Solar Energy

Researchers improve efficiency of next-generation solar cell material

Published

4 years ago

February 25, 2021

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Researchers improve efficiency of next-generation solar cell material

Perovskites are a leading candidate for eventually replacing silicon as the material of choice for solar panels. They offer the potential for low-cost, low-temperature manufacturing of ultrathin, lightweight flexible cells, but so far their efficiency at converting sunlight to electricity has lagged behind that of silicon and some other alternatives.

Now, a new approach to the design of perovskite cells has pushed the material to match or exceed the efficiency of today’s typical silicon cell, which generally ranges from 20 to 22 percent, laying the groundwork for further improvements.

By adding a specially treated conductive layer of tin dioxide bonded to the perovskite material, which provides an improved path for the charge carriers in the cell, and by modifying the perovskite formula, researchers have boosted its overall efficiency as a solar cell to 25.2 percent – a near-record for such materials, which eclipses the efficiency of many existing solar panels. (Perovskites still lag significantly in longevity compared to silicon, however, a challenge being worked on by teams around the world.)

The findings are described in a paper in the journal Nature by recent MIT graduate Jason Yoo PhD ’20, professor of chemistry and Lester Wolfe Professor Moungi Bawendi, professor of electrical engineering and computer science and Fariborz Maseeh Professor in Emerging Technology Vladimir Bulovic, and 11 others at MIT, in South Korea, and in Georgia.

Perovskites are a broad class of materials defined by the fact that they have a particular kind of molecular arrangement, or lattice, that resembles that of the naturally occurring mineral perovskite. There are vast numbers of possible chemical combinations that can make perovskites, and Yoo explains that these materials have attracted worldwide interest because “at least on paper, they could be made much more cheaply than silicon or gallium arsenide,” one of the other leading contenders. That’s partly because of the much simpler processing and manufacturing processes, which for silicon or gallium arsenide requires sustained heat of over 1,000 degrees Celsius. In contrast, perovskites can be processed at less than 200 C, either in solution or by vapor deposition.

The other major advantage of perovskite over silicon or many other candidate replacements is that it forms extremely thin layers while still efficiently capturing solar energy. “Perovskite cells have the potential to be lightweight compared to silicon, by orders of magnitude,” Bawendi says.

Perovskites have a higher bandgap than silicon, which means they absorb a different part of the light spectrum and thus can complement silicon cells to provide even greater combined efficiencies. But even using only perovskite, Yoo says, “what we’re demonstrating is that even with a single active layer, we can make efficiencies that threaten silicon, and hopefully within punching distance of gallium arsenide. And both of those technologies have been around for much longer than perovskites have.”

One of the keys to the team’s improvement of the material’s efficiency, Bawendi explains, was in the precise engineering of one layer of the sandwich that makes up a perovskite solar cell – the electron transport layer. The perovskite itself is layered with a transparent conductive layer used to carry an electric current from the cell out to where it can be used. However, if the conductive layer is directly attached to the perovskite itself, the electrons and their counterparts, called holes, simply recombine on the spot and no current flows. In the researchers’ design, the perovskite and the conductive layer are separated by an improved type of intermediate layer that can let the electrons through while preventing the recombination.

This middle electron transport layer, and especially the interfaces where it connects to the layers on each side of it, tend to be where inefficiencies occur. By studying these mechanisms and designing a layer, consisting of tin oxide, that more perfectly conforms with those adjacent to it, the researchers were able to greatly reduce the losses.

The method they use is called chemical bath deposition. “It’s like slow cooking in a Crock-Pot,” Bawendi says. With a bath at 90 degrees Celsius, precursor chemicals slowly decompose to form the layer of tin dioxide in place. “The team realized that if we understood the decomposition mechanisms of these precursors, then we’d have a better understanding of how these films form. We were able to find the right window in which the electron transport layer with ideal properties can be synthesized.”

