Solar Electricity
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Transcript of Solar Electricity
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Solar Electricity
Light energy, one photon at a time
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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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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 intensityThe brighter the light, the more energy it
carries.
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The Photoelectric Effect.
The Experiment.
Light source
Metal Plate
electrons
ElectricCurrent
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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.
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Photoelectric effect and light frequency
Frequency vs. Stopping Voltage
0
1
2
3
4
5
6
0.00E+00 2.00E+14 4.00E+14 6.00E+14 8.00E+14 1.00E+15
Frequency (Hz)
Vol
ts
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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.
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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 timeThe 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.
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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!
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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*sVisible light from 1.8 (red) to 3 eV (violet)
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Photovoltaic Cells
n-type Silicon
p-type Silicon
+ + + + +
- - - - - 1.5-2 V
electron
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The p-n junction
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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