluride (CdTe)made from Cadmium and Tellurium or copper indium diselenide or gallium aresenide

 

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Mostsolar cells are made from silicon. Silicon has the diamond structure -a perfect tetrahedralcoordinated crystal. Each silicon atom

 

in the lattice is in an identicalposition to every other atom and each has four nearest neighbors. The 4electrons in Si form bonds to

 

the nearest neighbors (as seen by the red bondsin the figure). Silicon is a semiconductor and is the element largelyresponsible for

 

the integrated circuit, which makes modern computers possible