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Novel technique for creating solar cells from perovskite

Posted: 18 Mar 2015     Print Version  Bookmark and Share

Keywords:Brown University  perovskite  solar cell  solvent-solvent extraction 

A team of researchers from Brown University has come up with an innovative way to make light-absorbing perovskite films for solar cells. The method involves a room-temperature solvent bath to create perovskite crystals, rather than the blast of heat used in current crystallisation methods.

A study published in the Royal Society of Chemistry's Journal of Materials Chemistry A revealed that the technique produces high-quality crystalline films with precise control over thickness across large areas, and could point the way toward mass production methods for perovskite cells.

SSE process

Production-friendly: The SSE process takes place at room temperature in less than two minutes, lending itself to mass production techniques. (Padture lab/Brown University)

Perovskites, a class of crystalline materials, have caused quite a stir in the clean energy world. Perovskite films are excellent light absorbers and are much cheaper 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 at a staggering pace in just a few years. The first perovskite cells introduced in 2009 managed an efficiency of only about 4 per cent, a far cry from the 25 per cent efficiency boasted by standard silicon cells. But by last year, perovskite cells had been certified as having more than 20 per cent efficiency. That rapid improvement in performance is promising, and researchers are racing to start using perovskite cells in commercial products.

There are a number of different ways to make the films, but nearly all of them require heat. Perovskite precursor chemicals are dissolved into a solution, which is then coated onto a substrate. Heat is applied to remove the solvent, leaving the perovskite crystals to form in a film across the substrate.

"People have made good films over relatively small areas, a fraction of a centimetre or so square. But they've had to go to temperatures from 100°C to 150°C, 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.

For example, the crystals often form unevenly when heat-treated, leaving tiny pinholes in the film. In a solar cell, those pinholes can reduce efficiency. Heat also limits the substrates on which films can be deposited. Flexible plastic substrates, for example, cannot be used because they are damaged by high temperatures.

Yuanyuan Zhou, a graduate student in Padture's lab, wanted to see if there was a way to make perovskite crystal thin films without having to apply heat. He came up with what is known as a solvent-solvent extraction (SSE) approach.

In his method, perovskite precursors are dissolved in a solvent called NMP and coated onto a substrate. Then, instead of heating, the substrate is bathed in diethyl ether (DEE), a second solvent that selectively grabs the NMP solvent and whisks it away. What's left is an ultra-smooth film of perovskite crystals.

Because there is no heating involved, the crystals can be formed on virtually any substrate, even heat-sensitive polymer substrates used in flexible photovoltaics. Another advantage is that the entire SSE crystallisation process takes less than two minutes, compared to an hour or more for heat-treating. That makes the process more amenable to mass production because it can be done in an assembly line kind of process.

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