The solar photovoltaic market is one of the fastest developing energy markets in the world. By 2030, the solar power industry will see a growth of a factor of 10. In order for solar energy to succeed, a new technology is required that can provide superior efficiencies and lower the costs below standard silicon PV panels. Quantum dot solar cells just might be that technology.

A quantum dot solar cell (QDSC) is a solar cell that uses quantum dots as the captivating photovoltaic material. It is used to replace bulky materials such as silicon, or copper indium gallium selenide. Quantum dots have bandgaps that are adjustable through a wide array of energy levels by changing the size of the dots.

A new type of quantum dot could lead to cheaper solar cells and better satellite communication (Image: University of Toronto)

Quantum dots are considered to be artificial atoms. Their energy levels are adjustable by altering their size, which in turn delineates the bandgap. The dots can be grown in a variety of sizes, allowing them to convey a variety of bandgaps without changing the underlying material or construction. Sizing is achieved by varying the fusion duration or temperature.

Because the bandgap of the quantum dots can be adjusted, quantum dots are desirable for solar cells. Frequencies in the far infrared that are characteristically difficult to achieve with traditional solar cells can be obtained using lead sulfide colloidal quantum dots. Half of the solar energy reaching the Earth is in the infrared region. A quantum dot solar cell makes infrared energy as accessible as any other.

Researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have shown that nanotechnology may greatly increase the amount of electricity produced by solar cells. Tiny nanocrystals—also known as quantum dots— yield as many as three electrons from one high energy photon of sunlight. When today's photovoltaic solar cells absorb a photon of sunlight, the energy gets converted to at most one electron, and the remaining energy is lost as heat.

Quantum dot solar cells have the potential for solar, or photovoltaic cells that reduce wasteful heat and capitalizes on the amount of the sun's energy that is converted to electricity.  This is significant toward making solar energy more cost-competitive with conventional power sources.

Existing solar cells go as high as 33% conversion efficiency, but production solar cells that are installed on roofs have on average a much lower efficiency. Therefore, if quantum dot solar cells could be manufactured cheaply they would be at least three times more efficient than existing manufactured solar cells. Solar cells based on quantum dots could convert more than 65 percent of the sun's energy into electricity.

Structure and working of Quantum dot solar cell

Quantum dots acquire surplus photon energy, which is usually lost to heat generation through a process called multiple exciton generation. The light rays enter through the transparent electrode of a quantum dot solar cell onto a light absorbing layer of dots in order to generate electron hole pairs. The charged particles then separate and eventually travel to their respective electrodes, producing electric current.

The following are the benefits of quantum dot solar cells. They have a favorable power to weight ratio with high efficiency. The mass and area savings as well as flexibility leads to miniaturization. Their power consumption is low. There is an increase of electrical performance at low production costs. Their use is versatile and can be used in windows, not just rooftops.  

There are some disadvantages of QDSC. Cadmium selenide-based quantum dot solar cells are highly toxic in nature and require a very stable polymer shell. Cadmium and selenium ions which are used in the core of quantum dots are known to be cytotoxic. Quantum dot metabolism and degradation within a human body is still chiefly unknown and studies have shown that quantum dots accumulate in the kidney, spleen, and liver.

In aqueous and UV conditions, degradation increases. The particles do not possess the high crystallinity seen in organically made quantum dots, but the process is easier, inexpensive, and more reproducible than organic synthesis.

Quantum dots present advantages over organic dyes, but quantum dots can have surface defects, which can influence the recombination of electrons and holes by performing as temporary traps. Researchers needed to understand why the charges got trapped in the material. The origin of traps is the way surface treatments affect the material. The key factor is distribution with well-controlled elemental ratios. The electrons will be happy when the distribution is just right.

The traps result in the blinking of the quantum dots and deteriorates a quantum yield, which is the ratio of production to absorption. The blinking effect can be reduced by having a shell around the core, but the shells can alter optical properties and it’s hard to regulate the size of the particles.

When positioned into live cells, quantum dots display aggregation, which can interfere with cell function, which could be killed during the delivery process. Although quantum dots are in the nanometer range, bioconjugation with different molecules will increase the size of the dots making delivery into cells more problematic.  

Quantum dot’s bandgap can be tuned by adjusting their size or composition. From a single material system, the bandgap can be adjusted from the visible to the infrared. The quantum dots are processed from a solution that is consistent with high-quantity, cost-efficient roll-to-roll processing technologies. This process will reduce expensive vacuum deposition, lower cell and module weight, and related costs.

There remains a lot of work to be done before quantum dot solar cells will be presented on a commercial basis, but the potential is great. A huge step forward has been made; and over the coming years, there is confidence that quantum dot solar cells will provide an efficient and stable method of exploiting solar power.

The use of quantum dots is intensifying as more is discovered about how they work and their distinct properties. The technology of solar cells is proceeding quickly and solar cells using quantum dots are seen as an encouraging solution for the future.

For additional information:

1. https://www.cornellcollege.edu/physics-and-engineering/pdfs/phy-312/colins-fungura-zasada.pdf

2. http://www.natcoresolar.com/core/wp-content/uploads/2014/04/Solar-cells-and-Quantum-Dots.pdf

The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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