US researchers make perovskite solar cells at room temperature
Research led by a Brown University PhD student has revealed a new way to make light-absorbing perovskite films for use in solar cells.The new method involves a room-temperature solvent bath to create perovskite crystals, rather than the blast of heat used in current crystallisation methods.
The study, published in the Royal Society of Chemistry's Journal of Materials Chemistry A, shows 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.
There are a number of different ways to make perovskite 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 centimeter or so square. But they've had to go to temperatures from 100 to 150 degC, 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.
The SSE approach also enables films to be made very thin while maintaining high quality. Standard perovskite films are generally on the order of 300nm thick. But Zhou has been able to make high quality films as thin as 20nm. The SSE films could also be made larger -- several centimetres square -- without generating pinholes.
"Using the other methods, when the thickness gets below 100nm 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 20nm because the crystallisation at room temperature is much more balanced and occurs immediately over the whole film upon bathing."
Those ultra-thin films are partially transparent (films of standard thickness are black and opaque), so they could be used to make photovoltaic windows, the researchers say. And by tweaking the perovskite precursor solution composition, Zhou has been able to make cells in different colours.
"These could potentially be used for decorative, building-integrated windows that can make power," Padture said.
The group plans to do more work to refine the process, but they are encouraged by the early results. Working with scientists at the National Renewable Energy Laboratory in Colorado, initial testing of cells made with SSE films showed conversion efficiency of over 15 percent. Solar cells based on semitransparent 80-nanometer films made using the process were shown to have higher efficiency than any other ultra-thin film.
"We think this could be a significant step toward a variety of commercially available perovskite cell products," Padture said.