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Organic photovoltaic solar cells could withstand harsh space environments

11 février 2025 à 15:00

Carbon-based organic photovoltaics (OPVs) may be much better than previously thought at withstanding the high-energy radiation and sub-atomic particle bombardments of space environments. This finding, by researchers at the University of Michigan in the US, challenges a long-standing belief that OPV devices systematically degrade under conditions such as those encountered by spacecraft in low-Earth orbit. If verified in real-world tests, the finding suggests that OPVs could one day rival traditional thin-film photovoltaic technologies based on rigid semiconductors such as gallium arsenide.

Lightweight, robust, radiation-resilient photovoltaics are critical technologies for many aerospace applications. OPV cells are particularly attractive for this sector because they are ultra-lightweight, thermally stable and highly flexible. This last property allows them to be integrated onto curved surfaces as well as flat ones.

Today’s single-junction OPV devices also have a further advantage. Thanks to power conversion efficiencies (PCEs) that now exceed 20%, their specific power – that is, the power generated per weight – can be up to 40 W/g. This is significantly higher than traditional photovoltaic technologies, including those based on silicon (1 W/g) and gallium arsenide (3 W/g) on flexible substrates. Devices with such a large specific power could provide energy for small spacecraft heading into low-Earth orbit and beyond.

Until now, however, scientists believed that these materials had a fatal flaw for space applications: they weren’t robust to irradiation by the energetic particles (predominantly fluxes of electrons and protons) that spacecraft routinely encounter.

Testing two typical OPV materials

In the new work, researchers led by electrical and computer engineer Yongxi Li and physicist Stephen Forrest analysed how two typical OPV materials behave when exposed to proton particles with differing energies. They did this by characterizing their optoelectronic properties before and after irradiation exposure. The first materials were made up of small molecules (DBP, DTDCPB and C₇₀) that had been grown using a technique called vacuum thermal evaporation (VTE). The second group consisted of solution-processed small molecules and polymers (PCE-10, PM6, BT-CIC and Y6).

The team’s measurements show that the OPVs grown by VTE retained their initial PV efficiency under radiation fluxes of up to 10¹² cm⁻². In contrast, polymer-based OPVs lose 50% of their original efficiency under the same conditions. This, say the researchers, is because proton irradiation breaks carbon-hydrogen bonds in the polymers’ molecular alkyl side chains. This leads to polymer cross-linking and the generation of charge traps that imprison electrons and prevent them from generating useful current.

The good news, Forrest says, is that many of these defects can be mended by thermally annealing the materials at temperatures of 45 °C or less. After such an annealing, the cell’s PCE returns to nearly 90% of its value before irradiation. This means that Sun-facing solar cells made of these materials could essentially “self-heal”, though Forrest acknowledges that whether this actually happens in deep space is a question that requires further investigation. “It may be more straightforward to design the material so that the electron traps never appear in the first place or by filling them with other atoms, so eliminating this problem,” he says.

According to Li, the new study, which is detailed in Joule, could aid the development of standardized stability tests for how protons interact with OPV devices. Such tests already exist for c-Si and GaAs solar cells, but not for OPVs, he says.

The Michigan researchers say they will now be developing materials that combine high PCEs with strong resilience to proton exposure. “We will then use these materials to fabricate OPV devices that we will then test on CubeSats and spacecraft in real-world environments,” Li tells Physics World.

The post Organic photovoltaic solar cells could withstand harsh space environments appeared first on Physics World.

Conditioning prepares aluminium-ion batteries for real-world use

Physics World

Par :Vijit Nautiyal

9 décembre 2024 à 15:12

Imagine a smartphone that charges faster, lasts longer and is more eco-friendly – all at a lower cost. Aluminium-ion batteries (AIBs) could make this dream a reality, and scientists are working to unlock their potential as a more abundant, affordable and sustainable alternative to the lithium-ion batteries currently used in mobile devices, electric cars and large-scale energy storage. As part of this effort, Dmitrii A Rakov and colleagues at the University of Queensland, Australia recently overcame a technical hurdle with an AIB component called the solid-electrolyte interphase. Their insights could help AIBs match, or even surpass, the performance of their lithium-ion counterparts.

