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The Limitations of Silicon’s Use in Solar Panels

In 2011, North Americans used 1.25*1020 joules worth of energy.(1) That is enough energy to power approximately 40 billion 100-watt light bulbs for an entire year. This energy was formerly generated through the burning of fossil fuels. However, solar energy could easily replace them because approximately 3.2*1020 joules worth of sunshine hits the Earth’s surface in a single hour. To put things in perspective, that is more energy than was consumed by the US over the course of an entire year!

Photovoltaics are composed of two sheets of semi-conducting material, usually silicon. The top sheet of a silicon solar panel is doped with phosphorus impurities and the bottom sheet with boron impurities. The phosphorus and boron doped layers are referred to as the n-type layer and the p-type layer respectively and when in proximity create an electric field in the junction between the two layers due to the recombination of electrons and holes(4). A silicon nitride layer is also placed on top of the panels because it is an antireflective optical coating that allows light to pass through instead of being reflected away or absorbed.

As light enters a solar panel, photons hit electrons within the panel and cause them to become excited and break off from their original atoms, leaving holes in their place. Electrons have a threshold, or “band gap” energy, which represents the minimum amount of energy necessary to knock electrons free from their atoms(6). If photons are absorbed within the junction between the n and p-type layers of the solar cell, the photovoltaic’s electron/hole pairs will remain separated by an electric field so electrons can enter the circuit and generate electricity.

Silicon solar panels are not efficient in converting light into energy because silicon has an indirect band gap that causes the panel to be very thick because it does not absorb light very well (6). This property both raises the cost of the solar panels as well as decreases the efficiency of the amount of sunlight they can convert to electricity. On average, solar panels convert less than 14% of incoming light waves into energy and even a slight change to the angle at which they are placed can reduce their efficiency by up to 50%.(2,3) Despite this, silicon is still used because it is so abundantly found in Earth’s crust, making it cheaper to use in solar panels.

Solar panels can also be made more efficient when the crystalline structure is monocrystalline and not multicrystalline. Monocrystalline silicon in solar panels is more efficient in energy conversion because of its well-organized crystal structure(5). Multicrystalline cells are much cheaper and easier to produce but are less efficient because of grain boundaries, which block carrier flows and provide unwanted paths for electrons(7).

Although further improvements to silicon could be made, other elements may be a better solution. One such element is called Gallium Arsenide, which is a promising candidate for future solar panels because when doped it has a higher electron mobility than in typical Si panels(9). GaAs also has a direct band gap, making it much more efficient at absorbing light than Si. Although Gallium Arsenide seems like a better option for solar panels, it can be dangerous to handle and very hard to process, making it much more expensive. With a little more research and some advances in processing techniques for Gallium Arsenide, the future of solar energy looks BRIGHT!

Works Cited:
1. http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=44&pid=44&aid=2
2. http://www.qrg.northwestern.edu/projects/vss/docs/power/2-how-efficient-are-solar-panels.html
3. http://www.wisegeek.com/how-do-solar-panels-work.htm
4. http://www.livescience.com/41995-how-do-solar-panels-work.html
5. http://webservices.itcs.umich.edu/drupal/recd/?q=node/156
6. http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/
7. http://pveducation.org/pvcdrom/manufacturing/multi-crystalline-silicon
8. http://science.howstuffworks.com/environmental/energy/solar-cell.htm
9. http://www.the-scientist.com/?articles.view/articleNo/9894/title/Gallium-Arsenide–Key-To-Faster–Better-Computing/

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