Presentation on theme: "The Photo Electric Effect"— Presentation transcript:
1The Photo Electric Effect Discovery, implications, and current technologyPresentation by Ryan Smith
2Discovery: 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.
3The Spark Gap Generator First observed the effect while working with a spark-gap generator ~ accidentally, of courseIlluminated his device with ultraviolet light:This changed the voltage at which sparks appeared between his electrodes!
4Hertz’s Spark Gap Generator: Light is causing an electrical response in metals
5Lenard 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:Light caused current to flow in an open circuit
7The 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.He found that by turning up the voltage he reached a point where the current in his circuit ceased to flow because no more electrons were making it to the other side of the gap. This was noted and dubbed the stopping voltage.
8Perplexing Observations: Lenard’s Findings:Thus he theorized that this voltage must be equal to the maximum kinetic energy of the ejected particles, or:KEmax = eVstoppingPerplexing Observations:The intensity of light had no effect on energyThere was a threshold frequency for ejectionClassical physics failed to explain this,Lenard won the Nobel Prize in Physics in 1905.
9Einstein’s Interpretation A new theory of light:Electromagnetic waves carry discrete energy packetsThe energy per packet depends on wavelength, explaining Lenard’s threshold frequency.More intense light corresponds to more photons, not higher energy photons.*For example, a dim blue light will eject electrons from a particular metal while a bright red light will not because the blue light comes in higher energy packets which are able to knock loose the electrons.This was published in his famous 1905 paper:“On a Heuristic Point of View About the Creation and Conversion of Light”
10Einstein’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 hvWhere h is planks constant and v is the frequency. The photon model also explains why there is no delay in the ejection of electrons…
12Quantum 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.”Einstein’s predictions were proved shortly after by the work of Millikan and others, and*This work won Einstein his Nobel Prize in 1922.*
13Quantum ImplicationsElectrons must exist only at specific energy levels within an atom Because the quanta of energy carried by photons can only be wholly absorbed by atoms, the theory demanded that
14Work Function ≈ Ionization Energy ΦΦΦ represents how hard it is to remove an electron…Electron volts (eV)Varies slightlyCorresponds to the energy difference between the vacuum level and the valence level on the energy diagram.essentially the first ionization energy - but slight differences occur due to forces from the enormous number of surrounding atoms in a metal
15Emergent Applications… Response is linear with light intensityExtremely short response timeFor example, night vision devices:Devices employing the photoelectric effect take advantage of… Electrons are carried away from the photocathode through a disk with millions of channels in it, amplified, then strike a phosphorescent screen on the other side, reproducing a real image.
16At Nearly the Same Time,Another Discovery is under way….
17Same basic principle as the photoelectric effect The PhotoVoltaic Effect:Same basic principle as the photoelectric effectHISTORYIn 1839, Alexandre Edmond BecquerelIn 1873, Willoughby SmithIn 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.Same basic principle as the photoelectric effect: Incoming photons excite electrons into a conductive energy level. Voltage results without the ejection of electrons.HISTORYIn 1839, Alexandre Edmond Becquerel discovered he could illuminate one of two metal plates in a dilute acid and alter the voltage produced by the cell.In 1873, Willoughby Smith notices a correlation between the illumination of selenium metal and its electrical resistance.In 1876, William Grylls Adams (with his student R. E. Day) discovered that illuminating a junction between selenium and platinum also has a photovoltaic effect, though in this case a voltage is actually produced, not altered.In 1883, the first “real” solar cell was built by Charles Fritts, forming p-n junctions by coating selenium with a thin gold layer.
18P- and N-type Materials N-Type: Requires doping a material with atoms of similar size, but having more valence electrons. ex/ Si:AsSi has 4 valence e-. Here one is replaced in the lattice by As, having 5 valence e-.
19P- and N-type Materials P-Type: Requires doping a material with atoms of similar size, but having fewer valence electrons. ex/ Si:Ga~ only 3 valence e-
20Donor and Acceptor Bands Dopants add quantum energy levelsTranslate into bands in the solid semiconductor.Formation of majority charge carriers on each side:N-TypeP-Typee- The majority charge carriers have excess energy that is not bound up in valence bonding with neighboring atoms. This higher energy allows them to traverse the crystal lattice. They are therefore able to respond to the electric field!e- *extra negative electrons*extra positive “holes”from electron vacancies
21Solar (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 ~Initially, electrons rush over to fill holes in the P-side while holes overpopulate the immediate area on the N-side of the junction:This repels the majority charge carriers on each side.An equilibrium is reached where a net charge concentration exists on each side of the junction.
22Solar (PV) Cells:A photon is absorbed by the material near the P-N junction, creating an electron/hole pair:
23The Electric Field Drives Current Minority charge carriers are attracted to the junctionMajority charge carriers are repelled
24Efficiency – the “Band Gap” Only the right frequencies of light let an electron cross the junction, or “band gap”.Limited by the match of the band gap to the solar spectrumMultiple junctions
26Hopes for the Future Multi-junction solar cells improve efficiency. Thin-film P-N junctionsolar cells reduce materialuse and cost.Bring the current price per watt down
27References:Austin, Geoff. Jan Photo Electric Effect. RetrievedEinstein, Albert. (1905). “On a Heuristic Viewpoint Concerning the Production and Transformation of Light.” Annalen der Physik, Vol 17, 132.Elert, Glenn. Photoelectric Effect. RetrievedHamakawa, Yoshihiro. (2004). Thin-Film Solar Cells: Next generation photovoltaics and its application. New York: Springer.Lenardic, Denis. A Walk Through Time. RetrievedU.S. DOE Photovoltaics Program. (2005). Photovoltaics Timeline. Retrievedn.a. n.d. Philipp Lenard – Biography. Retrievedn.a. n.d. The Photo Electric Effect. Retrievedn.a. n.d. The Electric Field In Action. Retrievedn.a. n.d. Timeline of Solar Cells. RetrievedRobertson, E F. O’Conner, J J. A history of Quantum Mechanics. RetrievedSmith, Willoughby. (1873). "Effect of Light on Selenium during the passage of an Electric Current". Nature, Vol ? 303.Available URL: