Yes, Solar Can Go Lower (Perovskite Solar Cells, That Is)

February 5th, 2016 by Tina Casey

Some solar industry observers have predicted that the cost of solar power will not continue to drop at its current rapid pace, but it’s still going to keep dropping, and with that in mind let’s take a look at a new breakthrough from EPFL, Switzerland’s Ecole Polytechnique Fédérale de Lausanne. It has concocted a perovskite solar cell that replaces a critical — but pricey — layer with a new material at only one-fifth the cost.

The Perovskite Solar Cell Bottleneck

For those of you new to the topic, perovskites are a class of synthetic crystal-ish minerals that share the structure and solar-friendly properties of naturally occurring perovskite. Perovskites are relatively cheap and easy to synthesize, and they have been drawing the attention of solar cell researchers around the world.

To ice the cake, perovskite solar cells can be manufactured in the form of a film, through relatively inexpensive solution-process methods.

Recent progress in the perovskite solar cell field has yielded certified power conversion efficiencies topping 20 percent, which is not quite up there with the best silicon solar cells but still a nifty tradeoff for the relatively low cost.

The cost of perovskite solar cells could go even lower without losing efficiency, except for a “bottleneck” consisting of the expensive materials used to form the hole-transporting layer of the solar cell (that’s the layer that receives positive charges from sunlight and passes them along).

A New Material For Perovskite Solar Cells

According to the EPFL, currently the best-performing perovskite solar cells have been made with materials that top €300 per gram to manufacture, a prohibitive price point in terms of commercial development.

The researchers came up with a low cost, alternate layer that seems to have outperformed their expectations. Instead of simply replacing the expensive layer without a loss of efficiency, the new layer resulted in a measurable improvement over solar cells made with the more expensive layer.

Here’s the rundown from the abstract:

In this work, we present a molecularly engineered hole-transport material with a simple dissymmetric fluorene–dithiophene (FDT) core substituted by N,N-di-p-methoxyphenylamine donor groups, which can be easily modified, providing the blueprint for a family of potentially low-cost hole-transport materials. We use FDT on state-of-the-art devices and achieve power conversion efficiencies of 20.2% which compare favourably with control devices with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Thus, this new hole transporter has the potential to replace spiro-OMeTAD.

Got all that? For more details you can check out the full article, just published in the journal Nature Energy under the title, “A molecularly engineered hole-transporting material for efficient perovskite solar cells.“

About Those Perovskite Solar Cells

We had an interesting discussion about perovskite solar cells last summer during a visit to EPFL, where we had a chance to sit down with the world renowned thin film solar expert Michael Graetzel.

Part of the discussion revolved around the use of a toxic material — lead — in perovskite solar cells. Lead is used in perovskite solar cells as a stabilizer, and until a substitute for lead comes up, Graetzel’s prediction is that lead-based perovskite solar cells would be limited to situations where cradle-to-grave tracking could ensure that lead from the solar cells would not enter the environment at any point during their lifespan.

Clean tech or not, materials lifecycle is critical issue for solar cells. Lead in particular is a touchy subject here in the US, where we’ve gotten a chilling reminder about lead hazards from the ongoing crisis in Flint, Michigan due to the potentially criminal mismanagement of the city’s water supply.

Speaking of lifecycle issues shadowing the clean tech field, get a load of this:

Gross, right? That’s actually not the problem. That’s a common, beneficial bacterium called Shewanella oneidensis. As described by researchers at the University of Wisconsin-Madison this little guy lives in the soil, where it converts metal ions to iron and other metals, which in turn are used as tasty snacks by other microorganisms.

The problem is that the compound nickel manganese cobalt oxide, which is apparently emerging as the material of choice for use as a catalyst in lithium-ion batteries, is toxic to Shewanella oneidensis.

One obvious solution is to keep such batteries out of landfills, but as the crisis in Flint demonstrates, the human factor can come into play with devastating results.

So far the researchers are treating the results of their bacterium study as a possible “red flag” that underscores the need to develop clean tech that is clean throughout its lifecycle.

That leads us all the way back around to perovskite solar cells. About two years ago we noticed that researchers were developing perovskite solar cells that deploy tin instead of lead.

We’re also curious to see how the cutting edge field of “hot-carrier” technology could influence the design and materials used in perovskite solar cells.

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Images: Top, “3-D illustration of FDT molecules on a surface of perovskite crystals” by Sven M. Hein (copyright EPFL); bottom, Shewanella oneidensis by Ella Marushchenko/University of Minnesota via University of Wisconsin-Madison.









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