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The shift from non-renewable energy to renewable energy has been gaining significance as the effects of global warming and climate change have become more pronounced. Out of all the renewable energy technologies, photovoltaics—known to many as solar cells—are the most widely used; only requiring sun rays to generate electricity. They are one of the easiest and cheapest renewable energy technologies to install and run. In this article will look at how photovoltaic systems convert sunlight into energy, i.e. electricity.
As a general device mechanism, the sunlight is converted into electricity via a semiconducting p-n junction, and these junctions are the backbone of these technologies among many others. A semiconductor is a material that has a conductivity between that of a highly conductive metal and an insulator—a sort of middle ground.
However, one of the key properties of semiconductor materials, and this extends to semiconductor junctions, is that an energy input can change it from an insulating state to a conductive state—a kind of periodic conductivity—and it is this switching of states that is beneficial to most technologies, as it enables the conductivity to be tuned for specific scenarios.
While many semiconductors change their electrical conductivity properties under an applied heat (thermal energy input), the semiconducting junction in a photovoltaic cell changes its conductivity when it is irradiated with photons of sunlight.
While there are now many different types of photovoltaic cell—such as organic solar cells, dye-sensitized solar cells, and quantum dots solar cells—which have slightly different working mechanisms, we’re focusing on the traditional, inorganic solar cell, as that’s the most widely used cell commercially.
Underlying Working Principles of Photovoltaic Cells
Solar cells (inorganic) are typically made up of doped silicon materials (also nanomaterials can now be used), and the junctions are formed by having a p-type silicon next to an n-type silicon. For reference, a doped material which is an n-type material has atoms that contain an extra electron in its atomic lattice, whereas p-type doped materials have atoms that have one less electron. This leads to the formation of excess electrons and holes for n-type and p-type materials, respectively.
Both charge carriers are involved with the energy conversion mechanism. It should be noted that only certain materials can be used to construct the junctions in a photovoltaic cell, as the materials must have the ability to undergo the photoelectric effect—which is the generation of a voltage in the presence of light.
When these two doped silicon materials are put next to each other they form a semiconducting junction. Located on one side of this junction is an abundance of holes, and on the other side is an abundance of electrons. In between these two charge carrier regions is an electrically neutral region known as the depletion zone—which acts as the junction interface between the two charge carrier regions. The depletion zone is formed when no sunlight is being shone on the photovoltaic cell.
The depletion zone itself is formed by the interaction and coming together between some of the electrons and holes. Both the charge carriers combine to form a neutral species that separate the other charge carriers from each other.
In addition to separating the charged species, the neutral zone also generates an internal electric field within the solar cell, and this electric field prevents the two charge carrier regions from completely combining together. This is very important because if these two regions completely collided with each other, the result would be a completely electrically neutral material that would not work as intended. This is because the migration of these charges under light stimulation is the reason why solar cells work and photovoltaic energy is produced, and an electrically neutral material would not generate an electrical current.
Generation of Photovoltaic Energy
An electrical current is generated when photons of light hit the solar cell because the photons of light transfer energy to the semiconducting junction, which in turn transfers energy to the free charge carriers on either side of the junction/depletion zone. When the charge carriers have an increased energy, their mobility increases so much so that they enter the depletion zone.
When the charge carriers enter the depletion zone, the depletion zone reduces in width. The width eventually reduces to the point where the internal electric field (that has arisen from the depletion zone) is no longer strong enough to counteract the motion of the charge carriers. This causes the electrons to move towards the holes, where they recombine. This charge carrier recombination process generates a constant electric current, which is also the generation of electrical photovoltaic energy that can be stored.
Once the electrical current is generated, it stays in this state while there is sunlight hitting the junction. When the charge carriers combine, it does increase the thickness of the depletion zone temporarily, but this only lasts until the next photon strikes which give more energy to the charge carriers. So, photovoltaic energy can be continuously harvested providing there is sunlight. The depletion zone doesn’t return to its natural resting state/thickness until the sunlight stops hitting it.
When there is no sunlight, the device ‘resets’ and the depletion zone will return to its original thickness and the charge carriers are once again separated. This process then starts again when the sunlight returns.
Sources
- NASA: https://science.nasa.gov/science-news/science-at-nasa/2002/solarcells
- Florida Solar Energy Center: http://www.fsec.ucf.edu/en/consumer/solar_electricity/basics/how_pv_cells_work.htm
- American Chemical Society: https://www.acs.org/content/acs/en/education/resources/highschool/chemmatters/past-issues/archive-2013-2014/how-a-solar-cell-works.html
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