Like lithium-ion batteries, AIBs contain an anode, a cathode and an electrolyte. This electrolyte carries aluminium ions, which flow between the positively-charged anode and the negatively-charged cathode. During discharge, these ions move from the anode to the cathode, generating energy. Charging the battery reverses the process, with ions returning to the anode to store energy.

The promise and the problem

Sounds simple, right? But when it comes to making AIBs work effectively, this process is far from straightforward.

Aluminium is a promising anode material – it is lightweight and stores a lot of energy for its size, giving it a high energy density. The problem is that AIBs are prone to instabilities as they cycle between charging and discharging. During this cycling, aluminium can deposit unevenly on the anode, forming tree-like structures called dendrites that cause short circuits, leading to battery failure or even safety risks.

Researchers have been tackling these issues for years, trying to figure out how to get aluminium to deposit more evenly and stop dendrites from forming. An emerging focus of this work is something called the solid-electrolyte interphase (SEI). This thin layer of organic and inorganic components forms on the anode as the battery charges, and like the protective seal on a jar of jam, it keeps everything inside fresh and functioning well.

In AIBs, though, the SEI sometimes forms unevenly or breaks, like a seal on a jar that doesn’t close properly. When that happens, the aluminium inside can misbehave, leading to performance issues. To complicate things further, the type of “jam” in the jar – different electrolytes, like chloroaluminate ionic liquids – affects how well this seal forms. Some electrolytes help create a better seal, while others make it harder to keep the aluminium deposits stable.

Cracking the code of aluminium deposition

In their study, which is published in ACS Nano, the Queensland scientists, together with colleagues at the University of Southern Queensland and Oak Ridge National Laboratory in the US, focused on how the aluminium anode interacts with the liquid electrolyte. They found that the formation of the SEI layer is highly dependent on the current running through the battery and the type of counter electrode (the “partner” to the aluminium anode). Some currents and conditions allow the battery to work well for more cycles. But under other conditions, aluminium can build up in uneven, dendritic structures that ultimately cause the battery to fail.

Photo of a battery being assembled from vials of chemicals in a laboratory, with a person's blue-gloved hand visible next to the vials — Work in progress: Assembling a cell for testing. (Courtesy: Dmitrii Rakov)

To understand how this happens, the researchers investigated how different electrolytes and cycling conditions affect the SEI layer. They discovered that in some cases, when the SEI isn’t forming evenly, aluminium oxide (Al₂O₃) – which is normally a protective layer – can actually aggravate the problem by causing the aluminium to deposit unevenly. They also found that low currents can deplete some materials in the electrolyte, leading to parasitic reactions that further reduce the battery’s efficiency.

To solve these issues, the scientists recommend exploring different aluminium-alloy chemistries. They also suggest that specific conditioning protocols could smooth out the SEI layer and improve the cycling performance. One example of such a conditioning protocol is pre-cycling, which is a process where the battery is charged and discharged in a controlled way before regular use to condition it for better long-term performance.

“Our research demonstrates that, like in lithium-ion batteries, aluminium-ion batteries also need pre-cycling to maximize their lifetime,” Rakov tells Physics World. “This is important knowledge for aluminium-ion battery developers, who are rapidly emerging as start-ups around the world.”

By understanding the unique pre-cycling needs of aluminium-ion batteries, developers can work to design batteries that last longer and perform more reliably, bringing them closer to real-world applications.

How far are we from having an aluminium-ion battery in our mobile phones?

As for when those applications might become a reality, Rakov highlights that AIBs are still in the early stages of development, and many studies test them under conditions that aren’t realistic for everyday use. Often, these tests use very small amounts of active materials and extra electrolyte, which can make the batteries seem more durable than they might be in real life.

In this study, Rakov and colleagues focused on understanding how aluminium-ion batteries might degrade when handling higher energy loads and stronger currents, similar to what they would face in practical use. “We found that different types of positive electrode materials lead to different types of battery failure, but by using special pre-cycling steps, we were able to reduce these issues,” Rakov says.

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