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The Photo Electric Effect. Discovery, implications, and current technology. Presentation by Ryan Smith. Discovery: Heinrich Hertz and Phillip Lenard. Hertz clarified Maxwell ’ s electromagnetic theory of light: Proved that electricity can be transmitted in electromagnetic waves.

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The photo electric effect

The Photo Electric Effect

Discovery, implications, and current technology

Presentation by Ryan Smith


Discovery heinrich hertz and phillip lenard
Discovery: Heinrich Hertz and Phillip Lenard

  • Hertz clarified Maxwell’s 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.

Back in 1887…


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!



Lenard goes further
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:



The experiment
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.


Lenard s findings
Lenard’s 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.


Einstein s interpretation
Einstein’s Interpretation

A new theory of light:

  • Electromagnetic waves carry discrete energy packets

  • The energy per packet depends on wavelength, explaining Lenard’s 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”


Einstein s relations
Einstein’s 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 - Φ

…Therefore light energy comes in multiples of hv


Graph of ke max vs frequency
Graph of KEmax vs. frequency


Quantum leap for quantum mechanics
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.*


Quantum Implications

Electrons must exist only at specific energy levels within an atom 


Work function ionization energy

Φ

Φ

Work Function ≈ Ionization Energy

  • Φ represents how hard it is to remove an electron…

  • Electron volts (eV)

  • Varies slightly


Emergent applications
Emergent Applications…

  • Response is linear with light intensity

  • Extremely short response time

  • For example, night vision devices:


At Nearly the Same Time,

Another Discovery is under way….


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.


P- and N-type Materials

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


P- and N-type Materials

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


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


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.


Solar (PV) Cells:

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


The Electric Field Drives Current

  • Minority charge carriers are attracted to the junction

  • Majority charge carriers are repelled


Efficiency – the “Band Gap”

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



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


References
References:

Austin, Geoff. Jan 2005. Photo Electric Effect. Retrieved 10-23-05.

http://www.eequalsmcsquared.auckland.ac.nz/sites/emc2/tl/pee/overview.cfm

Einstein, 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/photoelectric/

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.about.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.lancs.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/encyclopedia/Timeline-of-solar-cells

Robertson, E F. O’Conner, J J. A history of Quantum Mechanics. Retrieved 10-25-05.

http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html

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


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