C.8 Photovoltaic Cells And Dye Sensitised Solar Cells (HL) Flashcards
What must a molecule have to be coloured?
For a molecule to be coloured it must absorb light in the visible region of the spectrum.
Which molecules are able to absorb light in the UV region?
Molecules containing a conjugated system of alternating single and double bonds are able to absorb light in the UV region when an electron moves from a low energy level to a higher energy level.
What wavelength of light is absorbed if there is a longer conjugated system in a molecule?
The longer the conjugated system the longer the wavelength of light absorbed.
What happens to molecules with a short conjugated system?
Molecules with short conjugated systems have a fairly large energy gap between lower and upper energy levels and absorb UV light and appear colourless.
Why do molecules with longer conjugated systems appear coloured?
Molecules with longer conjugated systems have a smaller energy gap and can absorb longer wavelengths of light in the visible region they therefore appear coloured.
What is a semiconductor?
A semiconductor is a solid with intermediate electrical conductivity between that of a full conductor eg a metal and an insulator. Semiconductors are able to conduct electricity under particular conditions such as high temperatures or in the presence of sunlight.
Why do metals resistance increase as temperature increases?
Metals conduct electricity well because the delocalised electrons are free to move throughout the structure. Resistance in the metals arises because these electrons collide with the positive ions in the lattice. As the temperature increases the metal ions vibrate more and so there is essentially a larger cross section for the electrons to collide with and the electrical conductivity decreases (resistance increases).
Why do semiconductors resistance decreases as temperature increases?
At higher temperatures more electrons are promoted to the conduction band and are free to move.
Why do metals require less energy to remove electrons?
Metals tend to have lower ionisation energies than semiconductors in the same period this means it requires less energy to remove electrons from metals to generate free moving electrons. Semiconductors require more energy input to ionise and free the electrons.
What is silicon doping?
The semiconductor silicon can be modified by doping in order to increase its electrical conductivity.
What are the two methods of silicon doping?
By adding a group 13 element or by adding a group 15 element.
How does adding a group 13 element dope the silicon?
Adding an element such as boron means only three valence electrons are supplied instead of four creating positive holes in the valence band. Electrons are able to move into these holes and so electrical conductivity increases. As these charge carries are the positive holes these materials are called P TYPE semiconductors.
How does adding a group 15 element dope the silicon?
Adding an element such as phosphorus means five valence electrons are supplied. Four are used in bonding and the fifth is promoted into the conduction band increasing the electrical conductivity. As the charge carriers are negative electrons these materials are called N TYPE semiconductors.
What happens when an n type and a p type semi conductor are joined together?
A potential difference is built up at the junction between the two.
How can a photovoltaic cell power a device?
When light hits the n type semiconductor electrons are promoted to the conduction band. These electrons are not able to move directly from the n type to the p type because the voltage at the p-n junction prevents this. They must travel from the n type to the p type through an external circuit and this flow of electrons can be used to power an electronic device.
