photovoltaic materials and technology philip griffin ([email protected]) 3/02/10 university of...
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Photovoltaic Materials and Technology
Philip Griffin ([email protected])3/02/10
University of Tennessee- KnoxvilleDepartment of Physics
14 MW, 70,000 panel PV installation near Las Vegas, NV
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The photovoltaic effect was discovered in 1839 by Alexandre-Edmond Becquerel.
• In electrolytic cells made of two metal electrodes, small (measurable) amount of current flowed when the cell was illuminated.
• Effect only occurred for select materials.
Becquerel was only 19 when he discovered the photovoltaic effect…
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The first PV module was created at Bell Labs in 1954.
• Known as a “solar battery”
• Achieved an efficiency of 6%
• Individual cells were single crystals of doped silicon
• 1962- this type of solar cell was used to power communications satellites on long term space missions
http://www.porticus.org/bell/belllabs_photovoltaics.html
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In pure semiconductors, intrinsic carriers can be created by optical absorption.
• Photon with same energy as the band gap of the semiconductor an electron will be excited from the valence to the conduction band
• Excess or lack of photon energy will excite phonons in material and create heat (wasted energy)
• The optimum material will have a bandgap that is close to the sun’s maximum intensity energy
http://en.wikipedia.org/wiki/File:Solar_Spectrum.png
Material Gap (eV) Gap (nm)
Si 1.11 1117
CdTe 1.49 832
GaAs 1.43 867
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The modern solar cell owes its functionality to the p-n junction.
• n-type: free electrons leave donor atoms (+) ionized
• p-type: free holes leave donor atoms (-) ionized
• Near junction, free electrons/holes diffuse to balance ionized donor impurities
• On the n-side (p-side) of the junction, fixed positive (negative) charge accumulates
• An intrinsic electric field builds, opposing the recombination of electron hole pairsJ. L. Stone, Phys. Today 46, 22 (1993)
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Converting a p-n junction into a generator of electricity
J. L. Stone, Phys. Today 46, 22 (1993)
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Modern PV technology is based largely on bulk and thin film silicon.
A. Goetzberger, C. Hebling, H. W. Schock, Mat. Sci. Eng. R 40, 1 (2003)M. Gratzel, Nature 414, 338 (2001)
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Mono-crystalline silicon solar cells
• Manufacturing process is already well established by the microelectronics industry
• Large cylindrical bulk crystals grown by various techniques
• Bulk crystal is then sawed or cut into individual wafers
• On average, nearly half of the material is lost in cutting
http://en.wikipedia.org/wiki/File:Solar_cell.png
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Polycrystalline silicon solar cells
• Polycrystalline solar cells are cast in a mold or drawn into film-like ribbons
• This process creates multiple crystallites in one piece of material
• The boundaries between crystallites create an alternative pathway for carriers to recombine
• This reduces the efficiency when compared to monocrystalline silicon solar cells
http://en.wikipedia.org/wiki/File:Polycristalline-silicon-wafer_20060626_568.jpg
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U.S. PV energy production is minimal. Will its presence grow in the country’s energy profile?
Industry trends seem to indicate a possibility of this!
http://www.eia.doe.gov/aer/pdf/aer.pdf
http://www.eia.doe.gov/aer/pdf/aer.pdf
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Trends in PV research
http://en.wikipedia.org/wiki/File:PVeff%28rev110707%29d.png
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Large scale PV energy production will become reality only when the associated cost are decreased.
• Average cost of fossil fuel power in US is $0.1/kWh• Average cost of PV power is ≈ $0.3/kWh• If costs for PV installation can be brought below
$3/(peak)W, PV electricity prices will become competitive
http://www.eia.doe.gov/aer/pdf/aer.pdf
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Photovoltaic energy production is a safe, clean, and reliable way to create electricity.
• The main problem in the field today is to reduce the cost of cell production.
• Will novel technologies and materials accomplish this or will economies of scale accomplish this?
• A solar installation of land area ≈ 22,500 km2 could theoretically provide for the power needs of the entire US (land area 9,826,675 km2)
• US consumed 3.8 trillion kWh of energy in 2008