Solar cells – photovoltaic cells – were first employed for use on the satellite ‘Vanguard I’ in 1958. The Vanguard required power for seven years. The solution to this was the solar panels. The efficiency of these cells was 9%. With progress and improvements in materials science, PV cells have an efficiency of up to 42.8% today (Lund 2008). All forms of renewable energy are influenced by the power of the sun, solar power captures this energy in its rawest form.
PV cells draw their power from electromagnetic waves – photons. Each photon has a frequency and associated energy related by the equation E = hf. Here h is Planck’s constant, c speed of light, and f the photons frequency. As can be shown by some simple equations – as the frequency increases – the energy increases. Furthermore it should be noted when working with particles and electrons a commonly used value is electron volts – 1 electron volt = 1.602 x 10 -19 J (Walding 2004).
Bohr showed that electrons have a specific orbit in ratio to the amount of protons within that atom (Ion 2005). At the lowest energy state these electrons occupy the lowest orbit. When a photon of light comes into contact with an electron, the electron has the energy to occupy a higher orbit. Some photons have sufficient energy to free an electron from the atom – the photoelectric effect (Nobelprize.org 2011).
Albert Einstein theorized that a photon greater than the threshold frequency – minimum frequency to excite the electron – has the ability to eject a single election from the protons orbit. The kinetic energy of the electron can be found through the work function Ek = hf – W. W being the energy – work – lost to dislodge the electron. As the example shows if the threshold frequency has an energy of 1eV and the incident photon has an energy of 3eV the kinetic energy of the electron will be 2eV. This is the basis behind the theory of the photovoltaic cell (Walding 2004).
Two types of silicon are used to make electricity. N-type – which has a relatively low affinity for electrons – and P-type – have a higher relative affinity for electrons. When these types are brought into contact a depletion zone – PN junction – is formed. The excess of electrons in the N-type form with the P-types affinity towards them, creating a barrier (pveducation.org 2010).
PV cells require the use of ‘dope’. Usually the dopants boron and phosphorus. Due to the atomic structure of these three substances silicon can attain either a positive valency – P-type – or a negative – N-type. This is required to attain a PN Junction allowing an electric current to flow in only one direction. Furthermore the minority charge carrier – electron – from one section will pass through the junction to become a majority carrier while the majority carrier – hole – is unable (pveducation.org 2010). Here, conventional current is the movement of positively charged holes and not the electrons, although the system can be reversed to achieve the same amount of energy.
The following explains the proses that creates current. Firstly the photon is absorbed by the silicon and – depending on the frequency – will create an electron-hole pair. At this point several events can happen. The electron and hole can recombined, but in an ideal circuit the hole will pass through the PN Junction while the electron will pass through the exterior circuit. From this a potential difference is achieved (pveducation.org 2010). When several solar cells are connected in series a usable amount of electricity is obtainable.
Light absorption and efficiency
The efficiency of a solar cell is dependent on what frequency the silicon can absorb – the absorption coefficient. The higher the absorption coefficient the easier the photon is absorbed. As the graph shows, different semi conductive materials have different absorption coefficients in ratio to the wavelength in