What is Quantum Dot technology?

​One of the largest challenges for solar technology is to increase solar energy efficiency without making them practically expensive. 

​With the growing environmental issues associated with the burning of fossil fuels, there is always support for the alternate energy that is Solar across the globe. 

​One of the main advantages of solar technology is that the source of energy is free of cost and the solar panels are very modular which are easy to install practically anywhere where the sun is shining. 

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Why the efficiency is so low?
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​The most efficient solar cell has the efficiency of around 24% without making it extremely expensive.

To get a quick recap of this limitation, it is because of the maximum theoretical efficiency of the single layer silicon solar cell can be reached to 33.7% (Shockley-Queisser limit) with the ideal band gap of 1.34 eV.

The Shockley-Queisser limit assumes that one electron-hole pair is created by every falling photon on the silicon solar cell.

The silicon solar cell has the band gap of 1.1 eV resulting in the maximum efficiency of 33.3%. In simple words, it means that for 100 units of solar energy the 33.3 units of electricity are generated and the rest are lost or get wasted, limiting the efficiency to 33.3%.
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• If someone is not familiar with the basics of solar cell then here is a quick recap: The conventional solar cell is a PN junction and when the incoming photons of light, having energy greater than or equal to the band gap energy of the cell, hits the solar cell they get absorbed and the electrons are knocked off from the valance band and jumped to the conduction band, creating the electron-hole pair which results in the electric current.

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What are Quantum Dots?
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​The quantum dots are very small semiconductor particles of the size few Nano-meters (2-10 nm). Because of their small size, the physics of these particles like their optical and the electrical properties are governed by quantum mechanics.

It is their ability to absorb energy from the different regions of the visible spectrum by changing their sizes, making possible to increase the efficiency of the traditional solar cells beyond the Shockley-Quiesser limit.

By changing the size of the quantum dots solar cell, you can change the emission colour. The largest Nano-crystal will emit red colour while the smallest will emit violet and all the other colour appears in between of these two sizes.

(The band-gap of the nano-crystals is inversely proportional to their sizes that is the largest nano-crystal will have the smallest band-gap while the smallest nano-crystal will have the largest band gap.

That is why the smaller Nano crystals emit blue colour (Highest energy in the visible spectrum) while the larger Nano crystal emit the red end of the spectrum (lowest energy in the visible spectrum)
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The quantum dot solar cells of different sizes are spread over the traditional solar cell making it possible to absorb a greater portion of the spectrum, hence increases the overall efficiency of the cell.

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Higher efficiency possible with Quantum Dots
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​It is experimentally found that the quantum dots spread over the silicon solar cell can generate multiple electrons-hole pairs with a single photon of light, increasing the quantum yield thus making it theoretically possible to increase the efficiency beyond Shockley-Queisser limit.

Some tried to join or stack multiple semiconductors together in order to absorb most of the light from the spectrum but the method of making multi-junctions is very expensive and practically not feasible.

​In this arrangement, the semiconductors of different band gaps are joined together to capture the maximum portion of the sunlight thus increasing the efficiency of the solar cells. Although, the efficiency is increased the overall cost escalates, making it practically impossible for commercial applications.​

​While on the other hand, the quantum dot solar cells capture most of the spectrum at much lower. With such flexibility and the versatility in the design, the quantum dots offer much easy and the cheaper alternative than the multi-layer semi-conductors.

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Working of Quantum Dots
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​Quantum Dots works on the principle of the quantum confinement. The quantum confinement is the confinement of the exciton (the bound state of the electron-hole pair) to the dimension smaller than its Bohr radius (approximate distance between the nucleus and the electron of the hydrogen atom in the ground state).

When exciton gets confined, it behaves more like a particle in the box rather than the continuous energy which is seen in the bulk semiconductors. Now, these quantum dots have discrete energy levels.

In fact, the quantum dot solar cell works the similar way as the traditional single layer silicon solar cell. There is bandgap separating the valence band and the conduction band and the photon is absorbed in the similar fashion to excite the electron to the higher state, creating an electron-hole pair contributing to the electric current.

However, the difference is in its ability to generate multiple electrons with a single photon of light which is called the impact ionization. When the photon energy is higher than the bandgap energy (at least 2 times), the electron gets excited to jump to the conduction band.

Eventually, the electron releases the excess energy and gets settled to the bottom of the conduction band.

The excess energy instead of getting lost as heat and in the form of lattice vibrations which is generally in the case of the bulk semiconductors, is utilized to excite another electron from the valence band to reach the conduction band (This whole process is generally called as the impact ionization).

This phenomenon results in two electrons reaching the conduction band from the single photon of light.

However, the traditional solar cells also exhibit the same phenomenon of impact ionization but in their case, the rate of impact ionization is much slower than the rate of heat generation.

In other words, the excess energy released by the electrons is dissipated more readily in the form of heat and the lattice vibrations than used in exciting another electron.

While in the case of the QDSC (quantum dots solar cell) the rate of heat generation is significantly reduced due their discrete energy levels and the quantum confinement.

​Now when they receive photon energy much higher than their band gap energy, the multiple electrons are generated. This results in the increase in the efficiency of the quantum dot solar cells.       

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Quantum Dots and Toxicity
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​The different semiconductors can be used to create quantum dots like:

• Cadmium selenide
• Cadmium Sulphide
• Lead Selenide
• Lead Sulphide
• Indium Arsenide
• Indium Phosphide

However, despite their benefits, the main concern is the toxicity as their main constituents are dangerous heavy metals like Cadmium selenide and the lead sulphide. But is possible to create quantum dots which are non-toxic and still exhibit many properties of the heavy metals.

​The more promising options are Copper Indium Sulphide crystals with a protective coating of Zinc Sulphide

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Conclusion
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​Their lightweight, versatile, nature, increased efficiency, durability and cost effectiveness makes their future bright.

​Now, it will be interesting to see the implementation of the quantum dots on the commercial scale and researchers exploring other architecture of the quantum dots to further increase their efficiency.

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