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

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.

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

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.

Photoelectric effect and light frequency

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.

Einstein’s explanation  Published in 1905; Nobel Prize in  Light energy comes in packets, called “photons”  Each photon carries an energy equal to: – E ph = h f, where f is the frequency, and h = 6.63 x 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.

Photon Energy  E ph = h f –Red light has f = 4.5 x Hz –E red = (6.63 x J*s ) (4.5 x Hz) = 2.98 x 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 Joules  E red = 1.86 eV –Red photons have enough energy to charge up a 1.8 V battery!

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

Photovoltaic Cells n-type Silicon p-type Silicon V electron

The p-n junction

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