•A Method of increasing the number of mobile electrons and holes is to dope the material.
•Donor atoms contribute extra electrons that are less well bound than the usual valance electrons
–n-type, excess of negative carriers over positive
•Acceptor Atoms contribute too few electrons and so can trap electrons to contribute holes into the valance band.
–p-type, excess of positive carriers over negative.
•Electrons and holes move within the semiconductor in two ways
–Driven by concentration gradients
–Driven by potential difference.
Carriers move to reduce their electrical potential energy
•p-type and n-type material bought together
•On the n side we have an excess of electrons
•On the p side we have an excess of holes
•This creates a concentration gradient across the junction
•Electrons and holes diffuse across the junction
–Holes from p to n
–Electrons from n to p
•The leave behind layers of fixed charge on either side
•This sets up an electric field that resists further migration across the junction until  a dynamic equilibrium is reached
•The region around the junction is called the depletion region as it is now depleted of free carriers
•If an electron is not transported recombination will occur
•Radiative Recombination
–The decay of the electron is accompanied by the emission of a photon
•Non-radiative Recombination
–The electron decays via defect states in the band gap and it’s energy is given up as heat
•We want to collect and transport the electron before it recombines
•The band gap enables electrons to stay in the conduction band for long enough to be exploited.
•In a metal the continuum of available energy levels means that the electron would return to the ground state quickly.
•When an electron is promoted across a band gap it quickly decays to the lowest state in the conduction band but decay across the band gap is much slower.
–Thermalisation, Energy lost as heat.
•The promotion of electrons across the band gap is called photo generation.
•Photons with energy greater than the band gap will be absorbed to promote electrons.
•The energy of a photon can be related to its wavelength.
•Therefore the maximum cut-off wavelength for photon absorption is related to band gap as follows


•The semiconductor P-N Junction provides those three mechanisms
•Semiconductors are characterized by a band gap.
–A range of forbidden energies for electrons
–Separates the Valence band from the Conduction band
•Valence bands are normally full
•Conduction bands are normally empty
•Semiconductors typically have a band gap of around 1eV
•They are temperature and purity dependent
•At low temperature they act as insulators
•By supplying energy to them they can act as conductors
•The band gap is a result of the structure of the material
•All of the valance electrons are used in bonding.
•The conduction band is empty.
•By transferring energy to the electrons they can be promoted into the conduction band where they are free to move.
•This generates Electron-Hole Pairs
•Photovoltaic Cells produce electricity when exposed to sunlight.
•This relies on the photoelectric effect
•Light can be thought of as being a stream of photons.
•A photon interacting with matter can give up it’s energy to promote an electron to a higher energy level.
•If the electron does not immediately return to its ground state then we can exploit it’s energy as an electric current.
•For efficiency we need
–A high probability of photon absorption
–A charge separation mechanism
–Electronic transportation to an external circuit where it’s electronic potential energy can be exploited
  1. •1839: Nineteen-year-old Edmund Becquerel, a French experimental physicist, discovered the photovoltaic effect while experimenting with an electrolytic cell made up of two metal electrodes.
  2. •1873: Willoughby Smith discovered the photo conductivity of selenium.
  3. •1876: Adams and Day observed the photovoltaic effect in solid selenium.
  4. Although selenium solar cells failed to convert enough sunlight to power electrical equipment, they proved that a solid material could change light into electricity without heat or without moving parts
  5. •1953: Gerald Pearson at Bell Labs, makes first Silicon solar cell
  6. •1953: Daryl Chapin and Calvin Fuller refine concept, first solar cell to power electrical equipment
  7. •Mid 1950s: Efficiency doubles in 18 months But: 300 $/W compared to 50 cents/W for conventional power
  8. •Late 50s: Solar cells used as part of a hybrid power system for Vanguard satellite
  9. •Since the late 1960s Solar cells have been the accepted power source for satellites.
  10. •Early 1970s: Dr Elliot Berman lower quality silicon drops cost from 100 $/W to 20 $/W Allows use of solar power for terrestrial applications, like oil rigs and remote locations away from the grid.
  11. •Late 1970s: US Coastguard powers nearly all of it’s buoys and lighthouses using solar cells (other coast guards follow suit)
  12. •1978: Telecoms Australia install 13 solar powered repeater stations, following with 70 more.
  13. •By 1985 solar power is system of choice for remote communications
  14. •1980s: Rural communities benefit from solar power. 50% of the households in the outlying islands of Tahiti run on solar power
  15. •1980s: Centralised solar power plants are designed. Building integrated PV becomes more common
  16. •1990s – Present: Costs of solar power reduce further.
  17. •Solar power is used for applications like street lighting, powering bus shelters.