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How Quantum Dots could Make Solar Technology Better

published: 2011-07-12 9:16

Solar energy offers an attractive proposition: utilize this abundant energy well and the energy supply will be guaranteed.  This is an indication of how much insolation hits the earth, and even exploiting a fraction of this energy will sufficiently meet the demands of our societies. Accordingly, technology has been fashioned to harness this free energy, from the production of monocrystalline silicon solar cells to cadmium-based thin-film solar cells (such as cadmium telluride, CdTe) and copper-indium-gallium-selenide (CIGS) thin-film solar cells.

Unfortunately, the issue of efficiency has afflicted the industry for a very long time. Although each development has progressively improved the efficiency (and sometimes the cost) of solar energy technology, this progress has been muted by the sheer fact that current solar cells only absorb visible light; by exploring ways to increase the ‘capture zone’ of solar radiation, the efficiency of the cell might be increased.

The electromagnetic (EM) spectrum lists the range of frequencies found in electromagnetic radiation, from radio waves to high-energy gamma rays. Visible light (or just ‘light’ for short) allows for humans to see the world in color, and is found at the wavelength of between 380 nanometers (nm) and 780 nm. Light from the sun encompasses the complete EM spectrum, but current solar technology focuses mainly on absorbing visible light, meaning the current solar cell technologies are tuned to absorb solar radiation between 380 nm and 780 nm.

This disregards the light available that is in the infrared (700 nm to 1E6 nm, or 1 millimeter (mm)) and ultraviolet (100-380 nm) regions. One way to increase the efficiency of the solar cell, therefore, would be to tune the solar cell to absorb at either the ultraviolet region or at the infrared region or perhaps even to produce a solar cell that could absorb light at all wavelengths between 100 nm and 1 mm. To achieve this, researchers have turned to quantum science as a possible route, and more specifically, the use of quantum dots. 

A field of science free of boundaries

Quantum science explores the worlds of physics, chemistry, computer science, mathematics and quite recently, biology, allowing for exceptional and extraordinary hypotheses to be constructed; the peculiarities of quantum science allowed for the area of ultrafast supercomputers and the theory of quantum entanglement to be developed, and for concepts such as time travel and alternate particles (termed anti-particles) to be postulated.

The benefit to industry cannot be understated: from information and communications technology (ICT) to medical technology, industry will benefit from the commercialization of quantum-dot based technology by producing technology that will be quicker, more efficient, and more reliable.

This polarizing field of science has, through its oddities, allowed for the possibility of a near-perfect solar cell (thanks to quantum biology) and now, the possibility of a solar cell able to absorb visible light and other parts of the EM spectrum, for example, infrared light and ultraviolet light.

A Quantum dot holds the key to more efficient solar cells 

A quantum dot, in its basic form, is a fractional measure or portion of some material (or any matter) whose elemental component, the exciton (which can be looked upon as being the result of the bound state between the electron and hole) exists in the three dimensions of space.  This can be loosely translated as the ability for a dot to exist in one place and have the ability to move in three dimensions, up/down, left/right, and forwards/backwards.  One feature of an exciton is that it is able to transport energy with no alteration or interference of electrical charge, making their use in many applications, and in particular solar cells, considerably attractive.

Quantum dots are generally formed in colloidal suspensions and termed colloidal quantum dots (CQD), however, they are normally referred to as quantum dots.  Quantum dots exhibit electronic properties that float between that of a semiconductor and individual molecules and as such, will decide on the electronic properties according to the dimensions of the nanocrystal it becomes. This results in variable bandgaps, which in turn allow for tunability of the dot.

In general, smaller nanocrystals possess larger bandgaps that result in more energy being required to excite the nanocrystal. However, more energy for excitation results in more energy being released when the nanocrystal relaxes.  This control characteristic offers the prospect of producing solar cells with multiple layers on a gradient scale.

For manufacturers of solar cells, such as major players Motech Industries or Suntech Power, this degree of control might be beneficial in reducing costs associated with the manufacture of solar cells.

Quantum enhanced electron-ejected pathway

When sunlight strikes a solar cell, one of three possibilities take place. The first involves light passing through. The second involves light being reflected, or lost as heat. And the third is light being absorbed, initiating the chain of events that starts with an electron being ejected and ends with electricity being transferred to a load. For the absorbed photon from sunlight, one electron is ejected.

Quantum dots are different; the theory predicts that quantum dots will be able to ‘kick out’ three electrons, thereby increasing the efficiency threefold. This process was termed multiple exciton generation (MEG), and this is just one of the two factors that make quantum dot-based solar cells very promising; in addition to multiple exciton generation, quantum dots are also beneficial with respect to the issue of heat loss.

Multiple exciton generation (MEG) allows for the possibility of increasing the rate significantly due to multiple electrons being ejected as opposed to the single electron in traditional solar cells. The phenomenon of MEG has been demonstrated in a variety of nanocrystals from cadmium selenide to silicon, and lead selenide to lead telluride, indicating a very encouraging future for use in photovoltaic devices.

This nascent research is still many years (if not decades) away from commercialization, but the research is certainly one that will greatly enhance the efficiency of the solar cell and therefore be extremely attractive to manufacturers in industry.

