The traditional method for making light-absorbing perovskite films is blasting it with hot temperatures ranging from 100 to 150 degrees Celsius.
Researchers led by Yuanyuan Zhou, a Brown University Ph.D. student, have come up with a new way. Instead of using heat, this new method uses a room-temperature solvent bath to create perovskite crystals.
The results are outlined in a study published in the Royal Society of Chemistry’s Journal of Materials Chemistry A. The solvent bath can produce high-quality crystalline films. It also allows researchers to decide the thickness of the film over a large area. This method could ultimately lead to the mass production of perovskite solar cells.
Perovskite films are great at absorbing light. Plus, they are a lot cheaper to make than the silicon wafers seen in traditional solar cells.
There has been just one problem. They are inefficient. The first perovskite cells from 2009 had an efficiency of just 4%. While cheaper, they were no where near the 25% efficiency in standard silicon cells. But, that inefficiency gap has been disappearing. In 2014, perovskite cells hit 20% efficiency.
“People have made good films over relatively small areas — a fraction of a centimeter or so square. But they’ve had to go to temperatures from 100 to 150 degrees Celsius, and that heating process causes a number of problems,” said Nitin Padture, professor of engineering and director of the Institute for Molecular and Nanoscale Innovation.
The heat causes the crystals to form unevenly. In some cases, tiny pinholes form. And, that reduces efficiency.
The New Method
There are three main improvements made in the new method.
Crystals can form on virtually any substrate. Because no heat is involved, researchers can use nearly any substrate. Flexible plastic substrates could not be used in the traditional method because they are heat-sensitive.
Time. The old method could take an hour or more. Zhou’s method? Just two minutes. That sounds mass-production ready to me.
Very thin films. With the solvent bath, researchers are able to make high quality films as thin as 20 nanometers. Standard perovskite films are usually 300 nanometers thick.
“Using the other methods, when the thickness gets below 100 nanometers you can hardly make full coverage of film,” Zhou said. “You can make a film, but you get lots of pinholes. In our process, you can form the film evenly down to 20 nanometers because the crystallization at room temperature is much more balanced and occurs immediately over the whole film upon bathing.”
The next step for researchers is continuing to refine the new method. Early test results of the films show a conversion efficiency of over 15%. They want to get that closer to 25% before commercial adoption.
The image above is of traditional solar panels made with silicon.
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