Solar Cells

A team that includes researchers from U of T's Faculty of Applied Science & Engineering has created a solar cell that can stand up to high temperatures for more than 1,500 hours

An international team that includes researchers from the University of Toronto's Faculty of Applied Science & Engineering has created a perovskite solar cell that can stand up to high temperatures for more than 1,500 hours – a key milestone as this emerging technology moves closer to commercial application.

The team's findings were recently published in the journal Science.

“Perovskite solar cells offer new pathways to overcome some of the efficiency limitations of silicon-based technology, which is the industrial standard today,” says Ted Sargent, a professor in the Edward S. Rogers Sr. department of electrical and computer engineering who recently joined the departments of chemistry and electrical and computer engineering at Northwestern University.

“But due to its multi-decade head start, silicon still has an advantage in some areas, including stability. This study shows how we can close that gap.”

Traditional solar cells are made of high-purity silicon wafers that are energy-intensive to produce. In addition, they can only absorb certain parts of the solar spectrum.

By contrast, perovskite solar cells are made of layers of nanoscale crystals, making them more amenable to low-cost manufacturing methods. By adjusting the size and composition of these crystals, researchers can also tune the wavelengths of light they absorb.

It is also possible to deposit perovskite layers on top of each other, or even on top of silicon solar cells, enabling them to use more of the solar spectrum and further increase their efficiency.

Over the past few years, advances from Sargent’s lab and others have brought the efficiency of perovskite solar cells to within the same range as what is achievable with silicon. However, the challenge of stability has received comparatively less attention.

“We wanted to work at high temperatures and high relative humidity, because that would give us a better idea of which components might fail first, and how to improve them,” says So Min Park, a postdoctoral fellow in Sargent’s lab and one of three co-lead authors on the study.

“We combined our expertise in materials discovery, spectroscopy and device fabrication to design and characterize a new surface coating for the surface of the perovskites. Our data showed that it is this coating, made with fluorinated ammonium ligands, that enhances the stability of the overall cell.”

Perovskite solar cells typically contain a passivation layer, which surrounds the light-absorbing perovskite layer and acts as a conduit for electrons to move into the surrounding circuit.

But depending on its composition, as well as its exposure to heat and humidity, the passivation layer can deform in ways that impede the flow of electrons.

“Many groups use passivation layers made with bulky ammonium ions, a nitrogen-containing organic molecule,” says Mingyang Wei, a PhD graduate from the department of electrical and computer engineering who is currently a postdoctoral fellow at École Polytechnique Fédérale de Lausanne and co-lead author on the paper.

“Even though they form stable 2D structures at room temperature, these passivation layers can degrade at elevated temperatures, due to their intermixing with underlying perovskites. What we did was replace typical ammonium ions with 3,4,5-trifluoroanilinium. This new passivation layer does not intercalate into the structure of the perovskite crystals, making it thermally stable.”

The team then tested the performance of the cells using continuous measurements at a temperature of 85 Celsius, a relative humidity of 50 per cent, maximum power-point tracking and an illumination equivalent to full sunlight. In the paper, they report a T85 – the amount of time it takes for the cell’s performance to degrade to 85 per cent of its original value – of 1,560 hours.

“A typical value for a perovskite cell like this would be more like 500 hours,” Park says. “There are some teams who have reported measurements of more than 1,000 hours, but not at temperatures as high as this. Our design is a big improvement, and we were really excited to see that it worked this well.”

Park says the team’s passivation layer could be combined with other innovations, such as double- or triple-junction designs, to further enhance perovskite solar-cell performance.

“We still have a long way to go before we can fully replicate the performance of silicon, but the progress in this field has been very rapid over the last few years,” she says.

“We’re moving in the right direction, and this study will hopefully point the way forward for others.”

