James Martino110006471December 5, 14ESM 150 – Materials of the Modern WorldFreshmanElectrical Engineering

Solar Cells

Throughout the past hundred years people have been trying to harness the power of the sun and turn it into energy. One technology used to convert sunlight into electricity is the solar cell. I will begin this essay by discussing the history of the development of solar cells since the first solar motor was created in 1861 until the creation of the Solar Energy Research Institute in 1977. Next, I will evaluate how a solar cell works. After this I examine the three main types of solar cells used today including the materials used to make them, how they work, and what they are used for. I will end the essay with a short conclusion of the overall impact that solar cells have had on society.

Throughout history, humanity has been on a search for newer, more efficient ways of producing energy. This quest for energy has brought about incredible new technologies such as power plants, turbines, and reactors. Power plants have been used to convert many different materials into energy including coal, oil, and natural gas. Turbines are used to convert wind and water into energy and reactors are used to convert nuclear fuel into energy. Of all of the different technologies used to create energy, one of the most efficient may be solar cells which have been used publicly since the 1970’s, although the first solar motor was invented in the 1860’s [1].

The first solar motor was patented in 1861 by Auguste Mouchout, who was a mathematics instructor at the Lyce de Tours, while searching for a renewable alternative to coal [1][2]. Mouchout began this search for renewable energy because he thought “Eventually industry will no longer find in Europe the resources to satisfy its prodigious expansion. Coal will undoubtedly be used up. What will industry do then? [2]” Mouchout’s solar motor consisted of a glass enclosed iron cauldron which would trap the suns solar radiation inside to heat the water and, after adding a reflector to help concentrate the light, he was able to use it to power a small

steam engine. in 1872, after receiving financial assistance from Emperor Napoleon III, Mouchout further refined his invention by shaping the reflectors as a disk to increase the intensity of the light into the cauldron. The last major improvement that Mouchout made on his solar motor was increasing its water capacity to 100 liters and exchanging the single iron cauldron for a multi-tubed boiler. In 1881 the funding for Mouchout’s research was removed after solar power was deemed a practical failure due to France’s improved system for transporting coal and its improved relationship with England which was France’s main importer of coal

[3].The next major proprietor of solar energy was Frank Shuman, a chemist at the Victor G.

Bloede Company in West Virginia [3]. Frank Shuman saw the same problems with the continuous use of coal for power as Auguste Mouchout did and, in 1914, wrote “One thing I feel sure of and that is that the human race must finally utilize direct sun power or revert to barbarism. [4]” In 1880, Shuman became a chemist and by 1897 he created a parabolic trough and used it to boil a chemical, called ether, with a lower boiling point than water and run an engine with it. After improving the design for his parabolic trough Shuman created the Sun Power Company with the goal of creating a solar power plant that can run an industrial size steam engine. Shuman created a solar conversion system in 1911, which collected over 10,000 square feet of sunlight in Tacony, Pennsylvania. Although Shuman’s system required no money to create the energy like coal it was not adopted on a mass scale in North America due to the high initial cost of production [3]. Because of this Shuman went to England and got financial

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Figure 1 – Auguste Mouchout’s solar motor in 1872 (Image taken from "History First Solar Powered Engine France Auguste Mouchout 1865." Solarlife Green Economist. Solar Life, 5 Feb. 2012. Web.)

assistance to build a solar powered irrigation plant in Mahdi, Egypt in 1913. When completed, Shuman’s irrigation plant was comprised of five, sixty meter long parabolic troughs which tracked the sun through the sky to concentrate solar rays into five black boilers, which created enough steam to pump almost 23,000 liters of water every minute. After this amazing creation, Shuman got backing from UK investors to build a series of giant solar power plant in the Sahara desert, however due to World War I he lost his backing for his Sahara Project and his solar powered irrigation plant in Mahdi was scrapped for metal by Britain’s munitions industry only two years after it was built [5].

