Light-Activated Material Could Enable Heliotropic Solar Cells

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MEDFORD, Mass., March 17, 2021 — Light-activated composite devices developed by Tufts University researchers were shown in testing to execute precise visible movements and form complex 3D shapes. The materials performed without wires or other actuating materials or energy sources. The design combines programmable photonic crystals with an elastomeric composite that the Tufts researchers engineered at the macro- and nanoscale to respond to illumination.

The material is composed of two layers: a film with opal-like properties made of silk fibroin doped with gold nanoparticles (AuNPs) forming photonic crystals, and an underlying substrate of polydimethylsiloxane (PDMS), which is a silicon-based polymer.
Photonic material in the shape of a flower can move in response to light, closely tracking the angle of maximum exposure. Courtesy of Fio Omenetto, Tufts University.
Photonic material in the shape of a flower can move in response to light, closely tracking the angle of maximum exposure. Courtesy of Fiorenzo Omenetto, Tufts University.

The silk fibroin layer is highly flexible and durable, with a negative coefficient of thermal expansion (CTE), which means that it contracts when heated and expands when cooled. Conversely, PDMS has a high CTE and expands rapidly when heated. When the material is exposed to light, one layer heats up much more rapidly than the other, causing the material to bend as one side expands and the other contracts or expands more slowly.

“With our approach, we can pattern these opal-like films at multiple scales to design the way they absorb and reflect light,” said Fiorenzo Omenetto, corresponding author and the Frank C. Doble Professor of Engineering. 

Optomechanical devices that convert light to movement often require complex and energy-intensive fabrication or setups. Using the new devices, Omenetto said, the team was able to achieve exquisite control of light-energy conversion. It also generated macro-motion of the materials without the need for electricity (wires).

Team members patterned the photonic crystal films by applying stencils and then exposing the materials to water vapor to generate specific patterns. The pattern of surface water changed the wavelength of absorbed and reflected light from the film, causing the materials to bend, fold, and twist in specific ways when illuminated by laser light, according to the pattern’s particular geometry.

The researchers demonstrated the method by creating a “photonic sunflower,” with integrated solar cells in the bilayer film so that the cells tracked the light source. The devices maintained a common angle between the solar cells and the laser beam, showing optimal performance efficiency. As the system works the same with white light, it also shows potential in the solar power industry with advantages in its wireless form and light-responsive and heliotropic (sun-following) properties to enhance light-to-energy conversion efficiency.

The research was published in Nature Communications (

Published: March 2021
photonic crystals
Photonic crystals are artificial structures or materials designed to manipulate and control the flow of light in a manner analogous to how semiconductors control the flow of electrons. Photonic crystals are often engineered to have periodic variations in their refractive index, leading to bandgaps that prevent certain wavelengths of light from propagating through the material. These bandgaps are similar in principle to electronic bandgaps in semiconductors. Here are some key points about...
Research & Technologysolarsolar cellsMaterialsheliotropicsilk fibroinSilksilk fiberssiliconpolydimethylsiloxanePDMSphotonic crystalphotonic crystalslight-activatedTuftsNature Communications

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