U of T researchers create more efficient perovskite solar cell

Christopher.Sorensen — Wed, 12 Apr 2023 15:22:11 +0000

U of T researchers create more efficient perovskite solar cell

Christopher.Sorensen Wed, 04/12/2023 - 11:22

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From left Leiwei Zeng, Zaiwei Wang and Hao Chen show off samples of triple-junction perovskite solar cells that boast record gains in efficiency (photo by Tyler Irving)

Tyler Irving

Topic

Breaking Research

Resarch & Innovation

Electrical & Computer Engineering

Faculty of Applied Science & Engineering

Solar

Solar Cells

Sustainability

A team of researchers from the University of Toronto has created a triple-junction perovskite solar cell with record efficiency by overcoming a key limitation of previous designs.

The prototype represents a significant advance in the development of low-cost alternatives to silicon-based solar cells, which are the current industry standard.

“In addition to lower manufacturing cost, perovskites offer us the ability to stack multiple layers of light-absorbing materials on top of each other, and even on top of traditional silicon cells,” says Professor Ted Sargent, who recently joined the department of chemistry and the department of electrical and computer engineering at Northwestern University but maintains his lab at U of T’s Faculty of Applied Science & Engineering.

“In this work, we used rational design to address a critical challenge that can arise in this multi-layered paradigm, improving both efficiency and durability.”

Today’s solar cells are made from a single wafer of ultra-pure silicon, which is energy-intensive to produce. By contrast, perovskite solar cells are made using perovskite polycrystalline films that are coated onto surfaces with low-cost, solution-processing techniques similar to those used in the printing industry.

By varying the composition of the perovskite crystals within these films, each layer can be “tuned” to absorb different wavelengths of light, making efficient use of the entire solar spectrum. This is not possible with silicon, which always absorbs the same wavelengths.

Sargent’s group is among those developing new ways to unlock the potential of perovskite solar cells. Their previous work has included two-layered tandem cells, but their latest study, published in Nature, focuses on a three-layer design.

“Multi-layered cells are typically designed so that the top layer with wide-bandgap perovskites absorbs the most energetic photons, meaning high-frequency light with short wavelengths, toward the violet end of the spectrum,” says post-doctoral researcher Zaiwei Wang, one of four co-lead authors on the new paper.

“The next layer will absorb medium wavelengths and the bottom one will absorb longer wavelengths. But it’s in the top layer that we get the challenge of light-induced phase separation.”

The team used a type of perovskite material known as ABX3, which is made from a mix of different substances – including cesium, lead, tin, iodine, bromine and some small organic molecules. The top layer, in particular, is composed of mixed halide perovskites, which have a high proportion of bromine and iodine.

“What happens in light-induced phase separation of these mixed perovskites is that the bombardment of high-frequency photons causes the phases that are richer in bromine to get separated from those that are rich in iodine,” says Hao Chen, a post-doctoral researcher and co-lead author of the study.

“This leads to an increase in defects and a decrease in overall performance.”

To overcome this problem, the research team used detailed computer models to simulate the effect of altering the composition of the crystals. This work suggested two changes: removing the organic molecules for an all-inorganic perovskite structure and introducing the element rubidium.

“The introduction of rubidium suppresses the light-induced phase separation issue,” says Tong Zhu, another post-doctoral researcher and co-lead author.

“Our rubidium/cesium mixed inorganic perovskites show better light stability than [other] perovskite materials, including cesium-based inorganic perovskites and widely used organic-inorganic hybrid perovskites with similar band gaps.”

Using this knowledge, the team then designed and built a triple-junction cell with this composition. They measured its efficiency at 24.3 per cent with an open-circuit voltage of 3.21 volts. They also sent it to be independently certified by the National Renewable Energy Laboratory, which measured a quasi-steady-state efficiency of 23.3 per cent.

“In the past, triple-junction perovskite solar cells have demonstrated a maximum efficiency of around 20 per cent, so this is a big improvement. To our knowledge, this is also the first reported certification efficiency of triple-junction perovskite solar cells,” says PhD candidate Lewei Zeng, another co-lead author.