The next major leap in the creation of the solar cell was the discovery of silicon as a semi-conductor [2]. In the early 1950’s Daryl Chapin was working on a solar energy project and was experimenting on selenium, trying to surpass an efficiency level of one percent. Simultaneously, Calvin Fuller and Gerald Pearson were working on another different project when they discovered that silicon creates electricity when exposed to light [6]. After this discovery Fuller and Pearson joined with Chapin to create a new solar cell, using silicon, which

exceeded the efficiency of selenium. In 1954 Fuller, Pearson, and Chapin made solar power commercially possible when they introduced a solar cell that could power many things including a toy windmill, a twenty one inch Ferris wheel, and a radio with six percent efficiency. This final solar cell that they created was composed of long, thin strips of silicon mixed with arsenic, which was negatively charged, with a thin coating of boron on top, which was positively charged, and treated with a dull plastic coating to absorb the maximum amount of sunlight [7]. Despite its usefulness, this solar cell was still not very successful when it became commercially available in 1956, due to its

high initial price of three hundred dollars per watt [8].Solar Cells first practical use took place in 1958 when the United States sent the solar

powered satellite, the Vanguard I, into orbit. The vanguard project began in 1955 by an eight person group called the Stewart Committee. In July of 1955, the Stewart committee was

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Figure 2 – One of Frank Shuman's parabolic troughs in Maddi, Egypt (Image taken from Gornall, Jonathan. "The Promise of Solar Power, Made a Century Ago." The National. The National, 22 Jan. 2011. Web. 01 Dec. 2014.)

Figure 3 – Gerald Pearson standing next to his solar panel that is connected to a multi-meter (Image taken from "Bell Solar Panel Technology of the 1950's." The Invisible Agent. WordPress, 11 Mar. 2011. Web. 01 Dec. 2014.)

)

presented three options for putting a satellite into space. The first was named project orbiter which would use one of the army’s missiles to propel the satellite. Second was Project Vanguard which would use the navy’s Viking research rocket as a booster. The final option would make use of an Air Force rocket, which had never flown before, called the Atlas to boost the satellite [9].

The Stewart Committee decided on the Vanguard Project on August 3, 1955 and the first test launch took place on December 8, 1956. It took six more tries before the Vanguard I was successfully launched into orbit around the Earth on March 17, 1958 [9]. The Vanguard I rocket consisted of a three stage launch which released the Vanguard satellite at an altitude ranging

from two hundred to four hundred miles [10][11]. The satellite is a 3.25 pound, 6.4 inch diameter, aluminum sphere and contained a ten milliwatt battery powered transmitter and six square solar panels on the body of the satellite which powered a five milliwatt transmitter [10]. The battery powered transmitter stopped operating just two weeks after the Vanguard I was launched but the solar powered transmitter operated until May 1964, six years after the satellite was launched [10][11]. The use of the solar powered transmitter on the Vanguard I was a huge step in the advancement of solar cells, not only because it was the first satellite to utilize solar cells, but also because it was the first commercial use of solar panels [12].

Since the 1960’s solar cells have become the accepted power source for satellites around the world [8].

Despite the commercial use of solar cells on satellites, they were still not very popular among the average citizen, however in the 1970’s Dr. Elliot Berman found a way to create solar cells that were much less expensive. With funding from Exxon Corporation, Dr. Elliot Berman created a silicon solar cell out of lower grade silicon than was previously used on solar cells. This effectively lowered the cost of silicon solar cells from one hundred dollars per watt to twenty dollars per watt. Although using lower grade silicon decreased the efficiency of the solar cell it greatly increased the cost effectiveness of solar cells, making them more accessible to the average person [13].

The next great advancement of solar cells took place in 1977 when the government created the Solar Energy Research Institute, later renamed the National Renewable Energy Laboratory. The creation of the Solar Energy Research Institute was a big step towards the advancement of solar cells because it was the first time that the United States government embraced the use of solar cells [8]. Since 1977 the Solar Energy Research Institute has brought

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Figure 4 – The Vanguard I Satellite including the separation Mechanism (on top) (Image taken from "Vanguard 1." National Aeronautics and Space Administration. NASA, 26 Aug. 2014. Web.)

about many improvements on solar cells including lowering the average cost of solar cells to just below five dollars per watt by 2013 and creating a solar cell with 44.7 percent efficiency in 2014 [14][15].

