The Photo Electric EffectThe Photo Electric Effect

Discovery, implications, and Discovery, implications, and current technologycurrent technology

Presentation by Ryan Smith

Discovery:Discovery: Heinrich Hertz and Phillip Lenard Heinrich Hertz and Phillip Lenard

Hertz clarified Maxwell’s electromagnetic theory of light:Hertz clarified Maxwell’s electromagnetic theory of light:– Proved that electricity can be transmitted in electromagnetic Proved that electricity can be transmitted in electromagnetic

waves.waves.– Established that light was a form of electromagnetic Established that light was a form of electromagnetic

radiation.radiation.– First person to broadcast and receive these waves.First person to broadcast and receive these waves.

Back in 1887…

The Spark Gap GeneratorThe Spark Gap Generator

First observed the effect while working with a First observed the effect while working with a spark-gap generatorspark-gap generator ~ accidentally, of course ~ accidentally, of course

Illuminated his device with ultraviolet light: Illuminated his device with ultraviolet light:

– This changed the voltage at which sparks This changed the voltage at which sparks appeared between his electrodes!appeared between his electrodes!

Hertz’s Spark Gap Generator:

Lenard Goes Further…Lenard Goes Further…

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

Lenard’s Photoelectric Apparatus:

The Experiment:The Experiment:

By varying the voltage on a By varying the voltage on a negatively charged gridnegatively charged grid between between the ejecting surface and the collector plate, Lenard was able the ejecting surface and the collector plate, Lenard was able to:to:

– Determine that the particles had a negative charge.Determine that the particles had a negative charge.

– Determine the kinetic energy of the ejected particles.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:

KEKEmaxmax = eV = eVstoppingstopping

Perplexing Observations:

» The intensity of light had no effect on energy

» There was a threshold frequency for ejection

Classical physics failed to explain this,Classical physics failed to explain this,Lenard won the Nobel Prize in Physics in 1905.Lenard won the Nobel Prize in Physics in 1905.

Einstein’s InterpretationA 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 - ΦKE = hv - Φ

…Therefore light energy comes in multiples of hv

Graph of KEGraph of KEmaxmax vs. frequency vs. frequency

Quantum leap for quantum mechanics

• Wave-particle dualityWave-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.*

Electrons must exist only at specific energy levels within an atom

Quantum ImplicationsQuantum Implications

Work Function Work Function ≈ Ionization Energy≈ Ionization Energy

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

• Electron volts (eV)

• Varies slightly

ΦΦ

Emergent Applications…Emergent Applications…

Response is linear with light intensityResponse is linear with light intensity

Extremely short response timeExtremely short response time

For example, night vision devices:For example, night vision devices:

At Nearly the Same Time,

Another Discovery is under way….

The PhotoThe PhotoVVoollttaaiicc Effect: 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 junctionsp-n junctions by coating selenium with a thin gold layer.

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

P- and N-type MaterialsP- and N-type Materials

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

P- and N-type MaterialsP- and N-type Materials

•Dopants add quantum energy levels

•Translate into bands in the solid semiconductor.

•Formation of majority charge carriersmajority charge carriers on each side:

Donor and Acceptor BandsDonor and Acceptor Bands

N-Type P-Type

*extra negative electrons *extra positive “holes” from electron vacancies

e-

e-

Solar (PV) Cells:•Each material by itself is electrically neutral, however…•Joining P- and N-Type materials together creates an electric field at the junctionjunction between them ~

An equilibriumequilibrium is reached where a net charge concentrationnet 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 materialuse and cost.

•Bring the current price per watt down

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