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Solar Electricity

Solar Electricity. Light energy, one photon at a time. Outline. Photoelectric Effect The existence of photons Light energy in bundles Light energy creating an electric potential Photovoltaic cells Semiconductors Energy band gaps Silicon, Gallium Arsenide, Thin Films.

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Solar Electricity

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  1. Solar Electricity Light energy, one photon at a time

  2. Outline • Photoelectric Effect • The existence of photons • Light energy in bundles • Light energy creating an electric potential • Photovoltaic cells • Semiconductors • Energy band gaps • Silicon, Gallium Arsenide, Thin Films

  3. Light is a wave, or is it? • Waves carry energy in proportion to their amplitude • Light wave amplitude is Intensity. • So light energy should depend on intensity • The brighter the light, the more energy it carries.

  4. The Photoelectric Effect. • The Experiment. Electric Current electrons Light source Metal Plate

  5. Light making a current • Light shining on a metal can create a current: The Photoelectric effect. • Experiment: • VARY the light frequency (or wavelength) but keep the intensity the same. • OBSERVE the Voltage required to stop a current from flowing (the “stopping voltage”) • This should be a measure of the energy the electrons have as they leave the metal plate.

  6. Photoelectric effect and light frequency

  7. Analysis • Observations: • For each metal, there was a certain frequency below which no amount of light could cause a current. • Above that frequency, there was some current no matter how weak the light source. • The energy the electrons in the current had directly depended on the frequency of the light • Although MORE CURRENT could be produced with higher intensity light, the STOPPING VOLTAGE depended only on the frequency.

  8. Einstein’s explanation • Published in 1905; Nobel Prize in 1921. • Light energy comes in packets, called “photons” • Each photon carries an energy equal to: • Eph = h f, where f is the frequency, and h = 6.63 x 10-34 J*s is called Planck’s constant. • The electrons absorb one photon at a time • The electrons need a minimum of energy to escape the metal surface, hence the minimum f. • The photon energy above the minimum goes into the energy of the current, hence the increasing V.

  9. Photon Energy • Eph = h f • Red light has f = 4.5 x 1014 Hz • Ered = (6.63 x 10-34 J*s ) (4.5 x 1014 Hz) • = 2.98 x 10-19 Joules! • We can’t feel that, but what about an electron? • A new unit of energy: • 1 eV, the amount of energy required to move a single electron across a 1 Volt electric potential. • 1 eV = 1.6 x 10-19 Joules • Ered = 1.86 eV • Red photons have enough energy to charge up a 1.8 V battery!

  10. The light energy spectrum • Light frequency corresponds to photon energy. • The most convenient unit is eV. • h = 6.63 x 10-34J*s = 4.15 x 10-15eV*s • Visible light from 1.8 (red) to 3 eV (violet)

  11. Photovoltaic Cells electron n-type Silicon + + + + + 1.5-2 V - - p-type Silicon - - -

  12. The p-n junction

  13. PV-types and applications • Each cell produces .5 to 1.5 V, 8-24 cells in series produce 12 V. Highest efficiency – 23% • Types: • Crystalline Silicon: More efficient/expensive • Amorphous Silicon: Less efficient/expensive • Thin Film: Silicon or other semiconductors • Applications: • Remote locations • Homes more than ¼ of mile from electric grid. • Watches, calculators

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