Researchers from the University of Delaware have been awarded a $1 million grant from the W.M. Keck Foundation to pursue a novel idea that might enhance solar cells, medical imaging and even treatments for cancer.
The UD research team aims to develop new nanostructures that act like a ratchet to combine the energy of two red photons of light into a single blue photon, which has higher energy. Such an advance could improve solar cell efficiency to chemotherapy treatments.
The researchers hope to change the color of light by creating higher-energy colors like blue or green from low-energy colors of light like red.
This feat would be valuable for advancements in solar technology. A conventional solar cell is only capable of absorbing light with an energy that is above a specific threshold. Infrared light basically passes right through without the energy being tapped. If low-energy light could be converted into higher-energy light, the solar cell would be capable of capturing more of the sun’s abundant energy.
Based on their innovative method, the Delaware team estimates a 25 to 30% increase in the efficiency of commercial solar cells.
The team is led by Matthew Doty, associate professor of materials science and engineering and associate director of UD’s Nanofabrication Facility head. Joshua Zide, Diane Sellers and Chris Kloxin from the Department of Materials Science and Engineering; Emily Day and John Slater from the Department of Biomedical Engineering are Doty’s co-investigators.
“This prestigious $1 million grant from the Keck Foundation underscores the excellence and innovation of our University of Delaware faculty,” says Nancy Targett, acting president of the University. “Clearly, the University of Delaware is pursuing big ideas in renewable energy and biomedicine with the potential to benefit the world.”
“The University’s Delaware Will Shine strategic plan challenges us to think boldly as we seek solutions to problems facing society,” Domenico Grasso, UD’s provost, adds. “We congratulate the research team in the College of Engineering for this major award, and we look forward to their findings.”
Transforming the color of light
“A ray of light contains millions and millions of individual units of light called photons,” says project leader Matthew Doty. “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. 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.”
Doty states that the “photon upconversion” method is not new, however, the approach used by his team definitely is.
They plan to build a novel semiconductor nanostructure that will behave like a ratchet. It will have the ability to absorb two red photons in a sequence so as to thrust an electron into an excited state, leading it to discharge a single blue photon. These nanostructures can only be seen when magnified a million times under a high-powered electron microscope, as they will be extremely small.
“Think of the electrons in this structure as if they were at a water park,” Doty says. “The first red photon has only enough energy to push an electron half-way up the ladder of the water slide. The second red photon pushes it the rest of the way up. Then the electron goes down the slide, releasing all of that energy in a single process, with the emission of the blue photon. The trick is to make sure the electron doesn’t slip down the ladder before the second photon arrives. 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.”
Doty’s team will work on creating novel semiconductor structures that possess a number of layers of diverse materials, such as gallium bismuth arsenide and aluminum arsenide. Each of these layers will measure just a few nanometers in thickness. This customized set up will direct the electrons’ flow into states with varying prospective energy, thereby transforming the once-wasted photons into functional energy.
So far, the team has theoretically demonstrated that their semiconductors could realize an 86% upconversion efficiency. This is a huge improvement compared to the 36% efficiency displayed by the best materials currently available.
Doty states that the quantity of light absorbed and energy discharged by the structures can be tailored to match different applications, ranging from light bulbs to laser-guided surgery.
To create the extremely tiny nanostructures, one method that the team will employ is the molecular beam epitaxy, where layers of atoms are deposited one by one to build the structures. Each structure will be tested to observe its absorbing and emitting ability. The structure will be amended based on an analysis of these results in an attempt to improve performance.
T researchers will also create a milk-like solution containing millions of identical individual nanoparticles. Each nanoparticle will contain a number of layers of diverse materials. The multiple layers of this structure will realize the photon ratchet concept. Later, they hope to create upconversion “paint” that will easily coat solar cells, windows and other commercial items.
Improving medical tests and treatments
Although the primary aim of the three-year project is to enhance solar energy harvesting, Doty’s team hope to also investigate biomedical applications.
Several medical treatments and diagnostic tests, ranging from chemotherapy to CT and PET scans, depend upon the discharge of pharmaceutical drugs and fluorescent dyes. These payloads would ideally be delivered at specific disease sites and at specific times, but this is tough to control.
The research team will be working towards developing an upconversion nanoparticle that can be activated by light to discharge its payload. The goal here is to realize the moderated release of drug therapies deep inside diseased human tissue, while at the same time limiting the peripheral damage to healthy tissue by reducing the laser power needed.
“This is high-risk, high-reward research,” Doty says. “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. It’s amazing to think that this same technology could be used to harvest more solar energy and to treat cancer. We’re excited to get started!”