“Previous designs also tended to lose a lot of their performance in a matter of hours. By contrast, ours maintained 80 per cent of its initial efficiency even after 420 hours of operation – so that’s a big step in terms of durability as well.”

The team says that although further improvements to the performance will be needed before perovskite solar cells can compete with silicon in commercial applications, the latest study demonstrates a path forward.

“Theory tells us that perovskites have the ability to overcome a lot of the inherent limitations of silicon as a material,” Zeng says.

“But it’s not simply a matter of one displacing the other. There might be some applications better suited to perovskites, and some where silicon is better – or we could combine them both. There are a lot of exciting possibilities ahead.”

Researchers enhance durability of low-cost solar cells made from nano-sized crystals

Christopher.Sorensen — Thu, 23 Feb 2023 16:24:02 +0000

Researchers enhance durability of low-cost solar cells made from nano-sized crystals

Christopher.Sorensen Thu, 02/23/2023 - 11:24

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U of T Engineering post-doctoral researcher Chongwen Li holds up a sample of an inverted perovskite solar cell (photo by Remigiusz Wolowiec)

Tyler Irving

Topic

Breaking Research

Electrical & Computer Engineering

Faculty of Applied Science & Engineering

Research & Innovation

Solar Cells

Sustainability

An international team of researchers has developed a new technique to enhance the durability of inverted perovskite solar cells – an important step toward commercialization of an emerging photovoltaic technology that could significantly reduce the cost of solar energy.

Unlike traditional solar cells, which are made from wafers of extremely high-purity silicon, perovskite solar cells are built from nano-sized crystals. These perovskite crystals can be dispersed into a liquid and spin-coated onto a surface using low-cost, well-established techniques.

It is also possible to tune the wavelengths of light that get absorbed by the perovskites by adjusting the thickness and chemical composition of the crystal films. Perovskite layers tuned to different wavelengths can even be stacked on top of each other, or on top of traditional silicon cells, leading to “tandem” cells that absorb more of the solar spectrum than today’s devices.

The latest work, published in the journal Science, included researchers from the University of Toronto, Northwestern University, the University of Toledo and the University of Washington.

“Perovskite solar cells have the potential to overcome the inherent efficiency limitations of silicon solar cells,” says study co-author Ted Sargent, who recently joined the department of chemistry and the department of electrical and computer engineering at Northwestern University but remains affiliated with U of T Engineering, where he has a research lab.

“They are also amenable to manufacturing methods that have a much lower cost than those used for silicon. But one place where perovskites still lag silicon is in their long-term durability. In this study, we used a rational-design approach to address that in a new and unique way.”

In recent years, Sargent and his collaborators have made several advances that improve the performance of perovskite solar cells. But whereas much of this previous work focused on enhancing efficiency, their latest work looks at the challenge of durability.

“One key point of vulnerability in these types of solar cells is the interface between the perovskite layer and the adjacent layers, which we call carrier transport layers,” says Chongwen Li, a post-doctoral researcher who recently moved to U of T Engineering from the University of Toledo and is one of the paper’s lead co-authors.

“These adjacent layers extract the electrons or holes that will flow through the circuit. If the chemical bonding between these layers and perovskite layer gets damaged by light or heat, electrons or holes can’t get into the circuit – this lowers the overall efficiency of the cell,” Li says.

To address this issue, the international research team went back to first principles. They used computer simulations based on density functional theory (DFT) to predict what kind of molecules would be best at creating a bridge between the perovskite layer and the charge transport layers.

“Previous research has shown that molecules known as Lewis bases are good for creating strong bonding between these layers,” says Bin Chen, a post-doctoral researcher in Sargent’s lab who is now a research assistant professor at Northwestern University and a co-author on the paper.

“This is because one end of the molecule bonds to the lead atoms in the perovskite layer and the other bonds to the nickel in the carrier transport layers. What our simulations predicted was that Lewis bases which contained the element phosphorus would have the best effect.”

