# The Photo Electric Effect

## Presentation on theme: "The Photo Electric Effect"— Presentation transcript:

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:
Light is causing an electrical response in metals

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: Light caused current to flow in an open circuit

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. 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.

Perplexing Observations:
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. *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”

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 Where h is planks constant and v is the frequency. The photon model also explains why there is no delay in the ejection of electrons…

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.” Einstein’s predictions were proved shortly after by the work of Millikan and others, and *This work won Einstein his Nobel Prize in 1922.*

Quantum Implications Electrons 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

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

Emergent Applications…
Response is linear with light intensity Extremely short response time For 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.

At Nearly the Same Time, Another Discovery is under way….

Same basic principle as the photoelectric effect
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. Same basic principle as the photoelectric effect: Incoming photons excite electrons into a conductive energy level. Voltage results without the ejection of electrons. HISTORY In 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.

P- and N-type Materials
N-Type: Requires doping a material with atoms of similar size, but having more valence electrons. ex/ Si:As Si has 4 valence e-. Here one is replaced in the lattice by As, having 5 valence e-.

P- 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-

Donor and Acceptor Bands
Dopants add quantum energy levels Translate into bands in the solid semiconductor. Formation of majority charge carriers on each side: N-Type P-Type e-  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

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 ~ 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.

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”. Limited by the match of the band gap to the solar spectrum Multiple junctions

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 Photo Electric Effect. Retrieved 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 Hamakawa, Yoshihiro. (2004). Thin-Film Solar Cells: Next generation photovoltaics and its application. New York: Springer. Lenardic, Denis. A Walk Through Time. Retrieved U.S. DOE Photovoltaics Program. (2005). Photovoltaics Timeline. Retrieved n.a. n.d. Philipp Lenard – Biography. Retrieved n.a. n.d. The Photo Electric Effect. Retrieved n.a. n.d. The Electric Field In Action. Retrieved n.a. n.d. Timeline of Solar Cells. Retrieved Robertson, E F. O’Conner, J J. A history of Quantum Mechanics. Retrieved Smith, Willoughby. (1873). "Effect of Light on Selenium during the passage of an Electric Current". Nature, Vol ? 303. Available URL: