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  • Writer's pictureZhongyi Ho

Solar Cells: Silicon Cells VS Quantum Dot Cells

Updated: Jul 1, 2023

Edited by Kimberley Chee.



As the world transitions to a more sustainable future, coupled with constantly advancing technology, it is inevitable that new technology will be ubiquitous in the renewable energy industry. An example of this is the rise of both silicon solar (photovoltaic) cells, and quantum dot solar cells.


Before getting into the details of how a solar cell works, it is necessary to discuss why we should harness solar energy. Solar energy, in simple terms, is the energy or power generated from the sun. Light is generally described as a wave, but it can also be described as particles, also known as photons. Photons are particles with zero rest mass, and are known as ‘packets’ (or ‘quanta’) of energy that travel at the speed of light. Whenever we talk about light behaving as a particle, we are actually just describing a photon. Just like any other moving particle or object, photons have their own equation that demonstrate their energy, that is: E=ℎf where E stands for energy of a photon in Joules, ℎ stands for Planck’s Constant (6.63*10-34 Js) which is the constant that connects the energy carried by a single photon to its frequency, and f stands for frequency of the photon.


Figure 1: Demonstrating the difference between the energy equation for a particle and for a photon.

Solar cells are meticulously designed so they convert the energy of a photon to electrical energy. To give you an illustration of how both silicon solar cells and quantum dot solar cells generally work, it is when a photon gives energy to an electron in the solar cells (exciting it), the electron receives energy so it can escape from its current orbit and travel to a higher energy level, where it can then be converted to electrical energy via a series of methods, which will be described below.


Silicon solar cells are a type of photovoltaic cell that use p-type and n-type semiconductors to create a current. To understand how electricity is generated, it is necessary to investigate what happens in the p-n junction. Semiconductors have electrical conductivity between an insulator and a conductor, and semiconductor doping is the process of adding impurities into a semiconductor crystal to potentially create ‘holes’ and free electrons. In an n-type semiconductor, phosphorus, an atom with 5 electrons in the outer shell, is added in small quantities to the silicon semiconductor via n-type doping - this causes free electrons to be generated in the semiconductor crystal. In a p-type semiconductor, however, via p-type doping, boron, an atom with 3 electrons in the outer shell, is added in small quantities to the silicon semiconductor— this causes ‘holes’, or gaps where electrons could potentially fill in within the semiconductor crystal. As the n-type and p-type semiconductors are stacked on top of one another, the boundary between the two is what is known as the depletion region.



Figure 2: Shows the internal structure of the semiconductor crystal and the depletion region.

When light shines on the PV cell, the electrons will gain energy and be able to move freelycausing a charge difference between the two-types of semiconductors. If they were to be connected with wires in a circuit, a current would be generated.


Quantum Dot Solar Cells (QDSC), on the other hand, rely on quantum mechanics and use quantum dots as the photovoltaic material. Quantum dots, albeit similar to semiconductors, are only a few nanometres in size; they have adjustable band gaps, meaning more electrons can be released per photon. It is estimated that QDSCs are at least three times more efficient than traditional solar cells, considering how it can convert more than 65% of the Sun's energy into electricity, compared to traditional silicon solar cells, which only have an approximate 33% conversion efficiency.


Figure 3: Showing the relationship between quantum dot size and band gap energy.

Despite all the technological advancement, it still takes a long time to invent something new and efficient. With that being said, money is being poured into research and development to manufacture more efficient types of solar cells. With the rise of new types of solar cells, for example the perovskite solar cell​​s, there is no doubt that we will soon be able to harness more of the Sun’s energy it is just a matter of time until we become a more sustainable species on Earth.


 

References:

  1. Factors, F. (2021, July 13). Global Quantum Dot Solar Cell Market Share Will Reach USD 2,319.8 Million by 2026: Facts & Factors. GlobeNewswire News Room; Facts & Factors. https://www.globenewswire.com/news-release/2021/07/13/2261998/0/en/Global-Quantum-Dot-Solar-Cell-Market-Share-Will-Reach-USD-2-319-8-Million-by-2026-Facts-Factors.html

  2. ‌Sutter, P. (2022, March 10). What are photons? Livescience.com; Live Science. https://www.livescience.com/what-are-photons

  3. Photon Creation and Absorption – EWT. (2022). Energywavetheory.com. https://energywavetheory.com/photons/photon-interactions/

  4. Doping: n- and p-semiconductors - Fundamentals - Semiconductor Technology from A to Z - Halbleiter.org. (2022). Halbleiter.org. https://www.halbleiter.org/en/fundamentals/doping/

  5. Solar cells, their construction, and working. (2021, May 20). ASRMETA. https://www.asrmeta.com/solar-cells-their-construction-and-working/

  6. How a Solar Cell Works - American Chemical Society. (2013). 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#:~:text=When%20sunlight%20strikes%20a%20solar,to%20the%20p%2Dtype%20layer.

  7. Quantum Dot Solar Cells Are Coming | AltEnergyMag. (2014). Altenergymag.com. https://www.altenergymag.com/article/2018/05/quantum-dot-solar-cells-are-coming/28547#:~:text=The%20light%20rays%20enter%20through,respective%20electrodes%2C%20producing%20electric%20current.

  8. Boudouris, B. W. (2014, December 29). Introduction to Quantum Dots and Solar Energy Conversion Devices. Nanohub.org. https://nanohub.org/resources/21815/watch?resid=21816

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