Improved thin film harnesses indoor light to power devices

UNIVERSITY PARK, Pa. — Remember the solar calculator from middle school math class that had a small solar cell at the top? The idea behind that technology is not a relic of the past, but the subject of continuous innovation. Today’s indoor solar cells, known as photovoltaics, use new semiconductor materials, such as metal halide perovskite, to convert ambient light into electricity to power small electronic devices without needing batteries or a power outlet. Now, a team led by materials science researchers at Penn State has improved perovskite’s ambient light-absorbing ability and its capacity to remain stable over time even at high light intensities.

The researchers detailed their approach in APL Energy.

“The general idea is to try to reduce losses in device performance by finding the right perovskite material and experimenting with what composition performs well and what causes performance losses,” said co-corresponding author Nutifafa Doumon, assistant professor of materials science and engineering in the Penn State College of Earth and Mineral Sciences. “Instead of needing to absorb the broad spectrum of light from the sun, indoor photovoltaics only need to absorb the much weaker, narrower spectrum of indoor light.”

To improve the spectrum of light a photovoltaic device can absorb, known as the bandgap, researchers experimented with the material’s ratio of bromine to iodide, two components in perovskite’s crystal structure.

“After understanding how perovskite works and its light absorption profile, I changed the percentage of bromide and iodide incrementally, leading to six different material compositions,” said Justin Lin, doctoral student in materials science and engineering and lead author of the paper. “I tested the optical properties of all six and gradually shifted the material toward the wavelengths available from indoor lighting to absorb more indoor light. The composition containing 35% bromine was the champion.”

To make a thin film, researchers dissolve materials into a solvent, and as it dries, the perovskite forms a crystal structure. To improve the material’s structure and film quality, researchers used dichlorobenzene as an antisolvent instead of the commonly used chlorobenzene. This decision was based on findings from a 2024 paper by Doumon and Ivy Asuo, assistant professor of materials science and engineering.

“The grains of the thin film should be compact without voids and aligned in a specific direction to improve the quality of the film,” said Asuo, a co-corresponding author of the APL Energy study. “The antisolvent treatments help us drive the crystallization of perovskite, because you need that for a uniform, high-quality, defect-free thin film, required for photovoltaic performance.”

Finally, researchers added a salt, phenethylammonium bromide, or PEABr, on top of the perovskite layer, forming a passivation layer. The passivation layer adheres to the thin film’s surface to protect it from breaking down under high light intensities over long durations.

“Through testing, we learned that PEABr protected the perovskite layer from defects during the formation of the thin film and reduced loss in perovskite’s ability to absorb ambient light, even at very bright light over long periods of time,” Asuo said.

With the passivation layer, the researchers observed no decline in the device’s performance during 240 hours of testing at high light intensities. Based on that trend, the researchers estimate the device could remain stable for thousands of hours of use.

Doumon recently co-authored a paper in Nature Energy that detailed a common framework and best practices for researchers to measure the performance and stability of indoor photovoltaics that use new semiconductor materials.

The device detailed in APL Energy can maintain its performance over time at light intensities approximately 10 to 50 times higher than those required by the new consensus — equivalent to 8% to 50% of the Sun’s brightness.

Looking forward, researchers said they hope to see perovskite-based photovoltaics powering battery-free consumer products, but the technology still has a long way to go.

“Indoor photovoltaics have the potential to power the next generation of consumer devices like smart thermostats, wearable medical devices and TV remotes, but we’re not there yet,” Doumon said. “Because of these materials’ susceptibility to degrading quickly in harsher environments, we need to make them more durable. We also need to improve our ability to accurately measure them.”

In addition to Doumon, Lin and Asuo, the other co-authors are Souk Yoon “John” Kim, Penn State doctoral student in materials science and engineering, and Marvin Wu, professor of physics at North Carolina Central University.

The contributions to the work by Penn State researchers were supported by Penn State’s Materials Characterization Lab Core Facility of the Materials Research Institute, as well as the Penn State Institute of Energy and the Environment. The full list of funders is available in the paper. This content is solely the responsibility of the authors and does not necessarily represent the views of the funders.

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