24 July 2015

University of Delaware awarded $1m Keck Foundation grant to develop upconversion of red light to blue

The University of Delaware (UD) has received a $1m grant from the W.M. Keck Foundation to explore a new idea that could improve solar cells, medical imaging and even cancer treatments. The goal is to turn low-energy colors of light (such as red) into higher-energy colors (e.g. blue or green). 

Changing the color of light would give solar technology a considerable boost. A traditional solar cell can only absorb light with energy above a certain threshold. Infrared light passes through, with its energy untapped. However, if that low-energy light could be transformed into higher-energy light, a solar cell could absorb much more of the sun's energy. The team predicts that their novel approach could increase the efficiency of commercial solar cells by 25-30%. 

Based in UD's College of Engineering, the research team is led by project leader Matthew Doty, associate professor of materials science and engineering and associate director of UD's Nanofabrication Facility. Doty's co-investigators include Joshua Zide, Diane Sellers and Chris Kloxin (all in the Department of Materials Science and Engineering) and Emily Day and John Slater (both in the Department of Biomedical Engineering). 

Changing the color of light 

"The energy of each photon is directly related to the color of the light — a photon of red light has less energy than a photon of blue light," notes Doty. "You can't simply turn a red photon into a blue one, but you can combine the energy from two or more red photons to make one blue photon." 

For such a photon upconversion process, the team wants to design a new kind of nanostructure that will act like a ratchet: it will absorb two red photons, one after the other, to push an electron into an excited state, when it can emit a single high-energy (blue) photon. "The semiconductor ratchet structure is how we trap the electron in the middle of the ladder until the second photon arrives to push it the rest of the way up," says Doty.

The UD team aims to develop new structures containing multiple layers of different materials, such as aluminum arsenide and gallium bismuth arsenide, each just a few nanometers thick. This 'tailored landscape' will control the flow of electrons into states with varying potential energy, turning once-wasted photons into useful energy.

The team has shown theoretically that their structures could reach an upconversion efficiency of 86%, which would be a vast improvement over the 36% demonstrated by the best materials currently. Also, the amount of light absorbed and energy emitted by the structures could be customized for a variety of applications, from lightbulbs to laser-guided surgery, Doty says.

The team will use molecular beam epitaxy (MBE) to fabricate the nanostructures. Each structure will be tested to see how well it absorbs and emits light, and the results will be used to tailor the structure to improve performance.

However, the researchers will also develop a solution filled with millions of identical individual nanoparticles, each containing multiple layers of different materials that will implement the photon ratchet idea. Through such work, the team envisions a future upconversion 'paint' that could be easily applied to solar cells, windows and other commercial products. 

The research team aims to develop nanostructures that combine the energy of two red photons of light into a single blue photon.

Improving medical tests and treatments

While the initial focus of the three-year project will be on improving solar energy harvesting, the team will also explore biomedical applications. 

A number of diagnostic tests and medical treatments, ranging from computerized tomography (CT) and positron emission tomography (PET) scans to chemotherapy, rely on the release of fluorescent dyes and pharmaceutical drugs. Ideally, such payloads are delivered both at specific disease sites and at specific times, but this is hard to control in practice. 

The UD team aims to develop an upconversion nanoparticle that can be triggered by light to release its payload. The goal is to achieve the controlled release of drug therapies even deep within diseased human tissue while reducing the peripheral damage to normal tissue by minimizing the laser power required. 

"This is high-risk, high-reward research," Doty comments. "High-risk because we don't yet have proof-of-concept data. High-reward because it has such a huge potential impact in renewable energy to medicine… this same technology could be used to harvest more solar energy and to treat cancer." 

Tags: MBE

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