There are many different types of solar cells used today including silicon solar cells, multijunction solar cells, and polycrystalline thin film solar cells. These different types of solar cells are made out of different materials and in different ways but they usually consist of the

same general structure. The primary part of a solar cell is the semiconductor layer in the middle. The semiconducting layer separated into two different parts; the n-type semiconductor on top and the p-type semiconductor on the bottom. The n-type semiconductor is made by doping, or mixing, the semiconductor with another element that has slightly more valance electrons which gives it a negative charge. The p-type semiconductor is made by doping the

semiconductor with an element that has a slightly fewer amount of valance electrons, giving it a positive charge. When these two layers are put together all of the extra valence electrons from the n-type semiconductor rush to fill in the nearest empty spaces in the p-type semiconductor and as they fill in the spaces they create a neutral charged junction in between that the electrons cannot pass. Around the semiconductor are two contacts, the front contact above it and the back contact below it which are connected to whatever is being powered by the solar cell. Next, an antireflective coating is place over the front contact to make the solar cell more efficient by decreasing the amount of sunlight from being reflected. Lastly, a class cover is placed over the solar cell to prevent it from getting damaged [16].

When sunlight, or a photon, hit a solar cell it passes through all of the layers until it is absorbed by a single electron in the p-type semiconductor. This electron gets energized by the photon, breaks away from the atom that it is attached to, and, if the electron is given enough energy by the photon, jumps across the junction and attaches to an atom in the n-type semiconductor. When this electron ejects from the atom in the p-type semiconductor it leaves behind a hole which is almost immediately filled by an electron from another atom, creating a

chain reaction of electrons filling in holes. The electrons are eventually moved all the way to the contact which is responsible for creating a current that is sent through the object being powered by the solar cell [16].

Ninety percent of the world’s solar cells bought in 2008 are silicon solar cells with a

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Figure 5 – The basic composition of a solar cell (Image taken from "Solar 101 - Learn How Solar Power Work." Mysolarprojects. Mysolarprojects, n.d. Web.)

maximum efficiency of 27.6 percent [17][15]. Silicon solar cells use silicon as the semiconducting material because it is safe for the environment and it is one of the most abundant resources on Earth, comprising twenty six percent of the Earth’s crust. The top, n-type, layer of silicon is given a negative charge by doping the silicon with phosphorous which has on more electron than silicon while the bottom, p-type, layer is given a positive charge by being doped with boron which has one less electron than silicon. The front and back contacts in a silicon solar cell are made of silver because it is a good conductor while also being very cheap [17]. Until the last decade, the antireflective coating on a silicon solar cell was commonly made with titanium dioxide although now it is more commonly made with silicon nitride because it more efficiently sends light to the silicon layer [18].

Multijunction solar cells are much more efficient than silicon solar cells reaching 44.7 percent efficiency as of 2014 [15]. Multijunction solar cells are made up of multiple

semiconducting layers with a range of band gap values. In solar cells with a single junction there is a possibility that the incoming photons give the electrons too much energy causing them to overshoot, completely leaving the solar cell. Overshooting is the cause for most efficiency loss in a solar cell, creating heat but not creating any electricity. Multijunction solar cells effectively reduce the amount of energy lost to “overshooting” because it allows a greater range of wavelengths to be absorbed since there is a wider range of band gap values. In the 1960’s the projected theoretical maximums in efficiency were between 38 percent and 51 percent but after many advancements

in semiconductor technology the theoretical maximum efficiency jumped to 87 percent [19]. Although individual multijunction solar cells are more expensive than other types of solar cells, they actually save people money because not as many solar cells are needed to create electricity since they are so much more efficient [20].

The last major types of solar cells made today are polycrystalline thin film solar cells which are much smaller than other types of solar cells. There are three different types of thin film solar cells; amorphous silicon (a-Si), copper indium gallium deselenide (CIGS), and cadmium telluride (CdTe). A-Si thin films are basically a smaller version of silicon solar cells. These solar cells are much less efficient that other solar cells because they use a smaller amount of materials which decreases the amount of sunlight absorbed. Because of this a-Si solar cells

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Figure 7 - The current industry standard for multijunction solar cells, GaInP/Ga(In)As/Ge (Image taken from "III-V Multijunction Materials and Devices R&D." NREL: Photovoltaics Research. Alliance for Sustainable Energy, LLC, 24 May 2012. Web.)

are not useful for large scale solar projects like power plants but their small size makes them very useful for powering small objects like calculators [21].

