Researchers at the University of Pittsburgh have used nanoscale, grass-like structures to create a glass that lets through a large amount of light while appearing hazy. This is the first time glass has been made with high levels of haze and light transmittance at the same time. This new glass could help boost the performance of solar cells and LEDs.
The glass also has another notable quality. It can be switched from its hazy appearance to clear just by adding water. This makes it 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, leader of the research team. "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 glass achieves 95 percent light transmittance and a similarly high degree of haze at the same time. The researchers experimented 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.
This switchability was discovered almost by accident. “I was cleaning the new nanograss glass when I discovered that cleaning it with water made the glass become clear," said project lead, graduate student Sajad Haghanifar. While the discovery was incidental, it can be easily explained. "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."
Leu’s group developed this glass to improve the ability of solar cells to capture light and turn it into power. Nanostructure patterns can prevent light from reflecting off of the solar cell’s surface. These structures scatter the light that enters the glass. This helps more light reach the semiconductor material within the solar cell where it is then converted into power.
Within the glass, there is a unique pattern of nanostructures that looks a lot like grass. Because these structures are taller than previously used nanostructures, they increase the likelihood that light will be scattered. Even though the glass with the nanostructures appears opaque, tests showed that most of the scattered light makes its way through the glass.
Due to the highly hazy glass and high transmittance, this glass would also be useful for LEDs. LEDs work in a way that is basically the opposite of solar cells. LEDs use electricity that enters a semiconductor to produce light which is then emitted from the device. This new glass could increase the amount of light that makes it from the semiconductor, to the surroundings.
The researchers found that the shorter the nanograss, the higher the properties of antireflection are, while the longer nanograss tended to increase the haze. Glass with 4.5 micron-high nanograss showed a good balance of 95.6 percent transmittance and 96.2 percent haze for light with a 550-nanometer wavelength.
But more work is needed to estimate exactly how much the glass will cost to manufacture. The research team predicts that the glass will be inexpensive because it is simple to make. The nanostructures are etched into the glass using a process known as reactive ion etching, a scalable and straightforward method commonly used to make printed circuit boards.
To turn the glass into a smart window that switches from hazy to clear, a piece of traditional glass would be placed over the nanograss glass. Pumps could be used to flow liquid into the space between the two glasses, and a fan or pump could be used to remove the water. The researchers showed that, in addition to water, acetone and toluene can switch the glass from hazy to clear.
"We are now conducting durability tests on the new nanograss glass and are evaluating its self-cleaning properties," said Haghanifar. "Self-cleaning glass is very useful because it prevents the need for robotic or manual removal of dust and debris that would reduce the efficiency of solar panels, whether the panels are on your house or on a Mars rover."
A paper on this research was published in Optica.