After a series of controlled experiments, they found that different mixtures of intermediate compounds would form, depending on the acidity of the precursor solution. They also identified a sweet spot of precursor compositions that allowed the reaction to produce a much more effective film.

The researchers combined these steps with an optimization of the perovskite layer itself. They used a set of additives to the perovskite recipe to improve its stability, which had been tried before but had an undesired effect on the material’s bandgap, making it a less efficient light absorber. The team found that by adding much smaller amounts of these additives – less than 1 percent – they could still get the beneficial effects without altering the bandgap.

The resulting improvement in efficiency has already driven the material to over 80 percent of the theoretical maximum efficiency that such materials could have, Yoo says.

While these high efficiencies were demonstrated in tiny lab-scale devices, Bawendi says that “the kind of insights we provide in this paper, and some of the tricks we provide, could potentially be applied to the methods that people are now developing for large-scale, manufacturable perovskite cells, and therefore boost those efficiencies.”

In pursuing the research further, there are two important avenues, he says: to continue pushing the limits on better efficiency, and to focus on increasing the material’s long-term stability, which currently is measured in months, compared to decades for silicon cells. But for some purposes, Bawendi points out, longevity may not be so essential. Many electronic devices such as cellphones, for example, tend to be replaced within a few years anyway, so there may be some useful applications even for relatively short-lived solar cells.

“I don’t think we’re there yet with these cells, even for these kind of shorter-term applications,” he says. “But people are getting close, so combining our ideas in this paper with ideas that other people have with increasing stability could lead to something really interesting.”

Robert Hoye, a lecturer in materials at Imperial College London, who was not part of the study, says, “This is excellent work by an international team.” He adds, “This could lead to greater reproducibility and the excellent device efficiencies achieved in the lab translating to commercialized modules. In terms of scientific milestones, not only do they achieve an efficiency that was the certified record for perovskite solar cells for much of last year, they also achieve open-circuit voltages up to 97 percent of the radiative limit. This is an astonishing achievement for solar cells grown from solution.””

The team included researchers at the Korea Research Institute of Chemical Technology, the Korea Advanced Institute of Science and Technology, the Ulsan National Institute of Science and Technology, and Georgia Tech. The work was supported by MIT’s Institute for Soldier Nanotechnology, NASA, the Italian company Eni SpA through the MIT Energy Initiative, the National Research Foundation of Korea, and the National Research Council of Science and Technology.

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Solar Energy

Advancing safer lithium energy storage

Published

1 day ago

February 4, 2025

sarosh

Advancing safer lithium energy storage

by Erica Marchand

Paris, France (SPX) Feb 04, 2025

Charging our phones has become so routine that we rarely reflect on the breakthrough that made it possible. Rechargeable lithium-ion batteries, introduced commercially in the 1990s, propelled a technological revolution that earned their creators the 2019 Nobel Prize in Chemistry. This key innovation underpins the functionality of today’s smartphones, wireless headphones, and electric vehicles, making them both financially and environmentally practical.

As our devices grow more advanced, the demand for batteries that pack more power while remaining safe continues to rise. Yet engineering such power sources is far from simple. One promising design is the lithium metal battery, which could deliver more stored energy than standard battery types. Unfortunately, its potential is curtailed by a persistent issue: the emergence of tiny threads, or dendrites, that accumulate with each charge. When dendrites build up, they can form metallic connections that degrade battery functionality and pose a serious fire hazard. Until recently, researchers had limited approaches to probe and understand dendrite formation. In a new study led by Dr. Ayan Maity in the lab of Prof. Michal Leskes at the Weizmann Institute of Science’s Molecular Chemistry and Materials Science Department, scientists developed a novel method to identify the factors that spark dendrite growth, as well as to rapidly evaluate various battery components for improved safety and performance.

