Super speed extraction for solar efficiency

 By Amanda Saint

Researchers at the University of Cambridge, U.K., and the Politecnico di Milano, Italy, revealed that they have quantified the speeds at which future solar cells would have to operate in order to stretch what are presently seen as natural limits on their energy conversion efficiency. And they are very high speeds indeed…

The team of academics have published their findings in the Nature journal in a paper entitled: “Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy”.
In a press statement, Johannes Richter, a PhD student in the University of Cambridge Cavendish Laboratory’s Optoelectronics group, who is the paper’s lead author, said: “The timescale that we calculated is now the time limit that we have to operate within if we want to create super-efficient, hot carrier solar devices.”

New solar materials

Throughout the study, the team investigated photovoltaic devices based on a new type of material called perovskites, which are predicted to replace silicon in future photovoltaic devices.
These new perovskites are already almost as energy-efficient as silicon and the results of the study suggest that with further development, they could achieve unprecedented levels of super-efficiency. This is partly due to their size as they are much thinner than silicon cells, which typically are about a millimeter thick. In contrast, perovskite cells are only around one micrometer thick. To put this into context this is about 100 times thinner than a human hair.
Another plus in their favor is that they are also very flexible. The team believes that this means their application will be able to extend beyond being used to generate, and that they could eventually be used in things like tents and clothing. It’s expected that they will also be used in the next generation of electric vehicle batteries, sensors and lasers.

Perovskite crystals (Rob Lavinsky, Wikimedia)

Speedy solar conversion

So how are perovskites going to achieve these unprecedented levels of super-efficiency? The researchers say that in order to do so, they will need to turn sunlight into electrons and then extract these as an electrical charge within mere quadrillionths of a second.
They predict that moving electrons at this super-speedy rate would create “hot carrier” solar cells that can generate electricity much more efficiently. They do this by using the kinetic energy found inside electrons for a brief moment just after they are created and are moving at high speed.
According to the results of the team’s research, the amount of electrical energy that can be extracted from a hot carrier cell could achieve an efficiency rate of 30 percent, compared to standard silicon cells which typically have efficiencies closer to 20 percent.
But what does it mean for the solar industry? Potentially it means that eventually the industry could create more power for less cost, which has to be a good thing in a world needing to wean itself off of fossil fuels. But what are the drawbacks?

Perovskite solar cells (University of Oxford Press Office, Flickr)

Creating perovskites

Although their increased efficiency is expected to lower the cost of solar energy generation overall, at present one of the things holding perovskites back from large-scale commercialization is cost.
According to the Perovskite-Info portal, which was launched in 2015 soon after the discovery of perovskites, the most common electrode material used in perovskite solar cells is gold — making them costly to produce. They can be made more cheaply by substituting the gold but the cheaper perovskite solar cells have a short lifespan, meaning more have to be produced.
The portal, which is an information hub for perovskite professionals and enthusiasts, also reveals that perovskite solar cells deteriorate rapidly in the presence of moisture and that when doing so produce a byproduct that attacks metal electrodes. This can be managed by encasing the perovskite in protective material, but this then adds to the cell cost and weight.
It says that scaling up is another big challenge and that the high efficiency ratings that have been achieved using small cells are only good in the lab environment, as the size of the cells is too small to be used in solar panels.
Finally, it also raises the issue of toxicity. One of the breakdown products of perovskite is a substance called PbI, which it says is known to be toxic and possibly carcinogenic too. And then there’s the fact that many perovskite cells use lead — which has a negative environmental impact at the mining and processing stage and can cause lead poisoning in humans that are over-exposed to it.
There are a number of significant challenges that first need to be overcome for perovskites to achieve their wonder-material potential. But when they do, the solar energy industry could achieve a lot more, for a lot less, and helping to cut global carbon emissions even more significantly than it has already.

READ MORE: Cutting the cost of solar power by Robin Wylie

about the author
Amanda Saint
Journalist and content writer, specialised in engineering and technology with a focus on environmental sustainability, urbanisation and biotechnology.