Sometimes doping is good

One of the perpetual criticisms of solar power is the high cost versus traditional fossil fuel sources or nuclear power.  To be fair, these criticisms are largely true.  However, critics should keep an open mind about solar power as an energy source in the long run as exciting research is being done that could dramatically boost efficiencies and increase the power yield of solar cells, decreasing deployment costs.

I. Meet the Q-BIC 

Much recent work has revolved around "quantum dots" -- tiny metal and/or organic nanocrystals in the 1-20 nm range.  Quantum dots have unique properties and can actually produce more than one electron for every photon that hits them (a quantum efficiency > 100%) in a phenomenon called "multiple exciton generation" (MEG, for short).

Quantum dots
Quantum dots don't just look pretty, they have some handy physics quirks too!
[Image Source: Elec-Intro]

The latest breakthrough in quantum dots comes from the lab of Professor Vladimir V. Mitin at the University at Buffalo, New York.  Professor Mitin's new quantum dots harvest light in the infrared spectrum -- often underutilized in solar cells -- complementing existing photovoltaics.

But his special quantum dots do something more.  They're pre-doped with a negative charge, which helps them repel electrons.  Why would you want to repel electrons from your quantum dots?  

Well, imagine you have all your quantum dots exposed to visible light and they're busy "harvesting" the energy from the infrared portion of that light.  This "harvest" occurs by the infrared-range photons transferring their energy to an electron in one of the nanocrystal's atoms.  The electron is excited and "jumps" out of its orbit, joining a free flow current stream of electrons from various quantum dots.  The current flow is driven by a potential difference.

But imagine if one of the electrons in the stream passes by a quantum dot and sees one of the "holes" left when an excited electron departed.  It can sometimes fill in that empty space, in a phenomenon called recombination.  This is a bad thing, as all of a sudden your electrons go from producing useful current to malingering around in your nanocrystal.

By doping your nanocrystal, you're putting a lot of negative charge in it.  So even if your nanocrystal sheds some of its electron load, it's still has a lot of negatively charged electrons.  Like repels like, so this means electrons in the current stream tend to avoid the nanocrystals and recombination drops.

These special doped nanocrystal quantum dots are known as quantum dots with built-in-charge (Q-BICs).

Quantum Dot crystals
An electron micrograph of quantum dots (dark bumps in right most image) and an artist's sketch of a layered quantum dot cell (right images) are seen in this picture from an earlier Professor Mitin paper. [Image Source: Vladimir V. Mitin/University at Buffalo]

Professor Mitin didn't do this work alone.  The work was done by his core team, which also consisted of Andrei Sergeev and Nizami Vagidov, faculty members in UB's electrical engineering department; Kitt Reinhardt of the Air Force Office of Scientific Research; and John Little and advanced nanofabrication expert Kimberly Sablon of the U.S. Army Research Laboratory.

Professor Mitin isn't revealing the exact chemical stew used in the nanocrystalline Q-BICs, as he and his fellow professors have filed for a provisional patent on their work.  But his past studies [PDF] indicate that they're using indium arsenic nanodots, for at least some of their work.

III. What's Next

Professors Mitin, Sergeev, and Vagidov are joining together to found a startup company to market the solar cells, which they say can increase the conversion efficiency by 45 percent over traditional designs, between harvesting the infrared and fighting recombination of the infrared-derived current.  The new company is called OPtoElectronic Nanodevices LLC. (OPEN LLC.)

Eventually solar cells will likely make heavy use of quantum dots, as these little nanostructures allow high efficiency capture of targeted portions of the spectrum -- efficiency so high that it would violate the laws of physics if the nanocrystal was a traditional semiconductor.  By mixing nanodots, a cell could capture most of the visible light spectrum.  This latest development -- Q-BIC -- adds one more tool to improve such a design.
Quantum Dot mixture
A solar cell with a mixture of tuned quantum dots, perhaps doped Q-BICs would be a truly optimal third-generation solar cell.
[Image Source: Los Alamos Science & Tech Mag./U.S. Department of Energy's NNSA]

Solar may not win out in the long term with viable alternatives like nuclear fusion and algal biofuels on the way.  But developing efficient solar power will be a critical step for mankind in the creation of self-sustaining colonies on alien moons, asteroids, and worlds -- environments that often lack significant quantities of carbon and water (a source of fusion fuel) -- but that have an abundance of silicon and other mineral resources.

Source: University at Buffalo

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