International research collaboration produces all-perovskite parallel solar cells with high efficiency, record voltage

A collaboration between the Technical University of Toronto, Northwestern University and the University of Toledo has resulted in a fully perovskite parallel solar cell with extremely high efficiency and record-setting voltage.

The prototype device shows the potential of this emergence photovoltaic technology to overcome the major limitations associated with traditional silicon solar batterywhile providing lower production costs.

University of Toronto Engineering Professor Ted Sargent, who recently joined the Department of Chemistry and the Department of Electrical and Computer Engineering at Northwestern University, said: “Further improvements in solar cell efficiency are crucial. important to the ongoing decarbonization of our economy.

“Although silicon solar cells have made impressive advances in recent years, there are inherent limitations to their efficiency and cost, arising from material properties. Technology perovskite can overcome these limitations, but so far it has performed below its maximum potential.Our latest research identifies a key reason for this and points to a way forward. .”

Traditional solar cells are made from ultra-high purity silicon wafers, which are very energy-intensive to manufacture. Opposite, perovskite Solar cells are fabricated from nano-sized crystals that can be dispersed into a liquid and spin-coated on the surface using well-established, low-cost techniques.

Another advantage of perovskites is that by adjusting the thickness and chemical composition of the crystal film, manufacturers can selectively ‘tune’ the wavelengths of light that are absorbed and converted into electricity. , while silicon always absorbs the same part of the solar spectrum.

In a new paper published today in NatureAn international team of researchers used two different layers of perovskite, each tuned to a different part of the solar spectrum, to create what is known as a tandem solar cell.

“In our cells, the top perovskite layer has a wider band gap, which is good at absorbing the ultraviolet part of the spectrum, as well as some visible light“, said Chongwen Li, a postdoctoral researcher in Sargent’s lab and one of the five lead authors of the new paper.

“The bottom layer has a narrow band gap, which is more tuned to the infrared part of the spectrum. In between the two layers, we cover more spectral bands than silicon is capable of.”

The parallel design allows the cell to generate very high open-circuit voltages, thereby improving its efficiency. But the key innovation came when the team analyzed the interface between the perovskite layer, where light is absorbed and converted into excited electrons, and the adjacent layer, known as the electron transport layer.

“What we found was electric field on the surface of the perovskite layer—we call it the surface potential—is not uniform,” said doctoral student Aidan Maxwell, another co-author.

“The consequence of this is that in some places, excited electrons easily move into the electron transport layer, but in others they will just recombine with the holes they leave behind. Those electrons were lost in the circuit.”

To address this challenge, the team coated a substance called 1,3-propanediammonium (PDA) on the surface of the perovskite layer. Although the coating is only a few nanometers thick, it makes a big difference.

“PDA has a positive charge, and it can bring out the full surface potential,” said co-author Dr. Hao Chen, one of the other lead co-authors.

“When we added the coating, we got a much better energetic binding of the perovskite layer to the electron transport layer, and that led to a huge improvement in our overall efficiency.”

The group’s prototype solar cell covers an area of ​​one square centimeter and generates an open-circuit voltage of 2.19 electron volts, a record for an all-perovskite parallel solar cell. Its energy conversion efficiency measured at 27.4%, higher than the current record for traditional single-junction silicon solar cells. This battery is also independently certified at the National Renewable Energy Laboratory in Colorado, delivering 26.3% efficiency.

The team used industry-standard methods to measure the stability of the new cell and found that it maintained 86% of its original performance after 500 hours of continuous operation.

Professor Alberto Salleo, Chair of the Department of Materials Science and Engineering at Stanford University, who was not involved in the study, said: “Continuing to improve the efficiency and stability of next-generation solar cells is important. an important priority to decarbonize the power supply. .

“The team has developed a deep chemical understanding of what is limiting an important interface—the junction to the electron extractor layer—in the large gap section of perovskite solar cells. These insights come from science. Fundamental learning, made possible by innovative materials engineering strategies, will continue to propel the field forward.”

The researchers will now focus on improving efficiency even further by increasing the current flowing through the cell, improving stability, and expanding the area of ​​the cell so that it can be scaled down to scale. commercial rate.

Defining the key role of interfaces between classes also points the way towards potential future improvements.

“In this study, we focused on the interface between the perovskite layer and the electron transport layer, but there is another important layer that extracts the ‘holes’ that those electrons leave behind,” says Sargent. “.

“One of the cool things about my experience with this field is that learning to master one interface doesn’t necessarily teach you the rules for mastering others. I think there’s more to be discovered. have to be done.”

Maxwell says the ability of perovskite technology against silicon, although the latter has been off to a good start for decades, is encouraging.

“Over the past 10 years, perovskite technology has evolved almost as much as silicon technology has in the past 40 years,” he said. “Just imagine what it will be able to do in ten years.”