Our search for life on Europa may require these ultrathin solar cells - here's why

For instance, NASA's Europa Clipper spacecraft, scheduled for launch in 2024, will follow up on discoveries from the Galileo mission of the 1990s. Yet, while its sole mission is to focus on the icy moon, performing nearly 50 close passes, the craft will orbit Jupiter, not Europa itself, due to the intense radiation.

In addition, the Clipper's payload and other electronics will be enclosed in a thick-walled vault to protect them from radiation in Jupiter's magnetic field.

With this in mind, university of Cambridge researchers have now designed radiation-resistant ultrathin solar cells with the goal of improving spacecraft for these harsher environments.

Interesting Engineering (IE) spoke with Armin Barthel, principal scientist and a Ph.D. scholar at the university's Department of Materials Science and Metallurgy, to learn how the unique devices came to be and what future real-world applications may be expected.

Gallium arsenide (GaAs) solar cells have been powering spacecraft for decades

The primary power source for almost every spacecraft for the past 34 years has continued to be silicon or gallium arsenide (GaAs) solar cells.

During this time, power output levels increased from a few milliwatts (Vanguard 1, 10mW, 1958) to many kilowatts in the 1970s and 1980s (Skylab, 21 kW; MIR, 10 kW; Hubble Space Telescope, 5 kW).

Additionally, solar cells are seen as a "moving target" for rival technologies like nuclear and thermodynamic systems because of the consistent scientific advancements in enhancing their efficiency and, in particular, their radiation resistance.

What reduces a solar cell's electricity generation?

Still, one of the most problematic obstacles for solar cells is corpuscular, or particulate, radiation, particularly energetic protons, and electrons.

"When highly energetic particles like protons and electrons hit the solar cells, they introduce defects in the material, which reduce the efficiency of the devices," Barthel told IE.

Corpuscular radiation causes damage by displacing atoms. A significant portion of the displaced atoms transforms into defects in the crystalline semiconductors, which are the most common materials utilized in space photovoltaics.

These flaws frequently introduce changed states into the semiconductor's band gap and serve as recombination hubs.

Simply put, this corresponds to a decrease in photovoltage and photocurrent, which reduces the solar cell's electricity generation.

"The issue that this technology addresses is that of the reduction in power output of solar cells, when they are exposed to high energy radiation," explained Barthel.

Solar-powered satellites or landers to study Europa's habitability

The study highlighted that particle flux, species, and kinetic energy are only a few of the many variables that affect how quickly damage is caused and how severe it is. For different space environments, these elements can vary greatly.

The intensity of radiation for different Earth orbits and the orbit of Jupiter's moon Europa is demonstrated in the study's image above.

This shows the highly eccentric Molniya and Europa orbits experience a proton flow significantly greater than the other Earth orbits - Low Earth Orbit (LEO) and geostationary orbit (GEO). This is one of the reasons why medium earth orbits (MEO) are rarely employed.

Europa's harsh radiation damages spacecraft

Europa, in particular, orbits in one of the solar system's most severe planetary radiation environments and has only a tenuous atmosphere. As such, the study highlights that a solar-powered satellite or lander used for examining its habitability would benefit significantly from high radiation tolerance.

Europa's harsh radiation environment relates to its proximity to Jupiter. Around Jupiter, there is a radiation belt in the shape of a doughnut which forms as a result of charged high-energy particles being trapped and accelerated by the planet's strong magnetic field.

Europa, the smallest of the four Galilean moons encircling the planet, orbits about 417,000 miles (671,000 kilometers) from Jupiter and, as a result, is constantly exposed to its hostile, high-radiation zone.

The 'patterned' ultrathin cells were created using a process called photolithography

"The cells were fabricated [with gallium arsenide, GaAs] from a stack of semiconductor materials. This was grown for us by molecular beam epitaxy (MBE)," Barthel revealed. As the team did not do this themselves, IE is spared the details.

However, it is worth noting that MBE involves using atomic or molecule beams to deposit a thin single-crystal layer on a single-crystal substrate.