In the lab, the team tried out various formulations of phosphorus-containing molecules. Their experiments showed the best performance with a material known as 1,3 bis(diphenylphosphino)propane, or DPPP.

The team built inverted perovskite solar cells that contained DPPP, as well as some without. They subjected both types to tests that simulated the kind of conditions solar cells would experience in the field, illuminating them with light at a similar intensity to the sun. They also tried exposing them to high temperatures, both in the light and in the dark.

“With DPPP, under ambient conditions – that is, no additional heating – the overall power conversion efficiency of the cell stayed high for approximately 3,500 hours,” says Li.

“The perovskite solar cells that have been previously published in the literature tend to see a significant drop in their efficiency after 1,500 to 2,000 hours, so this is a big improvement.”

Li says the team has applied for a patent for the DPPP technique and has already received interest from commercial solar cell manufacturers.

“I think what we’ve done is to show a new path forward – that DFT simulations and rational design can point the way toward promising solutions,” he says.

“But there may be even better molecules out there. Ultimately, we want to get to a place where perovskite solar cells can compete commercially with silicon, which is the state-of-the-art photovoltaic technology of today. This is an important step in that direction, but there is still further to go.”

Quantum innovation advances low-cost alternative solar technology

Christopher.Sorensen — Thu, 07 Apr 2022 20:30:17 +0000

Quantum innovation advances low-cost alternative solar technology

Christopher.Sorensen Thu, 04/07/2022 - 16:30

Cutline

Post-doctoral researcher Hao Chen shows off a prototype inverted perovskite solar cell. The team leveraged quantum mechanics to improve both the stability and efficiency of this alternative solar technology (photo by Bin Chen)

Tyler Irving

Topic

Breaking Research

Electrical & Computer Engineering

Faculty of Applied Science & Engineering

Graduate Students

Research & Innovation

Solar Cells

Sustainability

A team of researchers from the University of Toronto’s Faculty of Applied Science & Engineering has leveraged quantum mechanics to optimize the active layer within a device known as an inverted perovskite solar cell – a technology that could one day result in mass-market solar cells that a fraction of those currently on the market.

At present, virtually all commercial solar cells are made from high-purity silicon, which takes significant energy to produce. But researchers around the world are experimenting with alternative solar technologies that could be manufactured and installed with less energy and at lower cost.

One of these alternatives, which is being studied in the Sargent Group lab, is known as perovskite. The power of perovskite materials comes from their unique crystal structure, which enables them to absorb light in a very thin layer and convert it into electricity efficiently.

“Perovskite crystals are made from a liquid ink and coated onto surfaces using technology that is already well-established in industry such as roll-to-roll printing,” says Hao Chen, a post-doctoral researcher in Sargent’s lab and one of four co-lead authors of a new paper published in Nature Photonics.

“Because of this, perovskite solar cells have the potential to be mass produced at much lower energy cost than silicon. The challenge is that right now perovskite solar cells lag traditional silicon cells in stability. In this study, we aimed to close that gap.”

Chen, along with his co-lead authors – PhD candidate Sam Teale and post-doctoral researchers Bin Chen and Yi Hou – are using a strategy based on an inverted solar cell structure.

In most prototype perovskite solar cells, electrons exit through a negative electrode at the bottom layer of the cell, with the “holes” they leave behind exiting through a positive electrode at the top.

Reversing this arrangement enables the use of alternate manufacturing techniques and past research has shown that these can improve the stability of the perovskite layer. But the change comes at a cost in terms of performance.

“It’s hard to get good contact between the perovskite layer and the top electrode,” says Chen. “To solve this, researchers typically insert a passivation layer made of organic molecules. That works really well in the traditional orientation, because ‘holes’ can go right through this passivation layer. But electrons are blocked by this layer, so when you invert the cell it becomes a big problem.”

