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Brown researchers shine new light on solar cell design

Materials science researchers improve perovskite solar energy cells to enhance efficiency, durability, viability

In the bustling laboratories of the School of Engineering, a team of innovative material scientists work to create a brighter, more sustainably powered tomorrow. Their recently published advancements to improve the durability of perovskite solar cells promise to advance renewable energy and solar technology.

The study, which was published in Science this May, examines the toughening of perovskite solar cells for commercial use. Improvements in the cells’ design have potential to provide a more cost-effective and efficient solar technology alternative.  

Solar (photovoltaic) cells are the building blocks of solar panels and attempt to capture light energy. When light strikes a material’s surface, it can transfer energy to the material’s outermost electrons, exciting them to a higher energy state. As these electrons return to their original, lower energy state, they release the energy they received from the light. Within solar cells, light excites a light-absorbing material layer to create pairs of electrons and holes, which are a lack of electrons. In additional layers, the electrons and the holes are transported to the electrodes to produce electricity, according to principal investigator Nitin Padture, professor of Engineering and director of the Institute for Molecular and Nanoscale Innovation. 

“Perovskite” refers to the class of materials used in the light-absorption layer in place of more commonly used materials, such as silicon, said first author of the study Zhenghong Dai GS. But despite their humble differences, perovskite solar cells have presented a major development in the future of solar technology since their invention in 2009.  

The cells’ potential comes largely as a result of their light-absorbing efficiency, which is far greater than that of traditional silicon solar cells. While a light-absorbing silicon layer is roughly 200 microns, twice the thickness of a human hair, perovskites are only about half a micron in thickness and are thus referred to as thin-film. 

Other thin-film alternatives, such as cadmium telluride, may be used in solar cells, but these materials require relatively rare and expensive compounds. Perovskites, on the other hand, are made of only a small amount of earth-abundant materials, which significantly reduces their cost. They are also manufactured through a less energy-intensive production process and are more effective at absorbing light than other materials. As a result, these cells require less material to produce the same amount of electricity. 

“The main hurdle of the (current) photovoltaic market is the cost of fabricating … solar panels,” Dai said. A cheaper alternative, such as perovskite cells, “can revolutionize the market.”

But while perovskite solar cells would be expected to outperform conventional solar panels, they easily degrade over time. “What makes them easy to make also makes them easy to break,” Padture said. The interfaces between a perovskite cell’s material layers are prone to fracture over time as energy moves across them. This destroys the cell’s ability to capture light energy. While a silicon cell may be used for over twenty years, a perovskite cell dies within a few months. 

Capitalizing on Padture’s unique background in mechanics and solar cells, his team strove to combat perovskite cells’ breakdown. The researchers first identified the perovskite cell’s weakest interface, its Achilles’ heel, before looking at how to toughen it, a feat they achieved with “molecular glue,” Padture said.

The team sought to strengthen the adhesion between the layers of the cells by gluing them together. Since traditional laboratory adhesives would destroy the cell’s properties, the researchers instead turned towards self-assembled monolayers. 

The monolayers are composed of molecular chains with two functional groups, an anchor group on one side which attaches to one surface and an iodine group on the other which attaches to the perovskite side of the interface. The monolayers, sitting perpendicular to the perovskite surface, effectively connect both sides of the interface.

The anchoring of many such chains throughout the interface provides a strong bond between the two material surfaces and facilitates energy transport within the solar cell. As a result, the self-assembled monolayer acts as a “molecular glue” that not only toughens the interface but also enhances the cell’s efficiency. 

Prior to the usage of self-assembled monolayers, the commercial viability of perovskite cells used in this study was limited by their short reliable usage time of only 700 hours. The study has boosted this to 4,000 hours, Dai said. 

Marina Leite, an associate professor of materials science and engineering at the University of California, Davis, who was not involved in this study, noted that the group’s use of their specific self-assembled monolayer  — known as the Iodine-Terminated Self-Assembled Monolayer, or I-SAM — boosts the interface’s toughness by about 50 percent, what she called a “major boost in stability.” 

But the team has only addressed one of the cell’s interfaces, and Padture identified at least four others that must be reinforced. He predicted that with continued advancements, perovskites may enter the commercial market within the next five to 10 years.

“The toughness of these bulky layers is like a chain. The strength of a chain lies in its weakest link. When you strengthen the weakest link, the next weakest link is going to break. So that’s what we’re working towards,” he explained. 

With a $1.5 million grant from the U.S. Department of Energy, the team plans to test the next weakest layers, conducting indoor and outdoor performance tests at the National Renewable Energy Laboratory’s large-scale testing facilities. 

Researchers noted that successful material science is interdisciplinary — it bridges the gaps between fields. “At the quantum level there is no boundary between chemistry or physics and engineering,” author of the study and Professor of Engineering Yue Qi said. 

Leite sees this reality reflected in this study, saying that it “demonstrates how the interplay between chemistry and mechanical behavior can pave the way for reliable, low-cost and high-performing solar cells.” 

The team, which also includes authors Srinivas Yadavalli GS, Min Chen GS and Ali Abbaspour Tamijani, has already seen cutting-edge and widespread implications of their work. For example, since perovskite cells can be tuned to absorb the spectrum of light that silicon cannot, engineers may layer perovskite cells atop silicon cells, potentially doubling the cells’ collective power output, Leite said. 

Due to their thin-film nature, perovskites may also be able to create flexible solar cells where traditional silicon cells would simply snap in half, Dai said. 

The study solves an almost decade-long challenge of interface treatment in perovskites that hindered their application in LEDs, Leite added. 

Because of its wide implications, the study was deemed by Wikipedia as one of the major scientific developments of 2021 and could help combat climate change effectively with efficient solar energy. 

Reflecting on the study’s potential, Padture said, “It’s quite exciting!”

Corrections: A previous version of this article inaccurately described the process through which solar cells function. Additionally, the article incorrectly stated that perovskite cells’ prior usage time was 700 hours. In fact, it is the perovskite cells used in the study that originally had this usage time. Also, the article  stated that perovskite cells’ potential is due to their “efficiency,” when, more accurately, it is due to their “light-absorbing efficiency.” The Herald regrets the errors.

Clarification: The article previously did not specify that Marina Leite was not affiliated with the study featured in this article.


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