Overview: The Quantum Dot
One of the primary challenges in the solar cell industry is depleting the cost per watt of electricity produced by solar power systems. The typical bulk of single-junction solar cells and photons with energy less than the semiconductor bandgap are not harnessed while those having much larger energies than the bandgap can produce hot-carries. Upon the thermalization process–where the cooling down takes place, the excess energy gets wasted and turn into heat.
Therefore, microstructures with a bandgap that is easy to be adjusted and adapted to match the spectral distribution of the solar spectrum are indeed crucial to the production process of the solar cells. Moreover, the solar photovoltaic market is rapidly developing and to make it more even successful, new technology is needed to provide predominant efficiencies without causing higher costs. This new technology is called “Quantum Dot Solar Cell or QDSC” which has tunable bandgaps and easily formed intermediate bands that can be used to create junctions using cheaper substrates like plastics, metal sheets or glass sheets.
What is Quantum Dot?
Quantum dots are considered as artificial atoms. These quantum dots are being used in replace to the bulky and heavy materials such as silicon, or copper indium gallium selenide (CGIS). The quantum dots are possible to adjust in a variety of sizes which also helps in tuning its energy levels, allowing them to convey different types of bandgaps without changing the underlying materials or construction. Sizing is achieved by varying the fusion duration or temperature. Quantum dots can be moulded into different types, it can be in two-dimensional sheets or three-dimensional arrays unlike the traditional semiconductor materials such as crystalline or amorphous.
What is Quantum Dot Solar Cell (QDSC)?
A quantum dot solar cell (QDSC) is a type of solar cell that uses quantum dots as the captivating or absorbing photovoltaic material. QDSC is used for replacing heavy materials such as silicon, or copper indium gallium selenide (CIGS).
Quantum dots are considered as artificial atoms. Their energy levels are adjustable by altering their size, which in turn delineates the bandgap. It is possible to grow the dots in a variety of sizes, allowing them to convey a variety of bandgaps without changing the underlying material or construction. In the future, QDSC can provide superior efficiency and bring down the cost of standard silicon PV panels.
QSDC can harness excess photon energy which is normally lost to heat generation through multiple exciton generation processes. Since the bandgap of the quantum dots can be tuned, these are perfect and ideal for solar cells. Through the use of lead sulphide colloidal quantum dots, the frequencies of the traditional solar cells far-infrared can be easily obtained. Since half of the solar energy reaching the Earth is in the infrared region, quantum dot solar cell makes the infrared energy possible to access by other regions.
In addition, quantum dot solar cells have a big potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to 66 percent by using hot photogenerated carriers to produce higher photocurrents or photovoltages.
Background of Quantum Dot Solar Cell (QDSC)
Quantum dots are under the special class of semiconductors, which are the nanocrystals. It is composed of a variety of periodic groups such as II-VI, III-V, or IV-VI materials and it can confine electrons through quantum confinement. When the quantum dots’ size finally meets the size of the material’s exciton Bohr radius, the quantum confinement effect becomes prominent and the energy levels of an electron can no longer be considered as a continuous band. They are now treated as discrete energy levels.
Consequently, quantum dots are now considered as an artificial molecule with energy gap and energy levels spacing dependent on the radius of its size. As the size of the quantum dots decreases the energy bandgap increases. Whereas, when the size of a quantum dot increases its absorption peak becomes redshifted due to the bandgap shrinkage. The tunable bandgap of quantum dots allows the construction of nanostructured solar cells which harvests more solar spectrum. In addition, quantum dots have large intrinsic dipole moments, which may lead to fleeting charge separation.
Also, quantum dots have been discovered to emit up to three electrons per photon due to the process of multiple exciton generation (MEG), which on the other hand standard crystalline silicon solar cells can only emit one. Based on the theoretical view, this could possibly boost solar power efficiency from 20 percent up to 65 percent.
Moreover, QDSC offers easy procedures and preparation. While it was suspended in the form of colloidal liquid they can be easily handled all throughout the production process, with a fumehood as the most compounded tools required. These are also typically synthesized in small batches but it can be manufactured on a larger scale. The dots can be distributed on a substrate by spin-coating either manually by hand or using an automated process. Large-scale production could use spray-on or roll-printing systems, significantly reducing module construction costs.
Quantum Dot Solar Cell in Relation to Photovoltaic System
Quantum dots acquire surplus photon energy, which is typically lost to heat generation through a process called multiple exciton generation. Generally, this type of solar cell works through the use of a photon by light travelling into the cell and hitting the particles of a quantum dot which in turn increases the energy of some of the electrons in the quantum dots. These excited electrons will be injected into the titanium dioxide and it will travel through the conducting surface of the electrodes.
While the electrons are traveling to the electrode’s conducting surface, they leave some holes in the quantum dots which will be filled by other electrons. The holes in the quantum dots are being filled by taking out the electrons from the electrolyte. The depleted electron from the electrolyte, in turn, will take the electrons from the counter electrode. As a result, a voltage will be created across the cell and it will induce a current.
Quantum dots are being used to complement the solar cell with a thin coating. Then the harnessed solar energy is then transported and converted into an electrical current to provide off-grid energy. The specific bandgap of quantum dots is proportional to the frequency of sunlight, where the light can be absorbed to excite the particles of nanocrystal.
Benefits and Drawbacks of QDSC to Solar Energy System
Solar photovoltaic panels that are using quantum dots solar cells are lightweight and versatile in nature compared and these have a comparatively low cost compared to those traditional photovoltaic. Unlike many ordinary solar cells, quantum dot cells manifest substantial and fair durability. They do not need to be exposed to high temperatures or to an inert atmosphere to produce energy. These solar cells have no signs of degradation at all after five months of utilization in normal weather conditions.
The manufacturing process of quantum dot photovoltaic cells has considerably less energy than other types of solar cells. Aside from that, the entire cell, excluding the electrodes can be placed at room temperature under normal conditions, without the use of any solution. This is indeed beneficial as it lessens the manufacturing processes required with the transport of materials.
However, with all of its advantages, there is still room for improvement. There are still works needed to be done before using the quantum dots for photovoltaic cells such as measuring up the other techniques currently used for photovoltaic cells like silicon-based cells. Moreover, the efficiency of quantum dots is lower than the standard rates of the solar energy industry. It only has approximately 9 percent of sunlight energy being successfully converted into electricity. Hence, it is still understandable with the fact that it was only given a short span of time to be used in photovoltaic cells, compared to the silicon-based cells which are being used for almost 6 decades.
Future Developments on Quantum Dot Solar Cell
The permanence of quantum dots solar cells has no solid research foundations but it was believed that future researches for this subject will definitely explain and expand this phenomenon so it can be used to greater advantage. Further research and test should be conducted still to open up the possibility of quantum dots utility and to make a solid foundation for this field.
This subject is just starting to bloom but it has potential in the solar industry. However, there are still more significant works to be done before the quantum dot voltaic cells may be offered in the market on a commercial basis, but the potential success they may carry for the future is undeniable. A great starting step has been made and over the coming years, there is a big hope that quantum dot voltaic cells may provide an efficient and effective method of harnessing solar power.
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