By Cindy Moffett
Imagine a fabric that could efficiently convert light into energy. Clad in our own lightweight, low-cost power plants, we could wear our own energy supply and never again worry about dead cell phone batteries.
Or consider the convenience of solar cloth for disaster relief operations in remote areas, where transporting the fuel needed to power heavy machinery is very expensive. Simply unrolling a solar-fiber blanket could capture enough energy from sunlight.
Science fiction? Pipe dream? No, it’s the brainchild of Ramki Kalyanaraman, associate professor in the Department of Materials Science and Engineering (MSE) and the Department of Chemical and Biomolecular Engineering. “This technology would make individuals more energy-independent and make possible other light-energy-harvesting products such as carpets, curtains, tents, and even textile-based solar farms,” he says.
In 2008, Kalyanaraman teamed with Gerd Duscher, an MSE associate professor who specializes in atomic and electronic structures, to improve the efficiency of silicon solar cells. “We realized that one of the fundamental challenges in this field is that the technology is based on planar devices, which means the solar cells are laid out on a flat surface. And light generally doesn’t get trapped very easily in a planar device,” Kalyanaraman says.
In fact, most solar panels convert sunlight to energy with an efficiency of only 12 to 18 percent. So the duo started thinking about how to improve those numbers.
Because fiber easily traps light, they wondered if they could make a solar cell in a hollow fiber. If so, the tubular solar cells would collect light from three dimensions rather than two, potentially giving them higher efficiencies.
After gathering support for their concept from a geoscientist, an optics expert, and a high-school teacher who had worked with hollow fibers, “the last piece of the puzzle was how to make these polymers of the diameter and material that we want,” Kalyanaraman says. They consulted MSE professor Gajanan Bhat, who confirmed it would be possible to make hollow fibers of various materials, transparencies, and mechanical properties.
Once the team was in place, they began to figure out how to make the material and get electrons out of it. Their design for an ultra-light, high-efficiency solar fiber calls for creating inorganic solar cells inside hollow polymer fibers 1 to 10 meters long and only 50 to 100 microns in diameter. These fibers would be as small as the threads woven into a dress shirt.
“The basic principle of a solar cell,” Kalyanaraman says, “is that light strikes materials, creating positive and negative charges. In order to transport the energy, you need a good conductor such as copper or aluminum. We would like to use such materials because they are abundant and economical.”
Coating their tubular cell design with a metal, however, would reflect light, whereas solar cells must absorb light to create electricity. After a good bit of design work, the team concluded that they should either use a transparent conducting oxide or a metal that only partly coats the tube.
“The fundamental challenge is to make the solar cell materials with quality that gives good performance,” Kalyanaraman says.
Several graduate students are integral to the search for the best materials and design for the solar fiber. Funded by UT’s Sustainable Energy and Education Research Center, Abhinav Malasi is researching plasmons—nanoparticles that scatter light efficiently. Tyler Smith is building a scaled-up prototype solar cell and Humaira Taz is investigating thin film techniques, which are crucial to depositing conducting materials into the threadlike tubes. They will use computational modeling to assess how well various metals trap light, how much current is generated, and how well it is transmitted.
“Since we are starting from scratch, we are learning to do the deposition on planar devices, figuring out how good the devices are, learning from that, and then transferring our knowledge into the fiber concept,” Kalyanaraman says. At a larger level, they have already made a successful device in a straw.
Duscher, Kalyanaraman, and their partners have created a startup company called Sunjoule Materials Inc. With support from a venture capitalist, Sunjoule has filed for a patent to protect the concept while research continues.
In September 2013 the group’s hollow-fiber solar cell concept won an honorable mention as well as a “most popular” nod, ranking twentieth out of 709 entries in NASA Tech Brief’s Create the Future design contest. The competition was created in 2002 to help stimulate and reward engineering design innovation.
Kalyanaraman hopes to have a working prototype by the end of 2014, and speculates actual devices could be on the market in five to ten years. He believes this project may only be the beginning for solar fiber.
“There are two other things that I have in mind,” he muses. “One is to also have a battery inside the fiber, so you have a solar cell and a battery. And eventually, it would be great to have a way to transport water. So then in these hollow fibers we could get energy from the sunlight, store it, and also transport water.”
By providing all three needs, the fibers would become a fairly complete life support system. In a world of rapidly diminishing resources, solar fibers may one day weave the cloth that keeps us alive.