The Photo Electric Effect

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The Photo Electric Effect

• Discovery, implications, and current technology

Presentation by Ryan Smith
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Discovery Heinrich Hertz and Phillip Lenard
Back in 1887

• Hertz clarified Maxwells electromagnetic theory
  of light
• Proved that electricity can be transmitted in
  electromagnetic waves.
• Established that light was a form of
  electromagnetic radiation.
• First person to broadcast and receive these waves.

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The Spark Gap Generator

• First observed the effect while working with a
  spark-gap generator accidentally, of course
• Illuminated his device with ultraviolet light
• This changed the voltage at which sparks appeared
  between his electrodes!

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Hertzs Spark Gap Generator
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Lenard Goes Further

• His assistant, Phillip Lenard, explored the
  effect further. He built his own apparatus called
  a phototube to determine the nature of the
  effect

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Lenards Photoelectric Apparatus
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The Experiment

• By varying the voltage on a negatively charged
  grid between the ejecting surface and the
  collector plate, Lenard was able to
• Determine that the particles had a negative
  charge.
• Determine the kinetic energy of the ejected
  particles.

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Lenards Findings

• Thus he theorized that this voltage must be equal
  to the maximum kinetic energy of the ejected
  particles, or
• KEmax eVstopping
• Perplexing Observations
• The intensity of light had no effect on energy
• There was a threshold frequency for ejection
• Classical physics failed to explain this,
• Lenard won the Nobel Prize in Physics in 1905.

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Einsteins Interpretation

• A new theory of light
• Electromagnetic waves carry discrete energy
  packets
• The energy per packet depends on wavelength,
  explaining Lenards threshold frequency.
• More intense light corresponds to more photons,
  not higher energy photons.

This was published in his famous 1905 paper On
a Heuristic Point of View About the Creation and
Conversion of Light
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Einsteins Relations

• Einstein predicted that a graph of the maximum
  kinetic energy versus frequency would be a
  straight line, given by the linear relation
• KE hv - F

Therefore light energy comes in multiples of hv
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Graph of KEmax vs. frequency
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Quantum leap for quantum mechanics

• Wave-particle duality set the stage for 20th
  century quantum mechanics.
• In 1924, Einstein wrote
• There are therefore now two theories of light,
  both indispensable, and - as one must admit today
  despite twenty years of tremendous effort on the
  part of theoretical physicists - without any
  logical connection.

This work won Einstein his Nobel Prize in 1922.
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Quantum Implications
Electrons must exist only at specific energy
levels within an atom ??
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Work Function Ionization Energy
F
F

• F represents how hard it is to remove an
  electron
• Electron volts (eV)
• Varies slightly

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Emergent Applications

• Response is linear with light intensity
• Extremely short response time
• For example, night vision devices

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At Nearly the Same Time, Another Discovery is
under way.
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The PhotoVoltaic Effect

• Same basic principle as the photoelectric effect
• HISTORY
• In 1839, Alexandre Edmond Becquerel
• In 1873, Willoughby Smith
• In 1876, William Grylls Adams (with his student
  R. E. Day)
• In 1883, the first real solar cell was built by
  Charles Fritts, forming p-n junctions by coating
  selenium with a thin gold layer.

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P- and N-type Materials

• N-Type Requires doping a material with atoms of
  similar size, but having more valence electrons.
  ex/ SiAs

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P- and N-type Materials

• P-Type Requires doping a material with atoms of
  similar size, but having fewer valence electrons.
  ex/ SiGa

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Donor and Acceptor Bands

• Dopants add quantum energy levels
• Translate into bands in the solid semiconductor.
• Formation of majority charge carriers on each
  side

N-Type
P-Type
e- ?
e- ?
extra positive holes from electron vacancies
extra negative electrons
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Solar (PV) Cells

• Each material by itself is electrically neutral,
  however
• Joining P- and N-Type materials together creates
  an electric field at the junction between them

An equilibrium is reached where a net charge
concentration exists on each side of the
junction.
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Solar (PV) Cells

• A photon is absorbed by the material near the P-N
  junction, creating an electron/hole pair

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The Electric Field Drives Current

• Minority charge carriers are attracted to the
  junction
• Majority charge carriers are repelled

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Efficiency the Band Gap

• Only the right frequencies of light let an
  electron cross the junction, or band gap.

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The Big Picture
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Hopes for the Future

• Multi-junction solar cells
• improve efficiency.
• Thin-film P-N junction
• solar cells reduce material
• use and cost.
• Bring the current price per watt down

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References
Austin, Geoff. Jan 2005. Photo Electric Effect.
Retrieved 10-23-05. http//www.eequalsmcsquared.au
ckland.ac.nz/sites/emc2/tl/pee/overview.cfm Einst
ein, Albert. (1905). On a Heuristic Viewpoint
Concerning the Production and Transformation of
Light. Annalen der Physik, Vol 17, 132. Elert,
Glenn. Photoelectric Effect. Retrieved 10-28-05.
http//hypertextbook.com/physics/modern/photoelec
tric/ Hamakawa, Yoshihiro. (2004). Thin-Film
Solar Cells Next generation photovoltaics and
its application. New York Springer. Lenardic,
Denis. A Walk Through Time. Retrieved 11-12-05.
http//www.pvresources.com/en/history.php U.S.
DOE Photovoltaics Program. (2005). Photovoltaics
Timeline. Retrieved 10-27-05. http//inventors.abo
ut.com/library/inventors/blsolar2.html
n.a. n.d. Philipp Lenard Biography. Retrieved
10-23-05. http//nobelprize.org/physics/laureates
/1905/lenard-bio.html n.a. n.d. The Photo
Electric Effect. Retrieved 10-06-05. http//www.la
ncs.ac.uk/ug/jacksom2/ n.a. n.d. The Electric
Field In Action. Retrieved 11-12-05.
http//www.sandia.gov/pv/docs/PVFEffElectric_Field
.htm n.a. n.d. Timeline of Solar Cells.
Retrieved 10-27-05. http//www.nationmaster.com/en
cyclopedia/Timeline-of-solar-cells Robertson, E
F. OConner, J J. A history of Quantum Mechanics.
Retrieved 10-25-05. http//www-groups.dcs.st-and.a
c.uk/history/HistTopics/The_Quantum_age_begins.ht
ml Smith, Willoughby. (1873). "Effect of Light
on Selenium during the passage of an Electric
Current". Nature, Vol ? 303. Available URL
http//histv2.free.fr/selenium/smith.htm