Brown researchers are shedding new light on the design of solar cells

In the busy laboratories of the School of Engineering, a team of innovative materials scientists are working on a brighter, more sustainable powered future. Their recently published advances in improving the durability of perovskite solar cells promise to advance renewable energy and solar technology.

The studythat was published in science investigates the toughness of perovskite solar cells for commercial use in May. Improvements in the design of the cells have the potential to provide a cheaper and more efficient solar technology alternative.

Solar cells (photovoltaics) are the building blocks of solar panels and try to capture light energy. When light hits the surface of a material, it can transfer energy to the outermost electrons of the material and excite them into a higher energy state. When these electrons return to their original, lower energy state, they give up the energy they received from light. In solar cells, light excites electrons in a layer of material known as the valence band. The released energy of the electrons is captured in additional layers and then “released from the solar cell”, says study leader 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 instead of more commonly used materials like silicon, said first author of the study, Zhenghong Dai GS. But despite their modest differences, perovskite solar cells have represented an important advancement in the future of solar technology since their invention in 2009.

The potential of the cells results primarily from their efficiency, which is far above that of conventional silicon solar cells. While a light-absorbing silicon layer is around 200 micrometers thick, twice as thick as a human hair, perovskites are only around half a micrometer thick and are therefore referred to as thin films.

Other thin film alternatives such as cadmium telluride can be used in solar cells, but these materials require relatively rare and expensive compounds. Perovskites, on the other hand, consist of only a small amount of earth-rich materials, which significantly reduces their costs. They are also made in a less energy-intensive production process and absorb light more effectively than other materials. As a result, these cells require less material to generate the same amount of electricity.

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

Although perovskite solar cells are expected to outperform traditional solar panels, they degrade easily over time. “What makes them easy to make also makes them easy to break,” said Padture. The interfaces between the layers of material in a perovskite cell tend to break over time as energy moves through them. This destroys the cell’s ability to capture light energy. While a silicon cell can be used for twenty years, a perovskite cell dies within a few months.

Drawing on Padture’s unique background in mechanics and solar cells, his team sought to combat perovskite cell breakdown. The researchers first identified the perovskite cell’s weakest interface, its Achilles’ heel, before delving into its toughness, a feat they achieved with “molecular glue,” Padture said.

The team tried to strengthen the adhesion between the layers of the cells by sticking them together. Since conventional laboratory adhesives would destroy the properties of the cell, the researchers turned to self-assembled monolayers instead.

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

The anchoring of many such chains over the entire interface ensures a strong bond between the two material surfaces and facilitates the transport of energy within the solar cell. As a result, the self-organized monolayer acts as a “molecular glue” that not only hardens the interface, but also increases the efficiency of the cell.

Prior to the use of self-assembled monolayers, the commercial viability of perovskite cells was limited by their short reliable service life of only 700 hours. The study increased this to 4,000 hours, Dai said.

Marina Leite, Associate Professor of Materials Science and Technology at the University of California, Davis, noted that the group’s use of their specific self-assembled monolayer – known as the Iodine-Terminated Self-Assembled Monolayer, or I-SAM – made the toughness of the Interface by about 50 percent, which she called a “great stability boost”.

But the team only looked at one of the cell’s interfaces, and Padture identified at least four more that need reinforcement. He predicted that with further advances, perovskites could hit the commercial market within the next five to ten years.

“The toughness of these bulky layers is like a chain. The strength of a chain lies in its weakest link. If you strengthen the weakest link, the next weakest link will break. We are working towards that, ”he explained.

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

The researchers found that successful materials science is interdisciplinary – it bridges the gaps between fields. “At the quantum level, there is no boundary between chemistry or physics and engineering,” said study author and engineering professor Yue Qi.

Leite sees this reality reflected in this study: “It shows how the interaction of chemistry and mechanical behavior can pave the way for reliable, inexpensive and powerful solar cells.”

The team, which includes writers Srinivas Yadavalli GS, Min Chen GS, and Ali Abbaspour Tamijani, has seen the innovative and far-reaching impact of their work. For example, because perovskite cells can be tuned to absorb the spectrum of light that silicon cannot absorb, engineers can layer perovskite cells on top of silicon cells, potentially doubling the cells’ collective output, Leite said.

Because of their thin-film nature, perovskites may also be able to make flexible solar cells where traditional silicon cells would simply break in half, Dai said.

The study solves a nearly decade-long interfacial treatment challenge in perovskites that has hampered their application in LEDs, Leite added.

Because of its far-reaching importance, the study was recognized by Wikipedia as one of the important scientific developments of 2021 and could use efficient solar energy to help combat climate change effectively.

Given the potential of the study, Padture said, “It’s pretty exciting!”