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Low-cost nanoparticle prod'n leads to better PV cells, LEDs

Posted: 20 Jun 2014     Print Version  Bookmark and Share

Keywords:nanoparticle  titanium-dioxide  solar cell  LEDs 

A team of researchers at Sandia National Laboratories has developed what an inexpensive way to synthesise titanium-dioxide (TiO2) nanoparticles and is seeking partners who can demonstrate the process at industrial scale for everything from solar cells to LEDs.

TiO2 nanoparticles show great promise as fillers to tune the refractive index of anti-reflective coatings on signs and optical encapsulants for LEDs, solar cells and other optical devices. Optical encapsulants are coverings or coatings, usually made of silicone, that protect a device.

Industry has largely shunned TiO2 nanoparticles because they've been difficult and expensive to make, and current methods produce particles that are too large. Sandia became interested in TiO2 for optical encapsulants because of its work on LED materials for solid-state lighting.

Current production methods for TiO2 often require high-temperature processing or costly surfactants, molecules that bind to something to make it soluble in another material, such as dish soap does with fat.

Those methods produce less-than-ideal nanoparticles that are very expensive, can vary widely in size and show significant particle clumping, called agglomeration.

Titanium-dioxide nanoparticles

Sandia National Laboratories researchers Dale Huber and Todd Monson have come up with an inexpensive way to synthesise titanium-dioxide nanoparticles, which could be used in everything from solar cells to LEDs. Photo by Randy Montoya

Sandia's technique, on the other hand, uses readily available, low-cost materials and results in nanoparticles that are small, roughly uniform in size and don't clump.

"We wanted something that was low cost and scalable, and that made particles that were very small," said researcher Todd Monson, who along with principal investigator Dale Huber patented the process in mid-2011 as "High-yield synthesis of brookite TiO2 nanoparticles."

Their method produces nanoparticles roughly 5nm in diameter, about 100 times smaller than the wavelength of visible light, so there's little light scattering, Monson said.

"That's the advantage of nanoparticles, not just nanoparticles, but small nanoparticles," he said.

Scattering decreases the amount of light transmission. Less scattering also can help extract more light, in the case of an LED, or capture more light, in the case of a solar cell.

TiO2 can increase the refractive index of materials, such as silicone in lenses or optical encapsulants. Refractive index is the ability of material to bend light. Eyeglass lenses, for example, have a high refractive index.

Practical nanoparticles must be able to handle different surfactants so they're soluble in a wide range of solvents. Different applications require different solvents for processing.

"If someone wants to use TiO2 nanoparticles in a range of different polymers and applications, it's convenient to have your particles be suspension-stable in a wide range of solvents as well," Monson said. "Some biological applications may require stability in aqueous-based solvents, so it could be very useful to have surfactants available that can make the particles stable in water."

The researchers came up with their synthesis technique by pooling their backgrounds, Huber's expertise in nanoparticle synthesis and polymer chemistry and Monson's knowledge of materials physics. The work was done under a Laboratory Directed Research and Development project Huber began in 2005.

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