Logo: University of Southern California

Jongseung Yoon Co-Leads Team Producing Flexible and More Efficient Solar Cells

New design for luminescent solar concentrators uses ultrathin, microscale silicon that can wrap around curved surfaces
By: Eric Mankin
January 22, 2013 —

Existing silicon solar cells convert sunlight to energy quite efficiently, but only if they are pointed directly at the sun. And they are flat and brittle, so they cannot be wrapped around structures that are rounded or irregular.

A team including the USC Viterbi School of Engineering’s Jongseung Yoon has improved an old idea for solving the first of these problems, and added a solution to the second.

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Mechanically flexible concentrator module has been created with a distributed array of silicon microscale solar cells embedded in thin, luminophore-doped waveguides, where the spacings and array configurations of the printed cells were optimally designed to match the intrinsic loss characteristics of conventional luminescent solar concentrators to enhance their performance
The old idea – first presented in 1976 – proposes what are called luminescent solar concentrators (LSCs). These incorporate “luminophores,” tailored bits of special substances that absorb sunlight and then re-emit it at a different frequency.

This reemitted light bounces within the LSC to strips of conventional solar cells mounted on the edges, which convert it into electricity.

The concept is elegant but the execution has been difficult. Optical inefficiencies, which result from re-absorption of the emitted light and light escaping from the LSC structure, have been problems. LSCs using conventional solar cells are bulky, so the geometry of key components could not be completely adjusted to optimize their performance. And the units were mechanically inflexible.

The alternative: Instead of conventional solar cells, which run from .1 to .5 mm in thickness, the design by Jongseung Yoon, an assistant professor in Viterbi's Mork Family Department of Chemical Engineering and Materials Science, and collaborators at the University of Illinois at Urbana-Champaign, uses microscale silicon solar cells. These are ultrathin (.01-.02 mm) silicon chips  directly embedded in a thin polymer layer doped  with luminophores, which is coated on transparent glass or plastic substrates.

Like conventional solar cells, these chips absorb light directly incident on their top surface. But additionally, if they are mounted on a reflective surface, light that goes through or around them without being absorbed and used can be sent back from the LSC structure for absorption through their bottoms and sides. They are also so thin that they can be mounted on rounded or other non-flat surfaces.

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Jongseung Yoon
In the design described in the paper published June 14 in  Nature Communications, interconnected arrays of such microcells sit on top of a layer of LSC material, converting incident solar light and then recapturing the light that goes around them into the LSC layers.  A reflector at the bottom sends unabsorbed light back through for another pass. The paper presents three variations on the theme.

The economic possibilities are clear, according to Yoon. Silicon cells are expensive. A thin grid of microcells over a layer of LSC can potentially capture energy at a much lower cost per square meter.

Such ultrathin LSC systems provide a 320% increase in power output, compared to the maximum power that can be obtained with the same amount of silicon in conventional cells without LSC effects. Yoon says the overall system performance can be further enhanced through optimization of luminophores, solar cells, as well as advanced design of waveguide structures.

“The unusual LSC designs reported here offer improved performance compared to conventional layouts,” the paper notes, “and a variety of engineering options with particular value in ultrathin lightweight bendable systems…. The rich ranges of design and materials possibilities suggest multiple paths for optimization.”

Yoon's main co-author on the paper was Lanfang Li of University of Illinois at Urbana-Champaign. Five other University of Illinois researchers contributed to the effort.