By adding cesium to a kind of crystal known as a perovskite, researchers say they can create tandem solar cells that may be much better at converting light to electricity than conventional solar cells while also being far more stable than previous comparable tandem cells.
Photovoltaic cells based on silicon currently make up about 90 percent of the global photovoltaic market. However, the rate at which the power conversion efficiency of silicon photovoltaics has improved has slowed dramatically, growing only from 25 percent to 25.6 percent in the past 15 years.
In order to create more efficient solar cells while making the most of the existing industrial capacity for silicon photovoltaics, researchers would like to create so-called tandem solar cells that combine silicon with other materials. One set of potential partners for silicon in these cells are perovskites, which are inexpensive and easily produced in labs.
Perovskites have recently exploded in popularity in the photovoltaic world because of how quickly they have caught up to silicon in efficiency, shooting up from under 4 percent to more than 20 percent in the last five years or so. Previous research suggested that metal halide perovskites could boost the efficiency of commercial silicon photovoltaic modules to more than 30 percent in tandem cells. This enhancement occurs because perovskites can absorb a wider range of light than silicon, including higher-energy blue light.
Scientists want to create perovskite cells with a band gap of about 1.75 electron-volts (eV). Such cells would only need photons each with only about 1.75 eV of energy to get an electrical current flowing, roughly the amount of energy seen in the weakest visible wavelengths of light. However, perovskite cells with such an optical band gap include materials that are unstable when exposed to light or heat.
Now researchers have improved the stability of perovskite cells by replacing some of the traditional formamidinium cations in the perovskites with cesium cations. The resulting crystalline structures are much more stable than their counterparts, the scientists find.
"Perovskite hybrid tandems will now be able to reach the highest possible efficiency whilst also being stable," study senior author Henry Snaith, a physicist at the University of Oxford in England.
With an optical band hap of about 1.74 eV, the perovskite cell demonstrated a power conversion efficiency of 17 percent on its own. When layered on top of silicon photovoltaics, the perovskite was able to boost silicon cell efficiency by 7.3 percent. The researchers suggest they could readily create a tandem cell made of silicon and their perovskite that could achieve more than 25 percent efficiency. With further minor improvements, they say a tandem cell with up to 30 percent efficiency is feasible.
"There still remains much commercial effort required to scale up to manufacture at high yield," Snaith cautions. "The remaining scaling-up challenges will mean that it will still be two to five years until large-scale manufacturing and deployment is underway."
The scientists added that more stable perovskites could also have applications in light-emitting devices. They detailed their findings in the Jan. 8 issue of the journal Science.
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.