Solar-Cell Material Defects Can Be Healed by Light, Humidity

An international research team has found that defects in the crystalline structure of perovskite—which is used to make solar cells—can be healed permanently by exposing them to light, improving the material in its generation of solar energy.

An international research team has discovered that defects in the crystalline structure of perovskite—which is used to make solar cells—can be healed permanently by exposing them to light, improving the material in its generation of solar energy.

This discovery—made by a team from the universities of Cambridge, MIT, Oxford, Bath, and Delft and built on work by some of the same researchers from last year—has implications for accelerating the development of perovskite-based solar cells with efficiencies on par with silicon-based cells, they said.

Perovskite—a calcium titanium oxide mineral composed of calcium titanate—is being eyed as a leading next-generation material in a type of solar cells called thin-film. Perovskite has advantages over materials used to develop these solar cells today, including higher efficiency and lower cost in the production of the cells.

One drawback to these types of cells, however, is that much efficiency is lost through defects, said Sam Stranks, who led the research while a joint fellow at MIT and Cambridge. Indeed, small defects in the crystalline structure of perovskites--called traps--can cause electrons to get “stuck” before their energy can be harnessed, he said. Stranks is now a researcher at Cambridge’s Cavendish Laboratory.


solar cells light

An international group of researchers has shown that defects in the molecular structure of perovskite (shown here)--a material which many see as the future of solar cell industry--can be “healed” by exposing the material to light and just the right amount of humidity. (Source: University of Cambridge)


The easier it is for electrons to move around in a solar-cell material, the more efficient that material will be at converting photons, or particles of light harvested from sunlight, into electricity, Stranks said. “We want to know the origins of the defects so that we can eliminate them and make perovskites more efficient,” he said.

Last year, Stranks and his team published a paper outlining their discovering that iodide ions—or atoms stripped of an electron so they carry an electric charge—migrate away from the illuminated region of perovskite material when it’s exposed to light. In this process, they take away most of the defects in that region along with them.

The problem with the study last year is that the effects were temporary; when the light was removed, the ions migrated back, taking the defects with them, Stranks said.

For the most recent research, the team made a perovskite-based device using techniques compatible with scalable roll-to-roll processes. However, before they completed the device, they exposed it to light, oxygen, and humidity, he said.

While humidity typically degrades perovskites, researchers found that when humidity levels were between 40 percent and 50 percent, and exposure limited to 30 minutes, there was no degradation, Stranks said. They deposited the remaining layers of the device after completing the exposure.

“It’s counter-intuitive, but applying humidity and light makes the perovskite solar cells more luminescent, a property which is extremely important if you want efficient solar cells,” he said.

When researchers applied light to the new device, electrons bound with oxygen--which formed a superoxide that can bind to electron traps and prevent them from impeding the movement of electrons. The perovskite surface, accompanied by water, also converted to a protective shell, which both removes defects from the surfaces while also locking in the superoxide.

What all this means for the perovskite-based cell is that the performance improvements achieved by correcting the defects are now long-lived, Stranks said.

“We’ve seen an increase in luminescence efficiency from one percent to 89 percent, and we think we could get it all the way to 100 percent, which means we could have no voltage loss,” he said. “But there’s still a lot of work to be done.”

Researchers published their new findings in the inaugural edition of the journal Joule, published by Cell Press.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 15 years.

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