Describes a new efficiency milestone that has been achieved by quantum dot solar cells, a kind of solar PV that uses the unique properties of quantum dots to capture photons and convert their energy into an electric current. This form of solar PV has languished for some time due to the low efficiencies that have been so far achieved. By finding a way to produce a double layer cell the research team at the University of Toronto has been able to achieve significantly higher overall efficiencies than has previously been possible using quantum dot technology. Quantum dot technology holds the promise of being able to ultimately produce very inexpensive solar cells, if the low efficiency problem can be overcome.
A research team at the University of Toronto has achieved an efficiency milestone of 4.2% conversion with a solar cell that is based on colloidal quantum dots (CQD) and has two discreet layers that enables it to capture both visible and near-infrared rays. In a paper published in Nature Photonics, the researchers reported that by capturing a broad range of light waves ranging from infrared through the visible spectrum they were able to boost the efficiency of their quantum dot cells well beyond levels that have previously been achieved.
While a 4.2% efficiency may not seem like much, but Ted Sargent, a professor of electrical and computing engineering who led the research at the University of Toronto said they hope to achieve 10% conversion efficiency in five years and keep improving from there, by adding more and more layers of quantum dots that are tuned to capture various wavelengths. One of the reasons why their two layer cell works is because they found a way to reduce electrical resistance between the layers. By introducing a transition layer, made up of four films of different transparent metal oxides, that keeps resistance between the layers very low, while also allowing light to pass through to the bottom layer.
However a lot of work is still required in order to eliminate what are known as “trapped states”, which are places within the quantum-dot material where electrons can become stuck and never make it to the electrodes where they can be usefully harvested in order to produce the desired current. If a way can be found to overcome this stubborn problem of “trapped states” multilayer quantum dot solar PV could theoretically reach as high as 50% conversion efficiency.
See related post: “Solar Takes Another Step Towards Grid Parity“, which speaks about how solar PV manufacturing continues along a trajectory of decreasing costs that will very soon cause it to reach grid parity.
What is a Quantum Dot Solar Cell?
A quantum dot is a nano-scale piece of matter, in this case a semiconductor, whose excitons are confined in all three spatial dimensions so that their electronic characteristics are closely related to the size and shape of the individual quantum dot crystal.Quantum dots can be tuned to absorb different parts of the solar spectrum by carefully varying their size, and for many years they have been seen as a promising approach to capturing solar power to produce electricity, in part because they can essentially be spray painted onto a substrate. So far however this approach has been hampered by low efficiencies. By discovering a way to combine two different types of quantum dots in two layers and in doing so improving the efficiency achieved the researchers may have opened the door up for colloidal quantum dot solar PV technology to become a contender in the evolving PV market still dominated by silicon based solar cells.
In fact even if the efficiency does not reach that which can be achieved by the best silicon solar cells, if the simple spray on roll to roll production techniques that quantum dot technology is suited for can produce cells for a much lower overall cost this kind of solar PV may grab market share.
The maximum theoretical efficiency for a single layer solar cell that is tuned to one wavelength is 31%, which results from the fact that the light waves that are of a different wavelength are either not captured at all or are converted with a much lower efficiency. In order to obtain higher conversion efficiencies ell manufacturers stack different materials that are designed to capture different wavelengths on top of each other. This process, which is known as tandem-junction PV, can raise the theoretical efficiency that can be achieved p to 42% from 31%.
This related post: “High Efficiency Solar Cells Can Be Made At a Much Lower Cost“, explores a new type of solar cell that comprise of arrays of thin silicon wires embedded in polymer substrate.
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