Quantum dots for reducing heat loss

As mentioned, photons from sunlight have one of three paths to follow and one such pathway is a photon being reflected, or simply referred to as heat loss. Solar cell technology is troubled with this issue so research into reducing this issue will be greatly beneficial to industry.

In 2010, researchers at the University of Texas and the University of Minnesota managed to integrate semiconducting nanocrystals of lead and selenium to move electrons quicker than before, and especially quicker than before the electrons are lost as heat. These quantum dots allowed for electrons to not only be captured before they were lost as heat but also channelled to the electron-accepting part of the solar cell. Experiments showed that the transfer time took less than 50 femtoseconds (1 femtosecond = 1E-15 second, or one quadrillionth of a second) and due to this very efficient transfer time, the rate of heat loss was reduced. Furthermore, experiments showed that the efficiency was boosted and rated at 66%, a considerable efficiency rate for a solar cell. This provides another example of what to expect with future generations of solar cells.

However, the use of quantum dots to reduce heat loss is not without issues. Chief among them is the issue of producing a connecting electrical wire that will be able to handle this enhanced electron capture rate; the wire connects the quantum dot-solar cell to the power unit and so the wire will need to be small enough for the quantum dot-solar cell but big enough to handle the electron flow and minimize heat loss. Research is currently underway on providing solutions to these issues and once achieved, the solar industry will no doubt be better by the advancement in reducing heat loss.

The device that multiplies quantum effects in generating solar energy

A different thread of research involved the use of graded layers of quantum dots to make a mark in solar cell technology.  Quantum dots have been used to further advance a second-generation solar cell technology: dye-sensitized solar cells (DSSC), a type of thin-film solar cell. DSSCs make use of the presence of quantum dots to covert sunlight into electrical energy.

In a traditional DSSC, titanium dioxide nanoparticles are covered in a specific molecular dye that is used for solar absorption.  The coated titanium dioxide is then placed under an electrolyte solution and with the platinum catalyst above the electrolyte solution, is then able to convert electricity from striking sunlight.  In the quantum version of this DSSC, the molecular dye that absorbs sunlight is replaced with quantum dots.  For example, in 2006, researchers used quantum dots of cadmium selenide and assembled a titanium dioxide film to produce a DSSC.

The efficiency recorded was very low, at 1.35%, but the science was proved correct. Since 2006, research in this area has proceeded at pace producing quantum dots of a variety of molecules, such as lead selenide and cadmium telluride. The research is many years away from being commercialized but it does offer a tantalizing peek at what the future of solar cells might look like.  DSSC manufacturers, which fall under thin-film manufacturers, will benefit by the boost to efficiency (by the use of quantum dots) and the reduction in associated costs (due to quantum dots being inexpensive).

A significant breakthrough came in June 2011. Researchers at the University of Toronto, led by Professor Sargent, developed a tandem-arranged colloidal quantum dot solar cell device called the graded recombination layer, and this illustrates the possibility of a solar cell absorbing both visible and infrared light. The device is made up of two layers, or two stacks; one layer absorbs the visible light and the other layer captures the infrared light.

This laboratory-scale device is said to have a theoretical efficiency rating of 42%, comparable to the Gallium arsenide multijunction solar cellbut as this graded recombination layer device (and on-going research) is still in the pre-commercialization stage, a cautious approach is required before making any bold assertions on the efficiency. What is certain is that the device has a recorded efficiency rating of 4.2%, but it is important to note that theoretically the efficiency can reach ten times this figure.

Efficiency issues aside, the research principles are really quite extraordinary. Conventional solar cells utilize single junctions in their composition, resulting in one p-n junction (where one P-type and one N-type semiconductor combine to form a junction). Tandem solar cells, otherwise known as multijunction solar cells, utilize many p-n junctions. In the case of the device created by professor Sargent, using n-type titanium dioxide and p-type lead selenide colloidal quantum dot film layers, the multiple p-n junctions are effectively and efficiently arranged so as to accommodate efficient transport of electrons between the two layers with no degradation to either layer.

The quantum dots used for the device are tuned so that the dots at the front cell and tuned to receive visible light and the dots in the back cell are tuned to receive infrared light. Thus, one has a device that is able to collect more photons from sunlight and thereby increase the efficiency of converting sunlight into electricity.

A quantum-based future for the solar industry

The research noted in this article points to a future generation of quantum-based solar cells offering higher efficiencies than what is currently available. In addition, quantum-based solar cells will reduce the cost of manufacturing solar cells as the material used for the quantum dots are inexpensive and relatively abundant. What is more is that manufacturers will not be required to change their machinery. Another benefit of using quantum dots is that it is a flexible material, offering the possibility of creating quantum-dot composite solar cells, perhaps even lowering the cost of manufacturer even lower.

The research into adopting quantum science into next generation solar cells is still relatively new, but with modern technology advancement, researchers will be able to produce solar cells that incorporate quantum dots that are appealing to both industry and consumers. With the characteristics of extremely high efficiencies and lower manufacturing costs, quantum-based solar cells do indeed offer the prospect of utilizing solar energy better.

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