The Photoelectric Effect and it’s use in Solar Cells

Updated website at: http://sciencehsc.com.au 

 

When electromagnetic radiation is shone onto the surface of a metal and if the incident photons are of higher work function than the metal, electrons would be physically ejected. This is due to the photoelectric effect. However more detail is required. A photon of higher energy (thus higher frequency, E=hf) than the work function of the metal will elastically collide transferring completely both momentum and energy to an electron on the surface of the metal. This electron will have enough energy to be physically ejected.

It is essential to note that :

  • The energy (or velocity) of the ejected electrons are dependent on the frequency of the incident photons
  • The quantity of the ejected electrons are dependent on the intensity of the incident photons

The threshold frequency is the minimum frequency of electromagnetic radiation to cause electrons to be ejected. Thus below this frequency, no electrons are emitted from the metal. This threshold frequency depends on the metal involved.

The kinetic energy of the ejected electron can be measured using a variation of Plank’s formula.
Energy=Frequency x Plank’s constantWork function of metal

It is also key to note that although Einstein was the first to correctly explain the photoelectric effect with the help of Plank’s idea of the quantisation of energy, it was Hertz who first observed this effect.

The photoelectric effect is a phenomenon occurring in everyday life. It is the underlying effect in solar cells and photocells (automatic doors). Below is an explanation of how a solar cell works.

Essentially, a solar cell is consisted of a junction between a P-type and N-type semiconductor that is exposed to light. As photons (usually UV-light) strikes the surface of the N-type layer, electrons are ejected due to the photoelectric effect. As there is a sandwich of a P and N-type semiconductor, a depletion zone is created as a result of the diffusion of electron and holes. This depletion zone is a electric field that prevents the electron flowing straight to the P-type semiconductor. The ejected electrons (being attracted to the P-type semiconductor since it’s positive and there is a potential difference) flows through the path of least resistance thus flows through an external circuit where a load is attached. As there is a flow of electrons, there is current (that is its definition). Also as electrons can only flow in one direction, due to the depletion zone, DC current is produced.

It is important to note that currently solar cells are only 11% efficient thus it is still economically unviable to employ commercially for an environmentally friendly fuel source. However as efficiency increases it will become our main source of energy.

Photocells on the other hand employs almost identical methods however is used to measure light intensity instead of producing DC current.

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