Inexpensive Solar Cell Coating May Lead To Big Increases In Solar Cell Efficiency, 1 Photon Knocks Loose 2 Electrons

A potentially very important development has just been announced in the field of solar cell technology. Researchers at MIT have successfully demonstrated a technique that allows a photon to knock two electrons loose, rather than only the usual one electron. The demonstration opens the door to solar cells with efficiencies higher than 30% in the near future, and potentially far higher than the 34% Shockley-Queisser efficiency “limit.”

For the past couple of decades, the Shockley-Queisser efficiency limit of 34% has been considered the maximum that a single optimized semiconductor junction could hope to achieve. But according to the press release from MIT, this limit could soon be shown to be irrelevant.

The principle behind the new technique has been understood since the 1960s, but had until now not successfully been put into practice, according to Marc Baldo, a professor of electrical engineering at MIT. The research team “was able, for the first time, to perform a successful ‘proof of principle’ of the idea, which is known as singlet exciton fission,” the press release stated.

Typically, in a photovoltaic (PV) cell, one photon only knocks loose one electron, and that loose electron is then harnessed to provide the electrical current. Whatever energy remains of the photon after knocking loose the electron is then lost as waste heat. But now, with the new technique, the extra energy is used to knock loose two electrons instead of just one, which also reduces waste heat. This makes for a much more efficient system.

This has been done before, in a way, as the researchers note: “Others have previously ‘split’ a photon’s energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth’s surface. The new work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels.”

All of this was made possible through the use of an organic compound known as pentacene. It has been known for awhile that this material has the ability to create two excitons from one photon, but until now that knowledge hadn’t led to its successful incorporation into PV technology, whereby more than on electron was knocked loose per photon.

As of now, since this was just a first proof of principle, the system hasn’t been optimized for efficiency — it’s currently around 2%. But according to the researchers, that should be pretty straightforward, and there appear to be no barriers.

The top-of-the-line commercial solar panels of today usually peak at an efficiency of about 25%, “but a silicon solar cell harnessing singlet fission should make it feasible to achieve efficiency of more than 30 percent,” Baldo says.

It’s worth noting that cost will likely not be an issue with regards to this technology, as it is simply an inexpensive coating placed on top of typical solar cells. There doesn’t seem to be much potentially holding this technology back, at first sight anyways.

Christopher Bardeen, a professor of chemistry at the University of California at Riverside (uninvolved in this research), says that this “very important” work “represents a first step towards incorporating an exotic photophysical process (fission) into a real device. This achievement will help convince workers in the field that this process has real potential for boosting organic solar cell efficiencies by 25 percent or more.”

The new research was just published this week in the journal Science.

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