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The Photo Electric Effect Discovery, implications, and current technology Presentation by Ryan Smith.

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Presentation on theme: "The Photo Electric Effect Discovery, implications, and current technology Presentation by Ryan Smith."— Presentation transcript:

1 The Photo Electric Effect Discovery, implications, and current technology Presentation by Ryan Smith

2 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…

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: KE max = eV stopping 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 - Φ …Therefore light energy comes in multiples of hv

11 Graph of KE max vs. frequency

12 Quantum leap for quantum mechanics Wave-particle dualityWave-particle duality set the stage for 20 th 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 Electrons must exist only at specific energy levels within an atom Quantum Implications

14 Work Function Ionization Energy Φ represents how hard it is to remove an electron… Electron volts (eV) Varies slightly ΦΦ

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 Photo Voltaic 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) p-n junctionsIn 1883, the first real solar cell was built by Charles Fritts, forming p-n junctions by coating selenium with a thin gold layer.

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

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

20 Dopants add quantum energy levels Translate into bands in the solid semiconductor. majority charge carriersFormation of majority charge carriers on each side: Donor and Acceptor Bands N-TypeP-Type *extra negative electrons *extra positive holes from electron vacancies e -

21 Solar (PV) Cells: Each material by itself is electrically neutral, however… junctionJoining P- and N-Type materials together creates an electric field at the junction between them ~ equilibriumnet charge concentration 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. ew.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. 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. n.a. n.d. Timeline of Solar Cells. Retrieved 10-27-05. cells cells Robertson, E F. OConner, J J. A history of Quantum Mechanics. Retrieved 10-25-05. Smith, Willoughby. (1873). "Effect of Light on Selenium during the passage of an Electric Current". Nature, Vol ? 303. Available URL:

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