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ScienceWise - Summer 2011

Connecting the dots

Article Illustration
Members of the team, Dr Fu lan, Dr Greg Jolley and Haofeng Lu
Article Illustration
The quantum dot layer gives the new solar cell a hugely improved response in the infrared

How nanotechnology could revolutionise solar power

Metals such as copper are excellent conductors of electricity because  they’re full of highly mobile free electrons. Insulators such as glass  on the other hand, have virtually no free electrons within their  lattice so have little ability to conduct.

In-between lie the  semiconductors like silicon. Semiconductors occupy a rather unique  place in that although in their pure state they have few free  electrons, these can be generated by light, heat or suitable chemical  additions known as doping. A key feature of semiconductors  is that they have an energy gap – a range of energies that electrons  are not allowed to occupy.

One increasingly important use of semiconductors is the creation of  photovoltaic solar cells to convert sunlight directly into  electricity. In such a cell the energy of photons of sunlight are used  to separate an electron from one of the atoms in the crystal lattice,  lift it over the energy gap enabling it to move freely and hence  conduct electricity.

However in addition to this electron, there is a hole left behind in  the sea of electrons surrounding the lattice atoms. These holes are  also able to move – although of course what’s really happening is  successive electrons are filling the hole leaving another hole where  they were. But the effect is that of a moving positively charged hole.

When it comes to making a solar cell from semiconductors, there are  two major challenges. One is to extract as much energy out of the  sunlight as possible so that ideally every single photon of light  creates an electron-hole pair. The other is to get these electron-hole  pairs to all migrate to the electrical contacts where they can do  useful work rather than simply recombining somewhere in the middle of the cell.
A cell made of a single material like silicon is inherently limited  because the band gap of silicon only enables it to absorb photons over  a certain energy range. Now for say a domestic solar panel this may be  fine, an efficiency of 10 or 20% may be acceptable. However in more  critical applications such as a space craft or solar car where you  really want the most power from the smallest lightest solar panel, it  may not be good enough.

To make cells more efficient engineers have turned to other  semiconductors like gallium arsenide and indium phosphide. These  composite materials offer a wider variety of energy gaps that can be  better tuned to sunlight. In addition by using a stack of two or more  cells different materials on top of each other, the one at the back  can capture some of the photon wavelengths that the one at the front  wasn’t able to use.

Dr Lan Fu leads a group of scientists at the Australian National  University looking at how complex compound semiconductors grown in  microscopically thin stacks can create highly efficient cells. But the  key to success here goes far beyond standard semiconductor materials  and structures. More significantly, it will rely upon the group’s  strong background in nanotechnology and quantum engineering.

One  technique they’re employing is the use of quantum dots – tiny blobs of  one semiconductor grown on the surface of another. They’re so small  electrons and holes trapped within them experience quantum effects. If  correctly engineered, a sheet of quantum dots can behave very much  like a conventional semiconductor however now the energy gap is  dictated not only by the chemistry, but also the geometry of the  materials. This means that custom bandgaps can be engineered that  would be quite impossible using conventional technologies.

The plan is to create an advanced but conventional multi layer  compound semiconductor solar cell then add an additional quantum dot  layer to the back. This additional quantum layer has the potential to  significantly increase the already high efficiency of the multi  junctions above by capturing the infrared photons that elude the bulk  materials above. Because the quantum dot technology is so flexible, it  should in principle be possible to tailor the cells absorption to  perfectly fit the sun’s spectrum.

The initial research has been focused on the study of single junction QD solar cell and the results have been very encouraging, the response of the  cells extending into the infrared well beyond the wavelengths that can  be utilised by conventional cells. The current from a solar cell is dictated by the number  of photons of light it absorbs and the efficiency with which the  carriers migrate to the contacts without recombining within the cell.  On all those points the prototype scores well.

However the greatest challenge facing QD solar cells is controlling the impact that the QD structure has on the cell voltage. The quantum effect in QDs has some influence on this since it makes the electrons and holes recombine easier as they travel through the device. Because power equates to voltage multiplied by current, this offsets some of the additional efficiency gained. 

“One of the challenges in this project is to design cells that can be used in multiple layers to harvest the full solar spectrum, yet to  configure those devices such that we don’t compromise the Voltage  generated.” Dr Fu says.
At this point in time the primary interest in such super high tech  cells is specialist applications such as space craft. However, as with  all technologies, refinement and volume production brings the costs  down and down. Thirty years ago it would have been totally  inconceivable to have a microprocessor in your vacuum cleaner or car,  yet now it’s common.
If cells of even 50% efficiency can be cheaply created, the average  house could be self sufficient for power with a panel no bigger than a  table cloth on the roof. And a self charging electric car that runs  for free and doesn’t pollute becomes a very real possibility. Perhaps  in the future you’ll pay more to park on the sunny roof of a  multistorey car park rather than less!

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