To venture into the depths of space, space agencies and companies need to first conquer the hurdle of dependable power sources.

Silicon and gallium arsenide are mostly the go-to materials for solar cells, but a new contender is emerging: organic solar cells.

Made from carbon-based materials, these cells offer several advantages: they’re lightweight, flexible, and potentially cheaper to produce. But their durability in the harsh space environment has been a major question mark.

A recent study from the University of Michigan investigated the impact of proton radiation — a major threat in space — on organic solar cells.

The previous research on organic solar cells in space primarily examined their overall efficiency after radiation exposure. However, the new study delves deeper by investigating the underlying molecular mechanisms that lead to performance degradation.

“Silicon semiconductors aren’t stable in space because of proton irradiation coming from the sun,” said Yongxi Li, first author of the study.

“We tested organic photovoltaics with protons because they are considered the most damaging particles in space for electronic materials,” Li added.

Flexible organic cells

Gallium arsenide is a popular choice for space missions due to its high efficiency. It can convert a significant portion of sunlight into electricity. Also, it can withstand the damaging effects of protons, which are abundant in space.

However, it also has certain disadvantages. For instance, gallium arsenide is an expensive material to produce. On the other hand, like silicon, it is relatively heavy and difficult to bend, which can limit its applications in space missions. 

Meanwhile, organic solar cells are known for their flexibility and lightweight nature. This makes them attractive for space applications where weight and the ability to conform to curved surfaces are important.

This research aims to assess the reliability of organic solar cells in the harsh space environment to determine their suitability for future space missions.

Electron trap

Interestingly, the study found that organic solar cells made with small molecules were highly resistant to proton radiation. The cells displayed no damage after three years of radiation exposure. 

However, those made with more complex polymer structures experienced a significant decline in efficiency, losing about half of their original performance.

The researchers discovered that proton radiation can break chemical bonds within the polymer, creating “electron traps” that hinder the flow of electricity.

“We found that protons cleave some of the side chains, and that leaves an electron trap that degrades solar cell performance,” said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Engineering at U-M.

The team found that these “traps” can be healed by gently heating the cells. This suggests the possibility of self-healing solar cells that could operate effectively in space.

“You can heal this by thermal annealing, or heating the solar cell. But we might find ways to fill the traps with other atoms, eliminating this problem,” said Forrest.

The study suggests that sun-facing organic solar cells could potentially self-heal at high temperatures (around 100°C or 212°F), which has been observed in laboratory settings.

“But questions remain: for instance, will that repair still take place in the vacuum of space? Is the healing reliable enough for long missions?” the researchers stated in the press release. 

Alternatively, researchers are exploring the possibility of designing materials that inherently resist the formation of performance-degrading electron traps.

The findings were published in the journal Joule.