With summer in full swing now, we mostly think of the sun in the context of beaches and tans (or sunburns for some of us . . . ). That same bright orb in the sky that’s giving you that warm summer glow, though, is emitting an enormous amount of energy – many times more than we would need to sustain the entire world. All we have to do is figure out how to harness it.
There are two big problems with the main ways in which we get our energy now. First, they’re primarily non-renewable, meaning that our limited store of these fuels will eventually be depleted. Some examples are petroleum, oil, and coal. Although some of these resources are extremely plentiful, like coal, and will last us a long while, others are running out quickly, forcing us to search for alternative forms of energy that will be able to sustain us indefinitely. Second, these resources are harmful to the environment because they give off toxic emissions, like heavy metals and carbon dioxide. If we continue using energy in this manner, there’s a chance that we could destroy the planet before we even need to worry about finding energy to sustain us farther into the future.
Enter solar power! Solar power would be a renewable energy source because it is used at the same rate by which it is produced. It’s “harvested” from sunlight, which doesn’t seem to be running out anytime soon. It’s also a clean energy source that doesn’t give off any carbon dioxide emissions.
Whenever we stand in sunlight, we can feel the sun’s energy hitting us in the form of heat (sometimes a little more than we would like in the summer). When sunlight hits a solar cell, also known as a photovoltaic cell (photo meaning “light” and voltaic meaning “electricity”), that energy can be transformed into electric current.
The most efficient photovoltaic cells are made of silicon, a semiconductor. When sunlight strikes the cell, the cell absorbs some of the energy of the light. This influx of energy knocks loose electrons from the silicon atoms, allowing them to move freely. Photovoltaic cells also contain one or more electric fields that force free-flowing electrons to flow in a specific direction in order to generate an electric current.
Silicon is particularly effective for use in solar cells due to its chemical properties. Silicon atoms have 14 electrons that are arranged in three shells around the nucleus. The first shell holds two electrons and the second shell holds 8 electrons. The third shell has the capacity to hold 8 more electrons, but only has 4.
All atoms are desperate to make their outer shells complete, whether this is accomplished by filling up the missing spaces with more electrons or getting rid of all the electrons in the outermost shell. Since silicon’s third shell is exactly half full, it doesn’t make much sense for it to get rid of four electrons (where are all of them going to go?). Instead, it shares these four electrons with four nearby atoms by overlapping and sharing their outer shells. By doing so, silicon creates a crystalline structure, which is an integral part of the photovoltaic cell.
When energy is added to silicon, it causes a few electrons to break free of these bonds and leave their atoms, leaving behind a hole in the crystalline structure. These electrons then bounce around in the crystalline lattice looking for another hole to fall into.
The problem with this is that there are very few holes in pure silicon. To solve this issue, solar cells purposefully have other atoms mixed in with the silicon atoms. One example of such is adding in a few phosphorous atoms, one for every million silicon atoms or so. Phosphorous has five electrons in its outer shell rather than four. It still bonds with its neighboring silicon atoms, but it has one extra electron that is left unbonded, kept in place only by the positive charge of the protons in the atom’s nucleus. When energy hits the photovoltaic cell, this extra electron easily breaks free, adding to the number of free carriers. Silicon with this particular kind of impurity is called N-type (“n” for negative) because of its excess of negative electrons bouncing around.
Impurities in silicon can be made in the opposite manner as well. Boron, which has only three electrons in its outer shell, can be added into silicon rather than phosphorous. Instead of having an excess of free electrons, this form of silicon would have an excess of free openings for electrons. This kind of silicon is called P-type (“p” for positive) because its lack of electrons creates a positive charge.
When N-type and P-type silicon interact with each other, it would make sense for the free electrons on the N-type side to see the free openings on the P-type side and make a mad dash to fill them.
This would be pretty counterintuitive, though, because then we’d be right back where we started with everything in perfect balance like pure silicon. What actually happens is that right at the junction where the P-type and N-type meet, the two types of silicon mix with each other and form a barrier. This barrier gradually makes it more and more difficult for electrons from the N side to cross over to the P side until finally, equilibrium is reached, creating an electric field completely separating the two sides. This electric field acts as a diode, allowing movement to occur in one direction only. In this case, the electric field permits electrons to flow from the P side to the N side, but not vice versa.
When sunlight hits the solar cell, it still forces electrons to jump out of their shells. With this electric field there, though, the electrons are forced over to the N side, leaving the holes in the P side, creating even more of an electrical imbalance inside the cell. What solar panels do, then, is provide an external current path allowing the electrons in the N side to take a route around the electric field barrier to get over to the P side holes.
The problem with solar energy is that it is much more expensive than our cheap, nonrenewable resources. Big silicon crystals are hard to grow so many companies have attempted to use newer materials with smaller and cheaper crystals. However, these crystals are not as good as silicon at converting sunlight into energy.
Regardless, even silicon solar panels have low efficiency rates. A typical crystalline silicon cell can convert 22 – 23% of light striking it into electricity, while the commercial solar panels that you can afford to use to power your house have efficiency levels of between 15 – 18% (Locke).
We’re not yet at the point where solar energy accounts for most of the world’s energy usage, but scientists are continuing to work to change that fact. The most efficient solar panel as of June 2013 was made by the Japanese technology firm Sharp and can convert 44.4% of incoming photons into electrical energy (Tom). Hopefully these numbers will continue to rise so we can look forward to an every-growing transition to renewable source of energy.
Written by Constance Kaita
Images courtesy of davestopher.com, cleanwater.org, popsci.com, rice.edu, planetminecraft.com, wisegeek.com
Locke, Susannah. “How does solar power work?” ScientificAmerican.com. 20 October 2008. Web. 16 July 2013.
Tom, David. “Sharp develops world’s most efficient solar panel.” TechSpot.com. 14 June 2013. Web. 16 July 2013.
Toothman, Jessika and Scott Aldous. “How Solar Cells Work.” HowStuffWorks.com. 1 April 2000. Web. 16 July 2013.