A First For Perovskite Solar Cells: Quantum Dots & “Exceptional” Efficiency

Next-generation perovskite solar cells were probably not what presidential candidate Hillary Clinton had in mind when she pledge that her first term as President would conclude with half a billion solar panels up and running in the US. However, perovskite technology has been progressing quickly, and it looks like this relatively low cost, easily synthesized solar-friendly material could find a spot in some of those new panels.

A First For Perovskite Solar Cells

Perovskite has been “shooting up the efficiency charts faster than anything researchers have seen before,” according to our friends over at the National Renewable Energy Laboratory.

In the latest development, a research team headed up by NREL has accomplished the first use of quantum dots to make perovskite solar cells.

Quantum dots are tiny particles of matter — so tiny you can count the number of atoms that compose them. Here’s an explainer from CleanTechnica last year:

Quantum dots are nanoscale particles of semiconductor materials. They’ve earned the moniker “artificial atoms” because their electronic properties can be precisely engineered, and because their behavior demonstrates unique behaviors when tasked with harvesting solar energy or giving off light…

The luminescent properties of quantum dots are beginning to attract attention in the medical field and in the electronics industry. This photo of the research team illustrates those properties of the quantum dots in solution:

In solar cells, quantum dots are being deployed to boost efficiency (here’s another example).

So…How’d They Do That?

One advantage of quantum dots is that they can be manufactured in bulk. They are produced by chemical reactions, not fabrication machines or other devices. That means there is a lot of potential for low cost, high volume manufacturing.

The NREL team made quantum dots of cesium lead iodide (CsPbI3), illustrating the all-chemistry approach:

The nanocrystals of CsPbI3 were synthesized through the addition of a Cs-oleate solution to a flask containing PbI2 precursor. The NREL researchers purified the nanocrystals using methyl acetate as an anti-solvent that removed excess unreacted precursors.

The purification process added a bonus feature by increasing the stability of the quantum dots:

Contrary to the bulk version of CsPbI3, the nanocrystals were found to be stable not only at temperatures exceeding 600 degrees Fahrenheit but also at room temperatures and at hundreds of degrees below zero.

That’s a critical difference. In bulk form, CsPbI3 is not stable enough to use in a solar cell at room temperature.

With the CsPbI3 quantum dots in hand, the next step in the process was to form them into a thin film with a thickness of 100-400 nanometers.

The result was a solar cell with 10.77% conversion efficiency “at an extraordinary high voltage circuit.”

What’s The Big Deal About 10.77% Efficiency?

Okay, so a solar conversion efficiency of around 10% doesn’t sound all that impressive. However, getting back to that thing about half a billion solar panels, if the idea is to saturate the energy landscape with solar panels within a relatively short amount of time, then the highest-efficiency solar panels are not necessarily going to do the trick.

They may be too expensive, or the manufacturing process will be too complex and slow, or the supply chain may be unreliable.

The idea behind the NREL research is to find a relatively abundant, low cost solar material that can be used in a high-volume manufacturing process.

With that in mind, researchers have been looking at CsPbI3 for some time. The problem has been that in bulk form, this material is unstable at ambient temperatures. The new NREL research provides a way forward.

Here’s the explainer from the the study, published in the journal Science under the title, “Quantum dot–induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics“:

CsPbI3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI3 (α-CsPbI3)—the variant with desirable band gap—is only stable at high temperatures. We describe the formation of α-CsPbI3 QD films that are phase-stable for months in ambient air.

What About The Pb?

If you caught that thing about lead — that’s the Pb in CsPbI3 — then you’ll recognize that there’s a problem with perovskite solar cells. For all their promise, they still rely on a notoriously toxic element as a building block.

That doesn’t necessarily mean they couldn’t be deployed safely in the mass market. It does mean, though, that the cradle-to-grave lifespan would have to be closely monitored and regulated.

Another safety measure would be to limit the use of lead-based perovskite solar cells to limited-access facilities and other secure locations.

In the meantime, researchers are working on a lead-free angle for perovskite solar cells, so stay tuned for that.

Follow me on Twitter and Google+.

Photos: top, perovskite solar cells (cropped) by Dennis Schroeder via NREL; bottom, “Ashley Marshall, Erin Sanehira and Joey Luther with solutions of all-inorganic perovskite quantum dots, showing intense photoluminescence when illuminated with UV light” via NREL.