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Cross section of the new solar cell, showing the two perovskite layers beige and red separated by a single-atom layer of boron nitride and the thicker graphene aerogel dark gray, which prevents moisture from destroying the perovskite Gallium nitride blue

Marking a major advance in inexpensive, easy-to-make perovskite solar cells, researchers at the University of California, Berkeley (UC Berkeley) and Lawrence Berkeley National Laboratory (Berkely Lab) have designed a new cell that has achieved 21.7% efficiency in the lab, higher than any previous perovskite cell.

Graded band gap — a new type of perovskite solar cell

This new so-called “graded band gap” solar cell combines two perovskite solar cell materials, each tuned to absorb a different wavelength of sunlight. Thus, this new type of sandwich architecture allows the perovskite solar cell to absorb nearly the entire spectrum of visible light.

”We are able to make these graded band gap solar cells by putting two different perovskite materials together and separating them with a nano-scale, atomically thin material called hexagonal boron nitride (h-BN),” explains Onur Ergen, the lead author of the Nature Materials paper “Graded bandgap perovskite solar cells." The UC Berkeley physics PhD candidate in the Zettl Research Group further emphasizes that h-BN plays a very important role in producing this graded band gap architecture. 

This major research advance could indeed catalyze the commercialization of perovskite solar cells. “These solar cells show there could be another player in the solar market other than silicon,” Ergen agrees. 

Being able to produce such perovskite-based solar cells at extremely low cost and with a high performance offers key advantages over existing silicon-based solar cells on the market today: “They are a thin-film-based technology and can be printed on any substrate including plastic,” Ergen notes, adding that the new approach can “easily be applied to a roll-to-roll process.”

Game-changing efficiency

“This is a high-performance solar cell with surprisingly high current output,” Ergen says about his team’s breakthrough graded band gap technology. “It is very stable, even under constant illumination, compared with other perovskite materials.”

According to the journal paper, the prototype layered perovskite solar cell already achieves an average steady-state efficiency of 18.4%, with a high of 21.7% and a peak efficiency of 26%. The lead author projects targeted efficiencies of a commercially viable, scaled version to be close to that peak efficiency: “We believe, in a couple of years, we can have solar modules more than 25% efficient using this technology.” 

But the perovskite cell’s record efficiency is not all Ergen is excited about. The pioneering technique is just as important to the success of this advanced solar technology. “Using the h-BN as a cationic diffusion barrier and adhesion promoter is a new and unorthodox approach to device fabrication and allows nano-scale control of material properties,” he says. “We believe it will open up a new experimental dimension, leading [into] as of yet unexplored physics, materials science and chemistry.”

Key real-world applications

Ergen expects perovskite-based solar cells will likely create a new market for solar cells: “Because they are very cheap, light and substrate independent,” he argues. “Maybe it will not last for 20 years on your rooftop. But it will be extremely cheap that you might replace it like you replace your tires. Overall, it will still be substantially cheaper than bulky silicon modules.” 

For that, Ergen hopes his team’s work will have powerful impact on the future of solar energy and the work of fellow solar researchers around the world. “We hope we provide a creative vision to solar energy research and encourage the solar community to look at different perspectives,” the PhD candidate says. “Thus, we, all of us, can find a way to develop very cheap solar panels that can allow us to power the world just through sunlight.”

What’s ahead 

The next step for the UC Berkeley/Berkeley Lab team will be to run theoretic models to fully understand the reason behind high current output and band gap narrowing on the device. At the same time, they want to produce solar cell panels that can power their department at the university — and then hopefully bring the technology to market.

Written by Sandra Henderson, Research Editor, Solar Novus Today

Labels: perovskite solar cell,University of California,Berkeley,Lawrence Berkeley National Laboratory,graded band gap solar cell,Zettl Research Group,Onur Ergen,h-BN

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