Incorporating solar panels into architecture could help make new buildings more energy-efficient and reduce their climate impact. But commercial solar cells generally come in only two, opaque colors–black and bluish black–which limits architects’ design options.
Scientists have developed several methods for making colorful solar cells, but these cells require complicated fabrication methods, are less efficient than current commercial cells, or both. Now, researchers have reported an easily applied microsphere-based coating that adds color to silicon solar cells while retaining over 95% of their efficiency (ACS Nano 2022, DOI: 10.1021/acsnano.2c05840).
Unlike colored dyes and pigments, which absorb certain colors of light and reflect the rest, the coating relies on structural color created by microspheres just a few hundred nanometers wide. Structural color arises from uniformly sized nanoscale features that are small enough to interact with and reflect specific wavelengths of light. So different sizes of microspheres reflect and produce distinct colors. And rather than absorb the other colors of the spectrum, these structures allow the rest of the light to pass through.
That makes the coating useful for adding color to solar cells, which generate more energy when more light hits them, says Tao Ma, a photovoltaics researcher at Shanghai Jiao Tong University who co-led the work. “With this technology, we don’t absorb solar energy, we just reflect a small part.”
To apply the coating, Ma’s team spray coated zinc sulfide microspheres onto commercial silicon solar cells. The tiny spheres self-assembled into a layer called a photonic glass, which made the solar cells appear red, green, or blue depending on the microspheres’ size. The three colored solar cells converted light into electricity with around 21.5% efficiency, compared to 22.6% for black, uncoated panels. The researchers also added multi-colored patterns to cells by simply placing stencils over the panel before spray coating with different microspheres.
Using structural color in this way has several advantages, says Kaline P. Furlan, a photonic materials researcher at the Hamburg University of Technology, who was not involved in the research. Because the color is a product of the microspheres’ tiny dimensions, the researchers can tune the color using the same zinc sulfide material. And “if the structure is kept intact, the color never fades,” she says.
Ma says he is confident that the process for making the nanoparticles can be scaled up at comparatively low cost, and the spray coating would lend itself easily to mass production. Now, he says his team’s goal is to develop more hues and more saturated colors, which Furlan says is an ongoing challenge in the photonic materials community.
Furlan says the spray coating process could be scalable depending on the material cost and availability, but cautions that zinc sulfide has been known to oxidize under ultraviolet radiation in humid conditions. She suggests that the researchers should carry out more detailed characterization of the microsphere coating after aging tests to determine whether oxidation is happening.