Brown University researchers have found a new way to make light-absorbing perovskite films for use in solar cells, by way of a room-temperature solvent bath rather than a blast of heat.

Perovskites, a class of crystalline materials, are said to be excellent light absorbers and are inexpensive to make than the silicon wafers used in standard solar cells. The efficiency of perovskite cells—the percentage of sunlight converted to electricity—has increased in just a few years.

The first perovskite cells introduced in 2009 managed an efficiency of about 4%, well below the 25% efficiency of standard silicon cells. By 2014, perovskite cells had been certified as having more than 20% efficiency. That improvement in performance is promising and researchers hope to use perovskite cells in commercial products, such as window-based photovoltaics.

"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," says Nitin Padture, professor of engineering and director of the Institute for Molecular and Nanoscale Innovation.

The research, published in the Royal Society of Chemistry's Journal of Materials Chemistry, shows that the method creates high-quality crystalline films, which have precise control over thickness across large areas. Such a control could enable mass production techniques.

"Using the other methods, when the thickness gets below 100 nanometers (nm) you can hardly make full coverage of film," says Yuanyuan Zhou, a graduate student in Padture's lab. "You can make a film, but you get lots of pinholes. In our process, you can form the film evenly down to 20 nanometers (nm) because the crystallization at room temperature is much more balanced and occurs immediately over the whole film upon bathing."

That 20 nm is in comparison to the standard perovskite films, which are generally about 300 nm thick when made through the heating process.

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