Yash
The Quantum Dots technology improves the efficiency of the solar cell.
One of the largest challenges for solar technology is to increase solar panel 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 and are easy to install practically anywhere where the sun is shining.
The efficiency limit
The most efficient solar cell has an efficiency of around 24% without making it extremely expensive.
To get a quick recap of this limitation, it is because the maximum theoretical efficiency of the single layer silicon solar cell can be reached 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 a band gap of 1.1 eV resulting in maximum efficiency of 33.3%. In simple words, it means that for 100 units of solar energy 33.3 units of electricity are generated and the rest are lost or get wasted, limiting the efficiency to 33.3%.
A quick Re-cap
If someone is not familiar with the basics of a solar cell then here is a quick recap.
The conventional solar cell is a PN junction.
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.
Hence, creating the electron-hole pair which results in the electric current.
Read: Know the I-V Charasteristics of a solar cell
What are Quantum Dots?
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 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.
Thus, making it 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 color.
The largest Nano-crystal will emit red color while the smallest will emit violet and all the other color appears in between these two sizes.
The Relation
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 Nanocrystals emit blue color (The highest energy in the visible spectrum).
While the larger Nano crystal emits the red end of the spectrum (lowest energy in the visible spectrum).
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 increasing the overall efficiency of the cell.
Higher efficiency is possible with Quantum Dots
It is experimentally found that the quantum dots spread over the silicon solar cell can generate multiple electron-hole pairs with a single photon of light.
Hence, increasing the quantum yield.
Thus making it theoretically possible to increase the efficiency beyond the 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.
The working of Quantum Dots
The Principle
Quantum Dots works on the principle of quantum confinement.
Quantum confinement is the confinement of the exciton (the bound state of the electron-hole pair) to a dimension smaller than its Bohr radius.
(It is an 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.
Discrete Energy Levels
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 a similar way as the traditional single-layer silicon solar cell.
There is a bandgap separating the valence band and the conduction band.
And the photon is absorbed in a similar fashion to excite the electron to a higher state.
Thus, creating an electron-hole pair contributing to the electric current.
Impact Ionization
However, the difference is in its ability to generate multiple electrons with a single photon of light which is called 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 at the bottom of the conduction band.
The excess energy instead of getting lost as heat and in the form of lattice vibrations.
This is generally in the case of the bulk semiconductors being utilized to excite another electron from the valence band to reach the conduction band.
This whole process is generally called impact ionization.
Increased Efficiency
This phenomenon results in two electrons reaching the conduction band from a 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 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 to their discrete energy levels and the quantum confinement.
Now when they receive photon energy much higher than their band gap energy, multiple electrons are generated.
This results in an increase in the efficiency of the quantum dot solar cells
Although the efficiency is increased.
The overall cost escalates, making it practically impossible for commercial applications.
Quantum Dots and Toxicity
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 toxicity.
As their main constituents are dangerous heavy metals like Cadmium selenide and lead sulfide.
But it is possible to create quantum dots that are non-toxic and still exhibit many properties of heavy metals.
The more promising options are Copper Indium Sulphide crystals with a protective coating of Zinc Sulphide.
Conclusion
Their lightweight, versatile, nature, increased efficiency, durability, and cost-effectiveness make their future bright.
Now, it will be interesting to see the implementation of the quantum dots on the commercial scale with improved financial feasibility.
And researchers are exploring other architectures of the quantum dots to further increase their efficiency.
