Researchers at the University of Toronto have manufactured and tested a new type of colloidal quantum dots (CQD), that, unlike previous attempts, doesn't lose performance as they keep in contact with oxygen. The development could lead to much cheaper or even spray-on solar cells, as well as better LEDs, lasers and weather satellites.

Quantum dot solar cells

A quantum dot is a nanocrystal made out of a semicondutor material which is small enough to take advantage of the laws of quantum mechanics. Quantum dots are at the center of a very new and rapidly evolving field of research, with the promise for applications in highly efficient solar cells, transistors and lasers, among other things.

In the case of solar cells, quantum dots are used as the absorbing photovoltaic material. The dots have the advantage of having a band gap that can be tuned simply by changing the size of the nanoparticles, and so they can be easily made to absorb different parts of the solar spectrum.

This makes them very attractive for multi-junction solar cells, where you could use a series of quantum dots of different size next to each other to absorb different areas of the spectrum. Crucially, this would drastically cut down the cost and complexity of manufacturing such cells.

The even less expensive option would be for single-junction quantum dot cells. Even here, using quantum dots has definite advantages. Because the band gap can be tuned at will, a single-junction cell can be made to absorb light in the far infrared, where half of the energy from our Sun lies. This would be challenging with standard solar cells, because we don't have materials with the adequate band gaps.

So far, the record efficiency for a quantum dot solar cell is only nine percent, which is roughly half the performance of commercial bulk silicon cells. However, this is a very new field in which progress has been both steady and rapid.

A better dot

Like in standard PV cells, CQD cells use p-type and n-type semiconductors to manipulate charge and generate electricity. However, the n-type quantum dot semiconductor tends to bind with oxygen atoms, giving up its electrons and turning into p-type, which renders the cell useless. N-type semiconductors made using soft matter are notoriously prone to oxidation within minutes of air exposure.

Now, a team led by post-doc researcher Zhijun Ning and Prof. Ted Sargent at the University of Toronto has manufactured and demonstrated a new type of CQD n-type lead-sulfide material that doesn't bind with oxygen, preserving the performance of the cell and opening up a world of new optoelectronic devices that capitalize on the best properties of both light and electricity, including better satellite communication and pollution detectors.

Ning, Sargent and colleagues tested a solar cell manufactured using their material, and achieved a high 8 percent efficiency, just shy of the current efficiency record for quantum dot cells.

"The field of colloidal quantum dot photovoltaics requires continued improvement in absolute performance, or power conversion efficiency," said Sargent. "The field has moved fast, and keeps moving fast, but we need to work toward bringing performance to commercially compelling levels."

Although eight percent efficiency is much less than commercially-available panels, quantum dot solar cells ultimately have the potential to become more efficient than their silicon counterparts because a single photon can be made to excite multiple electrons inside the cell.

With colloidal quantum dots, in which the nanoparticles are evenly distributed, we may eventually have high-efficiency spray-on solar cells that we could apply on our roofs to generate our very own power supply.

A paper detailing the advance was published in the journal Nature Materials.

Source: University of Toronto