Rechargeable batteries function by allowing positively charged ions to migrate between the anode (negative electrode) and the cathode (positive electrode) through an electrolyte. Charging forces the ions back into the anode, counter to the usual flow in a typical chemical reaction, thus preparing the battery for another cycle of use. Lithium metal batteries take a different approach by employing a pure lithium metal anode, enabling higher energy storage. However, lithium metal is chemically reactive and quickly forms dendrites when it interacts with the electrolyte. Over time, enough dendrites can short-circuit the battery and raise the likelihood of combustion.

One way to avoid fire risks is to replace the volatile liquid electrolyte with a solid, nonflammable one, often comprising a polymer-ceramic composite. While altering the ratio of polymer to ceramic can influence dendrite growth, finding the ideal formulation remains a challenge for extending battery life.

To investigate, the team employed nuclear magnetic resonance (NMR) spectroscopy, a standard tool for pinpointing chemical structures, and tracked both dendrite formation and the chemical interplay within the electrolyte. “When we examined the dendrites in batteries with differing ratios of polymer and ceramic, we found a kind of ‘golden ratio’: Electrolytes that are composed of 40 percent ceramic had the longest lives,” Leskes explains. “When we went above 40 percent ceramic, we encountered structural and functional problems that impeded battery performance, while less than 40 percent led to reduced battery life.” Intriguingly, batteries with that optimal ratio displayed more dendrites overall, but those dendrites were effectively confined in a way that prevented destructive bridging.

These insights prompted a larger question: what halts the extension of the dendrites? The team hypothesized that a thin covering on the surface of dendrites, called the solid electrolyte interphase (SEI), might be crucial. This layer, formed when dendrites interact with the electrolyte, can affect how lithium ions travel through the battery, and it can also either prevent or accelerate the movement of harmful substances between electrodes. Both of these factors, in turn, can stifle or foster further dendrite development.

Probing the chemical composition of such thin SEI films is inherently difficult, since they measure only a few dozen nanometers thick. The researchers tackled this problem by enhancing the signals in their NMR data using dynamic nuclear polarization. This specialized technique leverages the strong spin of polarized lithium electrons, bolstering signals from the atomic nuclei in the SEI and exposing its chemical makeup. Through this refined lens, the researchers discovered precisely how lithium metal interacts with polymer or ceramic materials, revealing that certain SEI layers can simultaneously improve ion transport and block hazardous substances.

Their findings pave the way to design sturdier, safer, and more powerful batteries that will store greater energy for a longer duration with reduced environmental and economic costs. Such next-generation batteries could power larger devices without having to increase the physical size of the battery itself, while also extending the battery’s life cycle.

“One of the things I love most about this study is that, without a profound scientific understanding of fundamental physics, we would not have been able to understand what happens inside a battery. Our process was very typical of the work here at the Weizmann Institute. We started with a purely scientific question that had nothing to do with dendrites, and this led us to a study with practical applications that could improve everybody’s life,” Leskes says.

Research Report:Tracking dendrites and solid electrolyte interphase formation with dynamic nuclear polarization-NMR spectroscopy

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Solar Energy

Role of barrier films in maintaining the stability of perovskite solar cells

Published

2 days ago

February 3, 2025

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Role of barrier films in maintaining the stability of perovskite solar cells

by Riko Seibo

Tokyo, Japan (SPX) Jan 31, 2025

Perovskite solar cells (PSCs) offer a promising advancement in renewable energy due to their high efficiency, lightweight, and flexible properties. However, their commercial viability is challenged by their vulnerability to environmental conditions, particularly heat and humidity.

To tackle this issue, a research team led by Professor Takashi Minemoto, a Fellow at the Ritsumeikan Advanced Research Academy, alongside Dr. Abdurashid Mavlonov from Ritsumeikan University’s Research Organization of Science and Technology and Dr. Akinobu Hayakawa from Sekisui Chemical Co., Ltd., conducted an in-depth study on the durability of PSC modules under harsh environmental conditions. Their research, published in Volume 286 of *Solar Energy* on January 15, 2025, was first made available online on December 17, 2024.