These beams are produced in K-cells (Knudsen cells) in an environment with an extremely high vacuum. MBE is widely used in the manufacture of semiconductor devices and in the development of nanotechnologies.

Additionally, the team incorporated an 80 nanometer (nm) light-absorbing GaAs layer key to the cells' radiation tolerance.

"To turn this stack into working solar cells, we had to remove and add material in specific locations. The key to being able to choose these locations precisely at small length scales is a technique called photolithography," Barthel explained.

IE learned that photolithography works by coating a sample with a thin layer of 'resist' material. When exposed to light, the material's solubility in a particular solvent changes.

This means that shining light on this resist material through a patterned mask and then dipping it in the solvent will enable the selective removal of some of it.

"Where the resist was not removed, you now have a protective layer on your sample that will stop the removal or addition of other materials in those areas," Barthel said.

Devices one thousandth the thickness of a human hair

As shown in the figure above, the surface of each cell (green squares above the surrounding gray area) is only 120 nanometres - about one-thousandth the thickness of a human hair.

"Essentially, the thickness of the devices is approximately one-thousandth the thickness of a human hair," confirmed Barthel.

The test: Scientists blasted the solar cells at a Nuclear Facility

Barthel and his team blasted the 'ultrathin' GaAs solar cells with three megaelectron volt (MeV) proton radiation. Their results demonstrated that integrated light control in the devices provided improved effectiveness and a prolonged lifetime due to radiation resilience.

The devices were 'attacked' with protons produced at the Dalton Cumbrian Nuclear Facility in the United Kingdom to simulate the effects of radiation in space.

Cathodoluminescence, a method that can estimate the extent of radiation damage, was used to compare the performance of photovoltaic devices before and after exposure.

The efficiency of the devices' ability to convert sunlight into power after being hit by protons was also tested by a second set of tests using a Compact Solar Simulator.

3.5 times less cover glass is needed for the ultrathin cells

"Our ultrathin solar cell outperforms the previously studied, thicker devices for proton radiation above a certain threshold. The ultrathin geometries offer favorable performance by two orders of magnitude relative to previous observations," stated Barthel.

Compared to thicker cells, the study successfully demonstrated that nearly 3.5 times less cover glass is needed for the ultrathin cells to deliver the same amount of power after 20 years of operation.

There's room for improvement

Barthel pointed out that one major limitation of ultrathin designs is that they absorb less light than thicker cells. This is because there is less material there to absorb the light. "If less light is absorbed, less power is generated. To combat this issue, light management is required," Barthel said.

"In this study, one of the cell designs includes a rear mirror, which reflects all the light that wasn't absorbed the first time it passed through the cell back up through the cell," he explained.

However, he also highlighted that more sophisticated light management structures could be made. These would trap light in the cells, allowing them to absorb even more. "Our group is actively researching these and how to integrate them with the devices," Barthel revealed.

The cells could make satellites' much lighter'

Europa aside, Barthel highlighted that medium earth orbits (MEOs) will get more attractive as low Earth orbit becomes more crowded. The study also underscores this importance by underlining, "it is necessary to develop space photovoltaics with greater radiation tolerance that can extend mission lifetimes in MEO."

"If we want to make use of harsher radiation environments in space, we need a cost-effective power supply for such an environment. Our cells were created with this goal in mind," he added.

"Since [our ultrathin solar cells] can withstand greater amounts of radiation, they would need less cover glass to shield them from radiation, which means that the solar arrays and satellites could be much lighter," Barthel told IE.

This would lead to a significant decrease in the mass of the solar panel and, consequently, the launch price for a satellite or rover using the array.

The next step

The next steps for the team will involve using what they have learned from this study to optimize the devices further.

So far, they've only looked at one thickness for their ultrathin cells. Barthel explained that this study's results will help them determine if there is a different thickness that could give a better compromise between radiation tolerance and light absorption.

"We are also interested in looking at stacking multiple ultrathin cells to improve power output and also at trying different material combinations," Barthel revealed.