Transcript photoeffect
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 waves. – Established that light was a form of electromagnetic radiation. – 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 to: – 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 Φ Work Function ≈ Ionization Energy • Φ represents how hard it is to remove an electron… • Electron volts (eV) • Varies slightly Φ 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: P-Type e- e- N-Type *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 down References: Austin, Geoff. Jan 2005. Photo Electric Effect. Retrieved 10-23-05. http://www.eequalsmcsquared.auckland.ac.nz/sites/emc2/tl/pee/overvi 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. 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-solarcells Robertson, E F. O’Conner, J J. A history of Quantum Mechanics. Retrieved 10-25-05. http://www-groups.dcs.stand.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