Nanograss Glass with Fluid-Induced Switchable Opacity Could Improve LEDs, Solar Cells

Researchers at the University of Pittsburgh, Pennsylvania used nanoscale grass-like structures to create glass that appears hazy but still allows a large amount of light to pass through.

This is a breakthrough achievement where glass has been developed with such a high degree of haze and light transmittance simultaneously – a unique combination of properties that could be used to increase the performance of LEDs and solar cells.

Another incredible quality of this glass is that it can be easily switched from hazy to clear by simply applying water. This can probably make it useful for developing smart windows that change opacity or haze to block the glare from sunlight or to control the privacy of a room.  

Switchable glass available today is quite expensive because it uses transparent conducting layers to apply a voltage across the entire glass, 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.

Paul W. Leu, University of Pittsburgh’s Swanson School of Engineering, leader of the research team.

The scientists have described their new nanograss-based glass In Optica – The Optical Society's journal for high impact research. They showed that the glass achieves a record light transmittance of 95% and a similarly high level of haze simultaneously. The team experimented with glass that was etched with nanograss structures which ranged between 0.8 and 8.5 microns in height with each “blade” measuring a few hundred nanometers in diameter.

The switchability was discovered by chance. Although the discovery was one of serendipity, it can be explained easily.

I was cleaning the new nanograss glass when I discovered that cleaning it with water made the glass become clear, 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.

graduate student Sajad Haghanifar, project lead.

Using nanograss to improve solar cells
The new glass was developed by Leu’s team to enhance the ability of solar cells to harvest light and convert it into power. With nanostructure patterns, light can be prevented from reflecting off the surface of the solar cell. The light entering the glass is also scattered by these structures, allowing more light to reach the semiconductor material inside the solar cell, where it is changed into power.

A unique pattern of grass-like nanostructures is used by the new glass. Since the structures are taller than the nanostructures used before, they increase the chance that light will be scattered. While the nanograss-based glass seems to be opaque, tests have demonstrated that most of the scattered light ultimately makes its way through the glass.

Since the glass has high transmittance and is also highly hazy, it could also be used for making LEDs, which function in a way that is basically the exact opposite of a solar cell. LEDs use electricity that enters a semiconductor to generate light, which is subsequently emitted from the device. The innovative glass could possibly boost the amount of light that makes it from the semiconductor into the surroundings.

Finding the right ‘grass’ height
The team observed that shorter nanograss enhanced the glass’ antireflection properties, whereas longer nanograss increased the haze. In addition, glass with 4.5-micron-high nanograss demonstrated a good balance 96.2% haze and 95.6% transmittance for light with a wavelength of 550 nanometer (yellow light, a component of sunlight).

While more research needs to be done to predict the actual cost of developing the novel glass, the scientists predict that since their glass is easy to make it will be inexpensive. A process called reactive ion etching is used to etch the nanostructures into the glass. This process is a simple, scalable method that is generally used for making printed circuit boards.

In order to convert glass into a smart window that switches from hazy to clear, a piece of standard glass needs to be placed over the nanograss glass. Pumps can possibly be used to allow the liquid to flow into the space between the two glasses, and a pump or fan can be utilized to remove the water. The research team also demonstrated that besides water, toluene and acetone can also be applied to change 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.”