The Photo Electric Effect

The Photo Electric Effect
Discovery, implications, and
current technology
Presentation by Ryan Smith
Discovery: Heinrich Hertz and Phillip Lenard
Back in 1887…
Hertz clarified Maxwell’s electromagnetic theory of light:
– Proved that electricity can be transmitted in electromagnetic
– Established that light was a form of electromagnetic
– First person to broadcast and receive these waves.
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!
Hertz’s Spark Gap Generator:
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:
Lenard’s Photoelectric Apparatus:
The Experiment:
By varying the voltage on a negatively charged grid between
the ejecting surface and the collector plate, Lenard was able
– Determine that the particles had a negative charge.
– Determine the kinetic energy of the ejected particles.
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
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 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 KEmax vs. frequency
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 represents how hard it is to remove an electron…
• Electron volts (eV)
• Varies slightly
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
• 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:
e- 
e- 
*extra negative electrons
*extra positive “holes”
from electron vacancies
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”.
The Big Picture:
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
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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.
U.S. DOE Photovoltaics Program. (2005). Photovoltaics Timeline.
Retrieved 10-27-05.
n.a. n.d. Philipp Lenard – Biography. Retrieved 10-23-05.
n.a. n.d. The Photo Electric Effect. Retrieved 10-06-05.
n.a. n.d. The Electric Field In Action. Retrieved 11-12-05.
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Mechanics. Retrieved 10-25-05.
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the passage of an Electric Current". Nature, Vol ? 303.
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