The CIGS thin film solar cell operates in a different way in that it uses electrodes instead of contacts. CIGS uses a layer of either molybdenum or metal foil as one electrode and a layer of zinc oxide as the other electrode. In between these electrodes are two more layers which act as the n-type and p-type layers in other types of solar cells; one layer is copper indium gallium deselenide, a semiconductor, and the other is cadmium sulfide. CdTe thin films are very similar in structure to CIGS thin films. In CdTe thin films one electrode is made from a layer of carbon paste infused with copper and the other is made from either tin oxide or cadmium stannate. The semiconductor used in these thin films is cadmium telluride, along with cadmium sulfide which are used as the n-type and p-type layers. CIGS thin film are very costly but they are also very efficient for thin films and even flexible which makes them useful for curved surfaces [21]. CdTe thin films are cheap but have a low efficiency and are stiff which makes them prone to fractures [22]. These types of thin films are not very commonly used because they are relatively new technologies and because they contain cadmium which is a highly toxic substance [21].

Throughout the past century people have been trying to find new ways of creating renewable energy sources. One renewable energy source that seems very promising is solar power. There are many different types of solar cells including the most common silicon solar cells, the most efficient multijunction solar cells, and the smallest polycrystalline thin film solar cells. Over approximately fifty years solar cells have become increasingly popular and are now used to power a wide range of technology from calculators to homes.

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Works Cited

[1] Smith, Charles. "History of Solar Energy." Solar Energy.com. SolarEnergy.com, n.d. Web.

[2] "History First Solar Powered Engine France Auguste Mouchout 1865." Solarlife Green Economist. Solar Life, 5 Feb. 2012. Web.

[3] Smith, Zachary A., and Katrina D. Taylor. Renewable and Alternative Energy Resources: A Reference Handbook. Santa Barbara, CA: ABC-CLIO, 2008. Print.

[4] "Nothing New under the Sun?: Solar Heating’s Philadelphia Story." NFR. WordPress.com, 4 May 2010. Web.

[5] Gornall, Jonathan. "The Promise of Solar Power, Made a Century Ago." The National. The National, 22 Jan. 2011. Web.

[6] "Solar Cell – Invention History & Story – From Selenium to Silicon." Circuits Today. Circuitstoday.com, 20 Sept. 2013. Web.

[7] "Milestone-Nomination:First Practical Photovoltaic Solar Cell." IEEE Global History Network. IEEE Global History Network, n.d. Web.

[8] Reece, Will. "The History Of Solar Power." Experience. Experience.com, n.d. Web.

[9] Lethbridge, Cliff. "Rockets and Missiles." Spaceline.org. Spaceline, Inc., n.d. Web.

[10] "Vanguard 1." National Aeronautics and Space Administration. NASA, 26 Aug. 2014. Web.

[11] Gruntman, Mike. Blazing the Trail: The Early History of Spacecraft and Rocketry. Reston, VA: American Institute of Aeronautics and Astronautics, 2004. Print.

[12] "History of Solar Energy." Exploring Green Technology. ExploringGreenTechnology.com, n.d. Web.

[13] Perlin, John. From Space to Earth: The Story of Solar Electricity. Ann Arbor, MI: Aatec Publications, 1999. Print.

[14] Feldman, David, Robert Margolis, Ted James, Ran Fu, and Sam Booth. "Photovoltaic System Pricing Trends." Photovoltaic System Pricing Trends (2014): n. pag. National Renewable Energy Labratory. Energy.gov, 22 Sept. 2014. Web.

[15] "Best Research-Cell Efficiencies." National Renewable Energy Labratory. NREL, n.d. Web.

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[16] Toothman, Jessika, and Scott Aldous. "How Solar Cells Work." HowStuffWorks. HowStuffWorks.com, 01 Apr. 2000. Web.

[17] Saga, Tatsuo. "Advances in Crystalline Silicon Solar Cell Technology for Industrial Mass Production." Nature.com. Nature Publishing Group - Asia Materials, n.d. Web.

[18] Tiwari, G. N., and R. K. Mishra. Advanced Renewable Energy Sources. Cambridge: RSC, 2012. Print.

[19] Cherucheril, George, Stephen March, and Avinav Verma. Multijunction Solar Cells. London: Taylor & Francis, 2011. Web.

[20] "III-V Multijunction Materials and Devices R&D." NREL: Photovoltaics Research. Alliance for Sustainable Energy, LLC, 24 May 2012. Web.

[21] Harris, William. "How Thin Film Solar Cells Work." HowStuffWorks. HowStuffWorks.com, Apr. 2008. Web.

[22] "Technology: Thin-Film Photovoltaics." SoloPower. SoloPower.com, n.d. Web.

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