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| Fig. 1: Unit cell of silicon. (Source: Wikimedia Commons) |
The first solar cell was created in 1941 by Russell Ohl, a materials scientist at Bell Labs. Its energy conversion efficiency was 1%, so for every 100 Joules of solar energy that fell upon it, it created 1 Joule of electrical energy. [1] Over the past 70 years, silicon solar cells have been pushing towards the maximum limit of 32% for silicon. [2] The world record stands at 25% for a single junction silicon solar cell. [1] Currently, the state of the art for commercial silicon solar cells comes from 3 separate companies that all distribute panels with 21% efficiency to their customers. [3] With the world craving a new source of energy besides fossil fuels, silicon solar cells will play a much larger role in the future.
An ideal solar cell has a direct band gap of 1.4 eV to absorb the maximum number of photons from the sun's radiation. Silicon, on the other hand, has an indirect band gap of 1.1 eV. Silicon is not the ideal solar cell, but it provides several advantages: silicon is very stable (it has the same crystal structure as diamond - see Fig. 1), it is not toxic, it is the second most abundant element in the earth's crust, and silicon engineering has already been heavily studied (making wafers, doping, patterning, and making electrical contacts are all well understood). [2] A single silicon atom has 14 protons, 14 neutrons, and 14 electrons; its electron configuration largely determines its properties. Silicon atoms bind together with other silicon atoms readily in the same diamond unit cell as carbon. This lattice repeats itself an incredible number of times (on the order of 1023 times).
The seemingly infinite lattice creates bands of allowed energy states; silicon creates a band gap where no electrons are allowed to exist (a band gap that is 1.1 eV wide). Semiconductors have this band gap where electrons can be excited through the band gap into the conduction band where they can be extracted for electricity. Conductors (metals) on the other hand can be understood as a sea of electrons where there is no band gap. Doping silicon with different elements changes the electronic structure and imbue new properties into silicon. Boron doping creates p-type silicon (excess holes/electron acceptors), and phosphorus doping creates n-type silicon (excess electrons). If you combine p-type and n-type silicon you create a p-n junction; p-n junctions separate electron and hole pairs to sweep away electrons as current (see Fig. 2).
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| Fig. 2: Here is a drawn band diagram of a p-n junction of a silicon solar cell showing electron-hole pairs being created by incoming photons with energies of hv and electrons being swept away to the right to be used for our electricity. (Source: A. Zhao) |
Engineering solar cells involve 4 main concerns: (1) absorbing as many photons as possible, (2) maximizing the number of carriers created and extracted, (3) minimizing materials cost, and (4) ensuring mechanical integrity. To address (1), solar companies may etch the surface of the solar cells into pyramidal shapes to maximize the amount of light that enters the solar cell. In addition, rear point contact cells have been developed so there are no metal electrodes on top preventing the reflection of sunlight. [4] Addressing (2) and (3) - maximizing extracted electrons and minimizing costs - vary from company to company and are trade secrets. But for (4), ensuring mechanical integrity, there are a variety of ways to ensure solar cells will last 25+ years for consumers to install without worrying about the failure or breaking of solar cells. Nitrogen doping is a method to increase the yield strength of silicon as well as using stronger metal back contact to stabilize the silicon solar cell. [5]
Solar power provided 17 gigawatts of electricity for the United States in 2010. The current SunShot initiative from the Department of Energy is to multiply the capacity by 12 times to 300 gigawatts by 2030. [3] Solar energy only provides 0.6% of the total energy consumed in the United States, so it must increase to wane off the dependence on fossil fuels. In 1977, the cost of installing solar panels was $76.67 per watt; today it costs 60 cents per watt. [3] Solar energy is finally coming out of its infancy and gearing up to compete with coal and petroleum - our mastery of silicon has played a large role in the come up of solar power.
© Andrew Zhao. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. A. Green, "The Path to 25% Silicon Solar Cell Efficiency: History of Silicon Cell Evolution," Prog. Photovoltaics 17, 183 (2009).
[2] J. Nelson, The Physics of Solar Cells (Imperial College Press, 2003).
[3] R. Ramesh, "SunShot Vision Study," U.S. Department of Energy, February 2012.
[4] J.M. Gee et al., "Back-Contact Crystalline-Silicon Solar Cells and Modules," Sandia National Laboratory, SAN 099-0491 C, 8 Sep 98.
[5] K. Sumino et al., "Effects of Nitrogen on Dislocation Behavior and Mechanical Strength in Silicon Crystals," J. Appl. Phys. 54, 5016 (1983).
