Researchers say they've developed a more efficient way to create a class of crystalline materials known as perovskites, which hold great promise in the clean energy world.

Perovskite films in solar cells are excellent light absorbers and are much cheaper to make than the silicon wafers used in standard solar cells, says a research team at Brown University.

The team says its method can create perovskite crystals using a solvent bath at room temperature, avoiding the necessary blast of heat required in current crystallization methods.

The technique can produce high-quality crystalline films while offering precise control of thickness over large areas, possible pointing the way to a method of mass production for perovskite solar cells, the researchers report in the Royal Society of Chemistry's Journal of Materials Chemistry.

The efficiency such cells offer, in terms of the percentage of sunlight they can convert to electricity, has improved at an exponential rate in just around 5 years, they say, heightening interest in an efficient method of producing the crystals.

It took decades to achieve the same efficiency levels with conventional solar panel materials, they note.

"People have made good [perovskite] films over relatively small areas - a fraction of a centimeter or so square," says researcher Nitin Padture, a professor of engineering.

However, nearly all existing methods require heat, and "they've had to go to temperatures from 100 to 150 degrees Celsius, and that heating process causes a number of problems," Padture says.

They include a tendency for crystals to form unevenly with tiny pinholes throughout the resulting film, which can reduce the efficiency of solar cells.

The use of heat during crystallization also limits the kinds of substrates films can be deposited on; for example, flexible plastic substrates that would be cheap and offer versatile cell shapes cannot be utilized because high temperatures would damage them.

The researchers have dubbed their technique a solvent-solvent extraction approach, or SSE.

In this method, precursors of perovskite are dissolved into a solvent known as NMP and then deposited on a substrate. The substrate is then immersed in another solvent, diethyl ether, which latches onto the NMP and takes it way, leaving behind an ultra-smooth film of perovskite crystals.

With no heating, almost any substrate can be used, with the added benefit that the solvent method takes less than 2 minutes, as compared to at least an hour in a heat-treating technique.

The resultant films are partially transparent, and can also be made in colors, suggesting they would be suitable for photovoltaic windows, Padture says.

"These could potentially be used for decorative, building-integrated windows that can make power," he says.

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