Solar-cell manufacturers typically don’t offer a gift-wrapping option, but they may start to do so. A new study reports that the right wrapping material and method can curtail decomposition processes that quickly ruin device performance and shorten service life.
Solar cells made with sunlight-absorbing, semiconducting perovskite materials have been shining in the photovoltaic spotlight in recent years. These metallo-organic materials such as methylammonium lead trihalides and formamidinium analogs are less expensive to make and process than crystalline silicon, a conventional photovoltaic material, yet they offer comparable performance. But when exposed to heat, moisture, and bright sunlight, perovskites decompose and undergo structural changes which cause the cells’ electrical output to fall and have impeded the devices’ commercialization.
Researchers have come up with ways to use films of epoxy, butyl rubber, ceramics, and chemical treatments to protect perovskite cells. But some of the most effective methods require specialized equipment and are costly. Others guard against high humidity or prolonged exposure to heat, but do not stand up to harsh tests that combine punishing conditions.
After experimenting with various materials and solar-cell encapsulation methods, a team led by Lei Shi and Anita W. Y. Ho-Baillie of the University of New South Wales reports that combining glass with poly(isobutylene) or a poly(olefin) does the trick. The researchers showed that fully encapsulating the cells—not just the edges, as in earlier studies—with an inexpensive pressure-tight wrap made of a thin glass-polymer sandwich enables the devices to pass the damp-heat and humidity-freeze cycling tests of the International Electrotechnical Commission (Science 2020, DOI: 10.1126/science.aba2412).
These accelerated aging tests mimic demanding outdoor conditions by exposing the cells to 85% relative humidity and repeated temperature cycling between –40 and 85 °C, conditions that could cause cells to delaminate from ice formation. The encapsulated cells’ conversion efficiency, a standard measure of performance, fell by less than 5% over the course of 1,800 h of the damp-heat test and 75 cycles of the humidity-freeze test.
Ho-Baillie, who now holds a position at the University of Sydney Nano Institute, also developed a gas chromatography/mass spectrometry method for analyzing gas products of perovskite decomposition. By identifying methyl halides, methylformamide, and other species, the researchers elucidated multiple decomposition pathways. They showed that their gas-tight encapsulation method causes decomposition reactions to quickly reach equilibrium and subside before the cells are damaged.
“The results reported here are important,” says Sang Il Seok of Ulsan National Institute of Science and Technology, because they show that perovskite cells can achieve high efficiency and long-term stability. He adds that with GC/MS, researchers can now accurately identify gas products and deduce decomposition routes.
This story was updated on May 22, 2020, to include Anita W. Y. Ho-Baillie's second affiliation at the University of Sydney Nano Institute.