Scientists develop solar cell working on the 'size quantization effect'

Mar 10, 2008 11:38 GMT  ·  By

It looks like scientists have finally cracked it! They argue that, by creating a solar cell out of differently sized quantum dots, solar cells could become much more efficient than they currently are. This so-called 'rainbow' design involves arranging quantum dots according to size, so that each one acts on a specific wavelength of the electromagnetic spectrum, thus harvesting most of the power light carries.

The unique rainbow solar cell design, actually relies on a basic physical effect, known as 'size quantization effect', meaning that a quantum dot is able to absorb and convert only a specific wavelength of the electromagnetic spectrum, which is in direct relation to the size of the dot. In conclusion, smaller quantum dots have the ability of converting shorter wavelengths of light, while larger quantum dots absorb only large wavelengths.

Constructing a solar cell by using only one of the two, is greatly inefficient. However, if one is able to combine differently sized quantum dots in the design of a solar cell, then light would be absorbed and converted over a wider spectrum.

The rainbow design is proposed by a team of researchers from the University of Notre Dame, amongst which Anusorn Kongkanand, Kevin Tvrdy and others. In their study, the team were able to create single layer quantum dots out of cadmium selenide on titanium dioxide tubes and nano films. Light is absorbed by the quantum dots and emits electrons in the process, which are injected into the titanium dioxide layer to be conducted to the output electrode.

However, the 2.3 and 3.7 nanometer cadmium selenide quantum dots only have maximum absorption peaks in the 505 and 580 nanometers. They immediately observed that, by shrinking the size of the quantum dot, the solar cells were able to convert light at a much higher rate, then by using large quantum dots, which in exchange absorbed most of the incoming light. It seems that the 3 nanometer quantum dots achieved the best performance, meaning they are the most likely candidates for raw material for future solar cells.

Additionally, the team concentrated on quantization effect, and set upon experimenting with two basic quantum dot variations, particle films and nanotube. By using 8000 nanometer long nanotubes as electrodes, the Notre Dame team succeeded in creating a structure that gave them inner and outer access to the quantum dots, thus enhanced efficiency.

The next step, they argue, is the development of rainbow solar cells, by creating a structure that uses multiple sized quantum dots, so that smaller quantum dots would sit towards the outer regions of the solar cell and absorb the blue light, while the red light penetrates to the large quantum dots below, to be converted.

Currently, the silicon solar cells can only achieve 15 to 20 percent efficiency at most, converting only the red light, while the blue light is being converted into heat and lost to the surrounding environment. Semiconductor quantum dots, on the other hand, can produce multiple charge carrier when receiving light, which could result in a solar cell conversion efficiency as high as 30 percent. Now, 30 percent is still a small efficiency, however compared to 15 percent it is a giant leap forward.

Prashant Kamat, one of the scientists participating in the study, writes that this efficiency can only be obtained by implying the usage of two strategies: by increasing the light absorption in the near infrared spectrum and through the use of multiple charge carrier generating quantum dots. Strangely, while the rest of us think about producing electric energy with the help of solar cells, the authors of the study say that the new ultra-efficient solar cell design is more useful in creating colored windows that change color by altering the size of the quantum dots, and maybe generate some electricity in the process.

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Image of a solar cell array
Basic skematics of the rainbow solar cell design
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