Researchers study promising solar cell materials — with laser focus

December 16, 2016 by Niecole Comeau, Dalhousie University
Researchers study promising solar cell materials — with laser focus
Doctoral students Sam Marsh and Charlotte Clegg, here in Dr. Ian Hill’s lab, have co-founded a spin-off company after working on this study. Credit: Dalhousie University

Perovskites, once on the fringes of solar cell research, have fast become the "it" materials for advancing solar power.

The golden ticket for successful is efficiency: the amount of sunlight a solar cell can convert to electricity. With perovskites—a particular type of crystal-structured materials—scientists have been able make with efficiencies of 22 per cent, up from about 3 per cent in just a few years. That kind of surge in efficiency is unlike any other material in the history of solar cell technology. That perovskites are poised to be low cost and easy to integrate into infrastructure makes them a game changer for the future of clean energy.

Excitement around organometal halide perovskite, the kind used in solar cells, is hard to miss among those in the field. But fundamental understanding of this mysterious material—and why it works so well—is limited.

Today, Scientific Reports published a study from Dal physicists that examines this perovskite, how it behaves when it absorbs light, and a special kind of energy that's central to its efficiency.

"We've used a technique that hasn't been applied to these materials before," says Kimberley Hall, lead author of the study and Canada Research Chair in Ultrafast Science.

Ultrafast lasers and spectroscopy

Drs. Hall and colleague Ian Hill, with their team of graduate students from the Department of Physics and Atmospheric Science, used four wave mixing spectroscopy to study organometal halide perovskite. This technique, together with the application of ultrafast laser technology, allowed the team to observe what happens within a few femtoseconds—one quadrillionth of a second—after light is absorbed into the material.

The team was particularly interested in particles called excitons: electrons in an excited but bound state, as if "stuck" in an atom. But electrons have to be able to flow freely in a material to create a current that can be harvested for electricity. In perovskites, the bound electrons are eventually knocked free, allowing the electrical current necessary for a functioning solar cell.

"How hard it is to rip this electron away is a very important quantity and characteristic of a solar cell," says Dr. Hall (left). "Unless you can break this thing apart with a pretty small amount of energy, you can't make current."

The energy responsible for freeing an electron is called the "exciton binding energy." Previous attempts to measure it have ended in contradicting results. Perovskites are very easy to make, which is why they are low in cost, but they also have a lot of defects within their chemical makeup. These defects make it difficult to accurately measure a variety of material properties, including the binding energy.

Researchers study promising solar cell materials — with laser focus
Dr. Kimberley Hall used ultrafast lasers in her lab to examine the exciton particles of perovskite, an emerging solar cell material. Credit: Danny Abriel

Using their , the Dal research team was able to sift through the mess and differentiate defect-bound excitons from others. They've provided the clearest picture of exciton binding energy within perovskites to date.

Despite the defects, perovskite efficiency is still remarkable. Dr. Hill and Dr. Hall note the current industry standard would never work as well with that many defects.

A key contribution to the field

The study, says Dr. Hill, clarifies the current body of literature examining perovskites.

"We're able to not only get a more accurate measurement, but to understand more about why there were these discrepancies in the literature," says Dr. Hill, physics professor and associate vice-president research at Dal.

Solar cell researchers will continue to explore this superstar material, and knowing the value of exciton will help them improve its design even further.

"Why do they work so well with these defects? Can we make other materials that would work similarly?" asks Dr. Hill. "To do that, we have to first understand how the charges behave and that's why this work is so fundamental."

Possible applications could see perovskites combined with silicon solar cells to create the highest efficiencies yet—higher than any one material is capable of on its own. Doing so would be developing what's called a multi-junction solar cell. (It's possible to make these solely with traditional materials, but they're so expensive that their use is limited to high-end satellites.)

In the meantime, Sam March and Charlotte Clegg, PhD students with Dr. Hall and Dr. Hill respectively, are in the process of starting up Rayleigh Solar Tech. It's a Canadian company based in Nova Scotia that aims to address the remaining barriers of large-scale perovskite solar cell production to boost the up-and-coming technology. Jon-Paul Sun, another PhD student not involved in this study, is also a co-founder.

"We realized that we have a unique opportunity to help bring this potentially revolutionary technology to market," says March.

This new research and spin-off company bring us a few steps closer to a future of more affordable solar power generation throughout the world.

Explore further: New way to make low-cost solar cell technology

More information: Samuel A. March et al. Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy, Scientific Reports (2016). DOI: 10.1038/srep39139

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not rated yet Dec 16, 2016
Would people please stop doing research on perovskite PVs that 'show potential' or 'unravel secrets' and just start manufacturing the blasted things? 22% efficiency is fine if they are going to be seriously cheaper than the usual production alternatives. If they're a quarter of the price then I can just buy twice as many, get the equivalent of 44% efficiency and still save 50%.
5 / 5 (1) Dec 16, 2016
I've read that the main problem with perovskite PV cells is they degrade relatively rapidly in sunlight. So, they have amazing efficiency when they are first created, but after a few years, they might not even be functional. Add to this the fact that commercially-available PV cells already surpass 16% efficiency and last for decades, and it starts looking like perovskite would, at best, be only an incremental improvement. I have no idea why this site pushes perovskite so hard when it is so far from commercialization and the rest of the industry has all-but eliminated its benefits.
not rated yet Dec 18, 2016
and just start manufacturing the blasted things?

The stuff is improving too quickly at the moment. Any kind of manufacturing setup takes years to get going, so the chance of betting on the wrong horse is high. Wait till the development curve flattens out and then we'll see these going to market in pretty short order.
not rated yet Dec 18, 2016
Would people please stop doing research on perovskite PVs that 'show potential' or 'unravel secrets' and just start manufacturing the blasted things?

It's like all the advances on batteries.... it will ALWAYS be 10 years before it gets to market and by then someone will have come out with something better and so the previous design won't actually ever get on the market and then we start that stupid cycle all over again............
5 / 5 (1) Dec 18, 2016
Would people please stop doing research on perovskite PVs that 'show potential' or 'unravel secrets' and just start manufacturing the blasted things? 22% efficiency is fine if they are going to be seriously cheaper than the usual production alternatives. If they're a quarter of the price then I can just buy twice as many, get the equivalent of 44% efficiency and still save 50%.

Patience - The 22% cells are tiny, and most perovskites are very moisture sensitive, but progress is being made on both fronts.

(And while 22% is good, it is not half as good as 44% - even if the cells were half the price you would still have twice glass and aluminum for the modules, twice the shipping and installation cost, twice the steel for supports, twice the cleaning/maintenance, and take twice the land).

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