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Spring 2016

Light Reflection for Efficient Collection

New reflector traps more light and increases efficiency of solar concentrators

Michelle A. Harris

Diagram of the luminescent solar concentrator (LSC). Arrows show the path of the fluorescent light as it reflects within the LSC to reach the solar cells. TIR is the total internal reflected light that reaches the solar cell. Escape cone refers to the range of angles at which the light is reflected out of the LSC. Copyright 2016: American Chemical Society

In the global shift towards renewable energy sources, solar cells are the primary devices for converting light energy to electricity. Most solar cell devices use a solar concentrator to capture more light. Typical solar concentrators require direct sunlight and thus must be moved to face the sun throughout the day. Luminescent solar concentrators (LSC) employ a type of molecule, a luminophore, to avoid this technical requirement. A research team at the Light-Material Interactions in Energy Conversion Center (LMI) has developed a reflective filter that increases the efficiency of LSCs by more than 20 percent, making a more viable and practical source of electricity.

The luminophore in the LSC absorbs a certain wavelength of the sun’s light but emits light at a different wavelength that then is reflected towards one or more solar cells. The luminophore absorbs both direct and indirect or diffuse sunlight. Other solar concentrators, such as geometric solar concentrators, use lenses and mirrors to concentrate sunlight. Lenses and mirrors need direct sunlight to efficiently reflect the light towards a solar cell. They cannot capture the diffuse light on cloudy days and only receive enough direct sunlight in certain regions of the world. An LSC increases the capture of sunlight without these limiting factors.

Although the LSC addresses one of the major roadblocks for greater light capture, it isn’t the only issue. The team decided to take on another aspect of efficiency: the loss of emitted light through reflection out of the concentrator. The light emitted by the luminophore in the LSC is reflected, and a portion makes it to a solar cell. However, a significant amount of the light is reflected at such an angle that it escapes the LSC and never reaches a solar cell. To further the advantage of the LSC, Ralph Nuzzo and his team have developed a reflective filter that increases the amount of light that reaches the solar cells.

The team at LMI created a reflective filter by alternating different layers of suspended nanoparticles. This type of reflector is called a distributed Bragg reflector. The two small particles used, silicon dioxide and tin dioxide, refract sunlight at different angles. The refractive index of the nanoparticles and the thickness of each layer determine the range of wavelengths reflected. Their methods produced a filter that selectively reflects the wavelength of emitted light back towards the solar cells while still allowing through the wavelengths of sunlight that the luminophore absorbs.

To measure the efficacy of their distributed Bragg reflector, the team at LMI measured the electric current output of the solar cells within the LSC. Comparing the electricity generated with and without the reflector, they found a marked 23 percent increase in efficiency with their filter. The results found using their reflective filter are promising for concentrators using the studied luminophore.

The researchers also studied the properties that make the reflector effective. They found optimal features for the wavelength range and angle of the reflected light that are crucial for developing the ideal reflective filter for other luminophores as well. The type of luminophore they used emits a different wavelength than it absorbs. This characteristic is key for efficient solar concentration. Optimizing the luminophore in combination with their reflective filter will lead to highly efficient concentrators.

“With optimization, the light trapping design of the LSC device has the potential of surpassing the typical geometric concentrators,” says Lu Xu, who was first author on the study.

By a carefully designed layer of nanoparticles, the LMI researchers significantly increased the efficiency of their targeted solar concentrators.

This research and related work at LMI and at the Center for Advanced Solar Photophysics, another Energy Frontier Research Center, can be applied to a variety of concentrators to increase the efficiency of widespread solar energy.

Acknowledgments

This work was supported by the Light-Material Interactions in Energy Conversion Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Xu L, Y Yao, ND Bronstein, L Li, AP Alivisatos, and RG Nuzzo. 2016. “Enhanced Photon Collection in Luminescent Solar Concentrators with Distributed Bragg Reflectors.” ACS Photonics 3(2):278-285. DOI: 10.1021/acsphotonics.5b00630

About the author(s):

  • Michelle A. Harris is a postdoctoral researcher in the Argonne-Northwestern Solar Energy Research (ANSER) Center at Northwestern University under Michael Wasielewski. Her research involves ultrafast spectroscopic studies of charge transfer in DNA hairpins and in donor-acceptor molecules for solar fuels applications. She received her B.S. in integrative biology from the University of Illinois at Urbana-Champaign in 2009. She did her dissertation research under Dewey Holten in the Photosynthetic Antenna Research Center (PARC) and received her Ph.D. in chemistry from Washington University in St. Louis in 2014.

How to Hoard the Sun

Scientists show new options for putting more light into solar cells

The team's solar concentrator includes a specialized filter that prevents sunlight from bouncing out before it can be put to work. Image by American Chemical Society and modified by Nathan Johnson, PNNL

Solar cells aren’t as greedy as we’d like them to be, but at the Light-Material Interactions in Energy Conversion Center (LMI), scientists are making them grab more light and put it to work making electricity. The team improved a concentrator that fits between solar cells and increased the device’s efficiency by 20 percent. The concentrator uses luminophores, which absorb indirect light as well as direct light, meaning a cell can take in the light that is bounced around at crazy angles by the atmosphere. They also designed a specialized filter that prevents the light from bouncing out. The team’s work adds pages to the solar design book, showing new ways to achieve more efficient cells, something we all need as global energy demands increase. The LMI is led by the California Institute of Technology.

More Information

Xu L, Y Yao, ND Bronstein, L Li, AP Alivisatos, and RG Nuzzo. 2016. “Enhanced Photon Collection in Luminescent Solar Concentrators with Distributed Bragg Reflectors.” ACS Photonics 3(2):278-285. DOI: 10.1021/acsphotonics.5b00630

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.