Discussing the study’s motivation, Prof. Minemoto stated, “Perovskite solar cells stand out as particularly promising due to their low-temperature wet-coating process and compatibility with flexible substrates, offering unique opportunities for the solar industry. However, the stability of perovskite is weak compared with conventional material, which can be improved by fabrication processes such as encapsulation with barrier films.”

For this research, the team analyzed the durability of flexible PSC modules made from methylammonium lead iodide (MAPbI3) and encapsulated them using polyethylene terephthalate (PET) substrates with barrier films of varying water vapor transmission rates (WVTR). These modules were subjected to a damp heat test at 85 C and 85% relative humidity to replicate long-term outdoor conditions.

After 2,000 hours of exposure, researchers measured photovoltaic (PV) performance and assessed module degradation using current-voltage characteristics, spectral reflectance, and electroluminescence imaging. The findings confirmed that high humidity caused the MAPbI3 layer to break down into lead iodide, obstructing charge transport and significantly reducing the efficiency of the PSC modules.

Moreover, the study demonstrated the critical role of barrier films in maintaining module stability. Notably, the module with the lowest WVTR barrier retained 84% of its initial power conversion efficiency, whereas modules with higher WVTR deteriorated rapidly, ceasing to function after just 1,000 hours.

“Our study is the first to report the durability of encapsulated flexible MAPbI3-based PSC modules. When considering solar energy applications for walls and rooftops with weight limits or for mobile platforms, flexible PSCs are a great alternative to the traditional silicon panels. Insights from our study could help industries optimize these modules for highly stable and durable constructs,” explained Prof. Minemoto.

This research underscores the essential role of barrier films in ensuring the long-term viability of flexible PSC modules, which could reshape the photovoltaic industry. By enabling energy generation in a variety of locations, these advancements help alleviate pressure on power grids. Additionally, enhancing the durability of PSCs expands their usability across different environments, further accelerating the global transition to cleaner and more sustainable energy solutions.

Research Report:Perovskite solar cell modules: Understanding the device degradation via damp heat testing

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Solar Energy

Enhancing Durability and Efficiency in Tin-based Perovskite Solar Cells

Published

2 days ago

February 3, 2025

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Enhancing Durability and Efficiency in Tin-based Perovskite Solar Cells

by Riko Seibo

Tokyo, Japan (SPX) Jan 30, 2025

Tin-based perovskite solar cells are being hailed as a promising alternative for next-generation solar energy solutions due to their high efficiency, flexibility, and the potential for low-cost printing. However, replacing lead with tin to avoid environmental issues linked to lead toxicity presents its own challenges. Tin’s propensity to oxidize quickly results in reduced performance and durability compared to lead-based counterparts.

Researchers have developed a method to enhance the stability of tin-based perovskite by incorporating large organic cations into the perovskite structure. This results in a unique two-dimensional layered configuration known as Ruddlesden-Popper (RP) tin-based perovskites. Despite its potential, the precise internal structure and the mechanism through which this configuration improves performance have remained unclear.

In this study, researchers employed electron spin resonance (ESR) to analyze the internal behavior of the RP perovskite solar cell during operation at a microscopic level. Their findings revealed two key insights about the interaction of the materials under different conditions.

First, when the RP perovskite solar cell was not exposed to light, the holes in the hole transport layer diffused into the RP perovskite. This movement created an energy barrier at the interface between the hole transport layer and the RP tin perovskite, preventing electron backflow and leading to better performance.

Second, when exposed to sunlight, the high-energy electrons produced by short-wavelength light (such as ultraviolet rays) moved from the RP tin perovskite to the hole transport layer. This transfer further elevated the energy barrier, thereby enhancing the device’s efficiency.

Understanding the mechanisms behind these performance improvements is crucial for developing tin-based perovskite solar cells with greater efficiency and longer lifespans. These findings could provide important insights for future advancements in the field of solar energy.

Research Report:Operando spin observation elucidating performance-improvement mechanisms during operation of Ruddlesden-Popper Sn-based perovskite solar cells