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The Photo Electric Effect

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Title: The Photo Electric Effect


1
The Photo Electric Effect
  • Discovery, implications, and current technology

Presentation by Ryan Smith
2
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.

3
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!

4
Hertzs Spark Gap Generator
5
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

6
Lenards Photoelectric Apparatus
7
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.

8
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.

9
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
10
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
11
Graph of KEmax vs. frequency
12
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.
13
Quantum Implications
Electrons must exist only at specific energy
levels within an atom ??
14
Work Function Ionization Energy
F
F
  • F represents how hard it is to remove an
    electron
  • Electron volts (eV)
  • Varies slightly

15
Emergent Applications
  • Response is linear with light intensity
  • Extremely short response time
  • For example, night vision devices

16
At Nearly the Same Time, Another Discovery is
under way.
17
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.

18
P- and N-type Materials
  • N-Type Requires doping a material with atoms of
    similar size, but having more valence electrons.
    ex/ SiAs

19
P- and N-type Materials
  • P-Type Requires doping a material with atoms of
    similar size, but having fewer valence electrons.
    ex/ SiGa

20
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
21
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.
22
Solar (PV) Cells
  • A photon is absorbed by the material near the P-N
    junction, creating an electron/hole pair

23
The Electric Field Drives Current
  • Minority charge carriers are attracted to the
    junction
  • Majority charge carriers are repelled

24
Efficiency the Band Gap
  • Only the right frequencies of light let an
    electron cross the junction, or band gap.

25
The Big Picture
26
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

27
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
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