Photovoltaic

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.