Published on October 3rd, 2017 | by Tina Casey0
Coal-Killing Perovskite Solar Cell Problem Solved (Almost!) With Graphene
October 3rd, 2017 by Tina Casey
When two “wonder materials” of the solar energy age combine, good things happen. That’s the takeaway from new research that leverages graphene to create a more durable, efficient — and super cheap — perovskite solar cell. There are many next steps before this next-gen solar technology pops up on a rooftop near you, but the new development does keep the promise of perovskites alive — and brings the market for coal-generated electricity one step closer to doom.
What’s All This About Perovskite And Graphene?
For those of you new to the topic, graphene is an atom-thin layer of graphite (if you still use a pencil, that’s the stuff) that has unique and powerful electrical properties.
Perovskite refers to any synthetic version of the naturally occurring mineral perovskite, which has amazing solar conversion potential.
As for how amazing that is, researchers like to enthuse over the rapid pace of improvement in synthetic perovskite solar technology so far, as an indicator of future progress:
Halide perovskites have demonstrated exceptional progress in PV cell performance — from 3.8% in 2009 to a certified 22% in 2016. Remarkably, such high-efficiency perovskite solar cells can be made from polycrystalline materials by solution processing.
If you caught that thing about solution processing, that’s where the super cheap comes in. Conventional silicon solar cells are fabricated in a relatively expensive process, which must be defect-free for top performance. In contrast, solution processing can be as simple as a “dip and dry” dunk — and some research indicates that perovskite solar cells can tolerate well even with defects.
A More Durable Perovskite Solar Cell
There’s always a catch, and this one is a big one: perovskites don’t like humidity. For that matter, they degrade under sunlight. That puts a severe crimp on their application in real world conditions.
However, researchers love a challenge, and the chase is on for a more durable version of the hothouse flower.
The latest research comes from EPFL, the Ecole Polytechnique Fédérale de Lausanne (the Swiss Federal Institute of Technology in Lausanne, Switzerland). Here’s the rundown:
EPFL scientists have greatly improved the operational stability of perovskite solar cells by introducing cuprous thiocyanate protected by a thin layer of reduced graphene oxide.
Got all that? Cuprous means copper and thiocyanate is a salt produced in the human body as a detoxicating mechanism (thank you, Free Dictionary).
Put them together and you have cuprous thiocyanate, which has a grid-like molecular structure and is used as a paint additive. If that sounds somewhat nasty, it is. The tradeoff for improved efficiency and lower cost will be the need for protective lifecycle regulation, unless nontoxic substitutes can be developed.
How Did They Do That?
Where were we? Oh right, the improved durability of the new perovskite solar cell.
The new research from EPFL was just published in the journal Science under the title, “Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies >20%,” the CuSCN being shorthand for cuprous thiocyanate.
The research team forced their new solar cell to endure a “crucial accelerated aging test” consisting of exposure to more than 1000 hours to full sunlight at 60°C.
The result was pretty good. At the end, the perovskite solar cell retained more than 95% of its starting conversion efficiency, which was a little more than 20%.
For those of you familiar with solar conversion efficiencies, yes, 20% is far lower than the best silicon or multi-junction solar cells. That’s okay, when your goal is to mass produce large volumes of solar cells that perform decently at an affordable price.
Here’s the challenge described by EPFL:
Given the vast chemical versatility, and the low-cost processability of perovskite materials, the PSCs [perovskite solar cells] hold the promise to lead the future of photovoltaic technology by offering cheap, light weight and highly efficient solar cells. But until now, only highly expensive, prototype organic hole-transporting materials (HTMs,selectively transporting positive charges in a solar cell) have been able to achieve power-conversion efficiencies over 20%.
The team hit on cuprous thiocyanate as a “stable, efficient and cheap candidate” to sub in for the more expensive materials. That’s been tried before with poor results, and the EPFL team discovered why: in the conventional approach, the cuprous thiocyanate is layered directly onto a layer of gold, resulting in degradation.
With that in mind, the introduced two new twists. One involved fabricating the solar cell with a solution-based method. The other consisted of depositing a thin layer of graphene oxide between the cuprous thiocyanate and a layer of gold.
According to the research team, the resulting stability surpasses that of the halide perovskite solar cells favored by many other researchers.
The Coal-Killing Perovskite Solar Cell Better Act Fast!
Perovskite researchers will have to act fast if they want to take credit for killing off coal. Here in the US, cheap natural gas has already proved to be a powerful force pushing coal out of the power generation marketplace, though with mixed results for the environment.
Unlike the coal industry, natural gas stakeholders are also in a position to argue that their fuel of choice is more adaptable to the diversified grid of the future.
Energy Secretary Rick Perry just came up with a couple of highly controversial (shocker!) proposals that would enable both coal and nuclear power plants to continue operating even when cheaper alternatives are available, but considering the strength of the oil and gas lobby neither of his ideas are likely to result in regulatory action.
Or, will they? Current Secretary of State Rex Tillerson is a powerful advocate for natural gas within the Trump Administration, having come to public service from his position as CEO of Exxon. In recent days, though, President Trump has been sending out signals that Tillerson may soon be heading for the exit.
Pass the popcorn.
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Image: Structure of β-CuSCN and cross-sectional SEM micrograph of a complete solar cell, by M. Ibrahim Dar/EPFL.