University of Pittsburgh researchers have fabricated prototype glass that admits a large amount of light while appearing hazy, a combination of properties that could improve the performance of solar cells and LEDs. The addition of water triggers the switch from hazy to clear, which could prove useful for creating smart windows that change haze or opacity to control the privacy of a room or to block glare from sunlight.

“Switchable glass available today is quite expensive because it uses transparent conducting layers to apply a voltage across the entire glass,” said Paul W. Leu of the University of Pittsburgh’s Swanson School of Engineering. “Our glass would be potentially less expensive to make because its opacity can be switched in a matter of seconds by simply applying or removing liquid.”

The new fused silica glass uses a unique pattern of nanostructures that looks much like grass. Because the The top image shows that text can be read through normal flat glass, while the glass etched with nanostructure scatters light, making the glass appear opaque. This glass could help boost the performance of solar cells and LEDs. Source: Sajad Haghanifar, University of Pittsburghstructures are taller than previously-used nanostructures, they increase the likelihood that light will be scattered. Although glass with the nanostructures appears opaque, tests showed that most of the scattered light makes its way through the glass.

The nanograss-based glass achieves a record 95 percent light transmittance and a similarly high degree of haze at the same time. The material was developed by experimenting with glass etched with nanograss structures from 0.8 to 8.5 microns in height with “blades” each measuring a few hundred nanometers in diameter.

The discovery of switchability was accidental and occurred when a researcher realized the glass became clear when cleaned with water. Explains graduate student Sajad Haghanifar, “The water goes between the extremely hydrophilic nanostructures, making the nanograss glass act like a flat substrate. Because water has a very similar index of refraction to the glass, the light goes straight through it. When the water is removed, the light hits the scattering nanostructures, making the glass appear hazy.”

The glass was designed to improve the ability of solar cells to capture light and turn it into power. Nanostructure patterns can prevent light from reflecting off the solar cell’s surface and also scatters light that enters the glass, enabling more light to reach the semiconductor material within the cell.

Shorter nanograss improved the antireflection properties of the glass while longer nanograss tended to increase the haze. Glass with 4.5-micron-high nanograss showed a balance of 95.6 percent transmittance and 96.2 percent haze for light with a 550-nanometer wavelength.

The research is published in Optica.