The team overcame this limitation by taking advantage of quantum mechanics – the physical principle that states the behaviour of materials at very small length scales is different from what is observed at larger ones.

“In our prototype solar cells, the perovskites are confined to an extremely thin layer – only one to three crystals in height,” says Teale. “This two-dimensional shape enables us to access properties associated with quantum mechanics. We can control, for example, what wavelengths of light the perovskites absorb, or how electrons move within the layer.”

The team first used a chemical technique established by other groups to produce a two-dimensional perovskite surface atop their solar cell. This enabled the perovskite layer to achieve passivation on its own, eliminating the need for the organic layer altogether.

To overcome the electron blocking effect, the team increased the thickness of the perovskite layer from one crystal in height to three. Computer simulations had shown that this change would alter the energy landscape sufficiently to enable electrons to escape into an external circuit, a prediction that was borne out in the lab.

The power conversion efficiency of the team’s cells was measured at 23.9 per cent, a level that did not fade after 1,000 hours of operation at room temperature. Even when subjected to an industry-standard accelerated ageing process at temperatures up to 65 C, the performance only decreased by eight per cent after more than 500 hours of use.

Future work will focus on further increasing the stability of the cells, including under even higher temperatures. The team would also like to build cells with a larger surface area, as the current cells are only about five square millimetres in size.

Still, the current results bode well for the future of this alternative solar technology.

“In our paper, we compare our prototypes to both traditional and inverted perovskite solar cells that have been recently published in the scientific literature,” says Teale.

“The combination of high stability and high efficiency we achieved really stands out. We should also keep in mind that perovskite technology is only a couple of decades old, whereas silicon has been worked on for 70 years. There are a lot of improvements still to come.”

U of T startup QD Solar secures international financing

lanthierj — Tue, 14 Feb 2017 13:17:43 +0000

U of T startup QD Solar secures international financing

lanthierj Tue, 02/14/2017 - 08:17

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Ted Sargent (photo by Roberta Baker, courtesy Faculty of Applied Science & Engineering)

Topic

Global Lens

Research and Innovation

Faculty of Applied Science & Engineering

Nanotechnology

QD Solar, a Canadian startup co-founded by U of T researchers Ted Sargent and Sjoerd Hoogland, has received a big boost in funding and an important international nod to help bring its solar technology to market.

On Monday, the company announced it had closed its first significant round of venture capital financing led by DSM Venturing, based in the Netherlands, with participation from existing investors, KAUST Innovation Fund and MaRS Innovation.

Coupled with the $2.55 million the company received from Sustainable Development Technology Canada last March, QD Solar “has the resources to advance, develop, test and de-risk our solar technology, while concurrently developing the manufacturing processes needed to bring this technology to market,” said Dan Shea, CEO of QD Solar and a former senior executive with Celestica and BlackBerry, in a news release.

QD Solar’s quantum dot-based solar cells use nano-engineered, low-cost materials that can absorb otherwise wasted infrared light. Solar panels with this technology can boost their overall power generation by 20 per cent, the company says.

In the future, QD Solar says, it intends to develop quantum dot-based solar material that can be applied to any flexible surface to generate energy.

The technology was developed in labs at U of T by Sargent, a University Professor in the Edward S. Rogers Sr. Department of Electrical and Computer Engineering at U of T and Canada Research Chair in Nanotechnology, and Hoogland, director of research, technology and innovation at the Sargent Group.

“We’re delighted to see QD Solar and this novel technology created at the University of Toronto receive this substantial investment from Canadian and international sources to advance their bold vision for a new clean energy product,” said Derek Newton, U of T’s assistant vice–president of innovation, partnerships and entrepreneurship.

The startup was supported by MaRS Innovation and U of T’s Innovations & Partnership Office, which provided seed funding, patent protection and helped the company meet international industry partners and investors.

Since 2011, U of T researchers have created more than 830 inventions, founded more than 90 research-based startups and generated $49 million from licensing revenues.