A group of researchers led by a Brown University student have developed a unique way to make light-absorbing perovskite films, which are crucial for creating a type of solar cell. The new method uses solvent bath kept at room temperature to create perovskite crystals – a different approach than traditional crystallization methods (which generally use blasts of heat). This new approach could essentially lead to the mass production of cost-efficient solar panels.
A study titled “Room-Temperature Crystallization of Hybrid-Perovskite Thin Films via Solvent-Solvent Extraction for High-PerformanceSolar Cells,” published in the Journal of Material Chemistry A, suggests the new technique produces crystalline films of high quality. The researchers were able to control the thickness across large areas, which could lead to the mass production of perovskite cells.
What are perovskites?
Perovskites are a class of crystalline materials that work well as light absorbers and are generally more cost efficient than the silicon wafers used in standard solar cells.
The Brown University study comes on the heels of a report from Stanford University scientists, who similarly suggest perovskites can reduce the cost of solar panels. They say stacking perovskite film onto conventional silicon solar cells increases its efficiency. Currently, silicon solar cells are the go-to source for creating panels. However, silicon photovoltaics have an efficiency of approximately 25 – a number that has more or less stagnated over the past 15 years.
In contrast, in only a few years, the effectiveness of perovskite cells (which is measured by the percentage of electricity that is successfully converted into sunlight) has drastically improved; the first perovskite cell, released back in 2009, had an efficiency of just four percent. However, these cells received a 20-percent efficiency certification by last year. This has attracted the interest of researchers, some of which are in the midst of figuring out a way to integrate perovskites into commercial products.
According to a report published in Renewable & Sustainable Energy Reviews, it can cost anywhere between $5 and $10.50 per watt to install a solar panel system in the United States. Therefore, a 4Kw capacity system could cost up to $42,000. Perovskites could hold the key to reducing that cost.
Issues with heating the persokite cells
Perovskite films can be created through a variety of ways, but heat is generally required regardless of the method. The process works as follows: A solution made up of dissolved perovskite precursor chemicals is coated onto a substrate. Heat is then used to remove the solvent and leave only the perovskite crystals.
So what’s the issue with using heat? It can cause the crystals to form unevenly, which in turn create pinholes in the film. Heat also rules out the number of substrates that can be used during the crystallization process.
“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,” engineering professor Nitin Padture, who leads the Institute for Molecular and Nanoscale Innovation, said in a statement.
The solvent-solvent approach
Graduate student Yuanyuan Zhou wanted a solution to the heating problem. As a result, he used what’s known as a solvent-solvent extraction (SSE) approach. It’s a relatively straightforward process that involves dissolving perovskite precursors into an NMP solvent that’s coated onto a substrate. Heat is substituted by diethyl ether (DEE), a second solvent that eliminates the NMP solvent and leaves only perovskite crystals.
Mass production of perovskites
The lack of heat opens the doors to a number of substrates, offering flexibility to those interested in making perovskite films. Additionally, the SSE approach takes approximately two minutes as opposed to an hour for heat-treating. This could allow for mass production of the films.
Another advantage to the SSE approach is that it can produce thin films while still maintaining a high quality; traditional perovskite films tend to measure 300 nanometers, whereas Zhou’s films can measure as little as 20 nanometers.
“Using the other methods, when the thickness gets below 100 nanometers you can hardly make full coverage of film,” he 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 researchers add that the transparent nature of their films mean they can be used for photovoltaic windows. Additionally, Zhou has been able to add different colours to the cells.
“These could potentially be used for decorative, building-integrated windows that can make power,” Padture added.
Padture and his team are continuing their research, in collaboration with scientists at Colorado’s National Renewable Energy Laboratory, to refine the process.