What is solar photovoltaic technology?
Solar photovoltaic, or PV, involves the use of certain materials that exhibit the ‘photovoltaic effect’ to use the light from the sun as the feedstock for generating electricity. Currently, the most common material used for the creation of solar PV material is silicon. This is because silicon is widely available (although not just any sand will do), is recyclable, and is non-toxic. Other upcoming materials of interest for the manufacture of solar PV modules include biological materials and perovskite (CaTiO3). Perovskite and biological solar modules, if successfully scaled, will allow increased production with decreased environmental impact.
How does it work?
The photovoltaic effect is exhibited by many different types of materials. If we examine the word ‘photovoltaic’ it can be broken into two parts: photo – meaning light; and volt – meaning electric potential. This means materials that, when exposed to light, can produce a voltage. Each material has a band gap which, when a photon from the sun with more energy than the band gap hits the material, the photon will transfer energy to an electron in the material resulting in an electron being ejected (Figure 1). Any excess energy from the photon results in heat. The band gap also determines the maximum efficiency for that material. Silicon has a band gap of 1.1, which is very good for producing electricity from sunlight.
What is sunlight?
Sunlight is comprised of three major parts: infrared which, is relatively low energy; visible light which is higher energy than infrared and increases down the colour spectrum. Red light with a long wavelength is low energy, violet with a short wavelength, is highest energy. Finally, ultraviolet light has the longest wavelengths and highest energy photons. You may be thinking, ultraviolet has the highest energy so that will be best for solar production, but that would be incorrect. Remember, the remaining energy once the band gap is crossed results in heat. Furthermore, UV light can penetrate many materials and will not result in electrons being ejected. For silicon, with a band gap of around 1.1, the visible spectrum is excellent for producing electricity. A substance with a much higher band gap, such as copper oxide with a band gap of 2.1, could utilize the higher energy UV light. Finally, it is also possible to combine different materials in order to expand the range of light that will result in electrons being ejected, which results in higher efficiency.
In summary, the photovoltaic effect can be broken into three parts: absorption, generation, and separation. The absorption stage is the photon with sufficient energy being absorbed by the photovoltaic material. The generation stage is the energy from the photon resulting in an ejected electron (charge carrier). Finally, the separation stage is isolating the charge carrier using contacts in order to allow the electricity to flow and do work.
How does a solar cell use the photovoltaic effect?
A solar cell absorbs the light from the sun and we know that this will result in a free electron. Within the solar cell, we also have a slightly positive, and slightly negative side which is achieved through ‘doping’ the cells with tiny amounts of boron and phosphorus. When an electron is freed from the silicon atoms, it will travel towards the positive side creating an area of relative negative, and an area od relative positive charge. If we supply a wire to the contacts on each side, we can create an electrical circuit and use the free electrons to do work.
Stay tunes for the next blog which will cover the PV system components and how they use the photovoltaic cells to create useful electricity for our household needs.