Photo of ultrathin on-chip solar cells. The cells are the green squares and include an ultrathin layer of light-absorbing GaAs, which is key to their radiation tolerance. The surface of each green square is only 120 nanometers, about one-thousandth the thickness of a human hair, above the surrounding gray area. The gold-colored grids are electrically conducting metal contacts. (Image: Armin Barthel)

Most space satellites are powered by photovoltaic cells that convert sunlight to electricity. Exposure to certain orbit radiation can damage the devices, degrading their performance and limiting their lifetime. University of Cambridge scientists have proposed a radiation-tolerant photovoltaic cell design that features an ultrathin layer of light-absorbing material.

When solar cells absorb light, they transfer its energy to negatively charged electrons in the material. These charge carriers are knocked free and generate a flow of electricity across the photovoltaic. Making photovoltaics thinner should increase their longevity because the charge carriers have shortened lifetimes and thus don’t have to travel as far.

As low-Earth orbit becomes more cluttered with satellites, middle-Earth orbits — e.g., the Molniya orbit, which passes through the center of Earth’s proton radiation belt — become more attractive. Radiation-tolerant cell designs will be necessary for such orbits.

Radiation-tolerant cells can also benefit the study of other planets and moons. Europa, a moon of Jupiter, has one of the most severe radiation environments in the solar system. So, landing a solar-powered spacecraft on Europa will require radiation-tolerant devices.

The team built two types of photovoltaic devices using the semiconductor gallium arsenide: An on-chip design built by layering several substances in a stack, and the other involved a silver back mirror to enhance light absorption.

The devices were bombarded with protons — generated at the Dalton Cumbrian Nuclear Facility in the U.K. — and the devices’ irradiation performances were measured via cathodoluminescence. A second set of tests using a Compact Solar Simulator were carried out to determine how well the devices converted sunlight to power after the proton bombardment.

“Our ultra-thin solar cell outperforms the previously studied, thicker devices for proton radiation above a certain threshold. The ultra-thin geometries offer favorable performance by two orders of magnitude relative to previous observations,” said Author Armin Barthel.

The team believes that the improved performance of these ultra-thin cells is because the charge carriers live long enough to travel between terminals in the device. Compared to thicker cells, nearly 3.5 times less cover glass is needed for the ultra-thin cells to deliver the same amount of power after 20 years of operation. This translates to a lighter load and significant reduction in launch costs.

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