Most of the energy we use to power technology come from finite sources which are not sustainable. This energy which may be in the form of either fossil fuels, coal or even nuclear fuels and so on will eventually be used up. However renewable sources such as solar power will not run out…..until the sun runs out of hydrogen fuel but that’s another story.
We are able to harness the light energy radiated by the sun by the used of solar cells. A solar cell is an electronic device made of semiconductors which exhibit the photovoltaic effect to convert light energy into electrical energy. Semiconductors are materials which lie between conductors and insulators. A conductor is a material which is composed of atoms in which electrons are easily freed from the nuclei. Even though it is able to form a current, it remains electronically neutral as there are the same number of positive protons and negative free electrons. An insulator, on the other hand, is a material which is composed of atoms which hold more tightly onto their electrons so they have no free electrons like conductors. Current is a measure of the rate of flow of charge through a material, with the electrons being the charge carriers transporting energy across a circuit.
Semiconductors only conduct under certain circumstances such as when they are heated or doped. The vast majority of semiconducting material used is silicon with four valence electrons and in its natural state silicon form a covalent lattice which does not conduct. Increasing the temperature supplies an electron to escape the covalent bond creating two new charge carriers, a negatively charge electron and a comparable positive hole where the electron used to occupy. Doping decreases the semiconductor’s resistance allowing it to conduct. Intrinsic (pure) semiconductors are introduced with tiny amounts of impurities creating extrinsic semiconductors. The impurities are atoms with either one more or one less valence electron than the four valence electrons of semiconductors supplying free charge carriers.
Individual silicon atoms have their own energy levels in which their electrons orbit in and gaps between the levels which electrons are unable to occupy. When there are countless atoms of silicon together the levels merges into bands. The top band being the conduction band in which electrons are free to move around the material. The next band down is the valence band in which electrons are tightly bound to their atoms, unable to move.
The following explanation is explained using band theory and then also with the electron and hole theory, for a clearer picture.
The intrinsic semiconductor or pure conductor is solely made of silicon atoms and has a low conductivity. In extrinsic semiconductors, such as n-type and p-type the impurity atoms has its own energy levels which merge to form different bands than the intrinsic semiconductor, as seen above.
The donor band which lies just below of the conduction band has electrons which can be easily excited over the small energy gap to the conduction band giving rise to a larger current.
The acceptor band which is directly above the valence band bridges the energy gap between the valence and the conduction band. More electrons are able to jump to the conduction band via the acceptor band.
As an extension to what has been previously mentioned, the two types of semiconductors used in solar cells are:
N-type semiconductor are created by doping the silicon lattice with an element such as arsenic with 5 valence electrons. After forming four bonds with four silicon atoms, there is an extra electron left over. So every impurity atom contributes a free electron to conduct. The majority of free charge carriers are electrons and negative, giving name to n-type.
P-type semiconductor are created by doping silicon with an element with 3 valence electrons such as indium. After forming 4 bonds with four neighbouring silicon atoms there is a hole created as there are not enough electrons. The hole is relatively positive and is the majority free charge carrier.
Placing the two semiconductors together forms a p-n junction. At the junctions, free electrons from the n-type diffuse over to fill the holes in the p-type becoming free of free charge carrier and creating a depletion zone acting as a barrier.
Solar cells use this principle, connected with the n-type on top closest to light and the p-type underneath. When light is shone one to the cell, it is in packets of energy called photons. The photons release their energy down in the p-type layer which gives electron the energy to jump up to the top n-type layer, overcoming the depletion layer. The moving electron travels around in a circuit generating electricity.
In this way, solar cells are able to continually generate electricity. However they are limited to low conversion efficiencies meaning a smaller proportion of energy from the sun can be converted into usable electricity. The maximum theoretical solar cell efficiency according to the Shockley-Queisser limit is around 33.7%. This is based on single junction solar cells, which are only suited capture photons from the sun of a particular frequency. Like tuning into a particular channel of radio, it is only focused on a narrow range of frequency and does not utilise the remaining frequency of photons.
So whilst solar energy is the future we are still limited by the manufacturing costs of solar cells and their efficiency, but the future shall shine bright.
Author – Jiangmin Hou
Jiangmin is a 5th year high school student currently studying five STEM subjects at Scottish Higher level-Mathematics, Physics, Biology, Computer Science and Chemistry. She is interested in pursuing a degree in Medicine after completion of Secondary Education.