Photovoltaic Materials and Technology

Philip Griffin (pgriffi5@utk.edu)3/02/10

University of Tennessee- KnoxvilleDepartment of Physics

14 MW, 70,000 panel PV installation near Las Vegas, NV

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…

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

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

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)

Converting a p-n junction into a generator of electricity

J. L. Stone, Phys. Today 46, 22 (1993)

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)

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

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

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

Trends in PV research

http://en.wikipedia.org/wiki/File:PVeff%28rev110707%29d.png

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

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