A record-breaking polymer solar cell made by researchers at the University of California, Los Angeles, converts 10.6 percent of the energy in sunlight into electricity. The performance of the cell surpasses the previous record, 8.6 percent, set in July of last year by the same group.
Polymer solar cells are flexible, lightweight, and potentially inexpensive, but their performance lags behind that of conventional cells made from inorganic materials such as silicon. The goal of the researchers, led by Yang Yang, professor of materials science and engineering at UCLA, is to make a polymer solar cell that can compete with thin-film silicon cells. Yang's record-breaking cell, enabled by a new photovoltaic polymer developed by a Japanese company, Sumitomo Chemical, is a sign that researchers are getting better at making solar cells from these finicky materials.
The new plastic solar cell combines two layers that work with different bands of light—a polymer that works with visible light and one that works with infrared light. "The solar spectrum is very broad, from the near infrared through the infrared to the ultraviolet, and one single solar-cell component can't do it all," says Yang.
The best inorganic solar cells are also multilayer devices, but making multilayer organic solar cells has been difficult. Polymers can be printed from solution, like printing ink on paper, which is both a primary advantage of the technology and a liability, says Alan Heeger, who shared the Nobel Prize in 2000 for his co-discovery of conductive polymers. "There are no high temperatures involved, and manufacturing is simple," he says. But figuring out the right solvents to print each layer in a cell without bleeding into the one below it is tricky. The more layers, the more complex the problem becomes. Matching the electrical properties of each layer is also a challenge, as has been connecting them together.
Yang says he wants to make a polymer solar cell with an efficiency of 15 percent. He notes that efficiency numbers typically drop by about a third when solar cells are taken out of the lab and sold in working modules. A polymer solar cell that tests at 15 percent efficiency in the lab is likely to make a module with 10 percent efficiency, which Yang believes is good enough to compete with thin-film silicon solar.