Columbia Chemists think they can double solar cell efficiency
A team of scientists at Columbia University has demonstrated the possibility of greatly boosting the efficiency of hybrid organic inorganic perovskite (HOIP) solar cells. They reported the results in the journal Science.
"Among the materials being explored for next generation solar cells, HOIPs have emerged a superstar," said Ziaoyang Zhu, a professor of chemistry at Columbia who led the research. "Until now no one has been able to explain why they work so well, and how much better we might make them. We now know it's possible to make HOIP-based solar cells even more efficient than anyone thought possible," he added.
Over the last seven years, scientists have managed to rapidly increase the efficiency with which HOIPs can convert solar energy into electricity, from 4 to 22 percent. This newly published work, Zhu says, suggests that "scientists have only just begun to tap the potential of HOIPs to convert the sun's energy into electricity."
The maximum theoretical efficiency silicon solar cells might reach has been calculated to be roughly 33 percent. It takes hundreds of nanoseconds for energised electrons to move from the part of a solar cell that infuses them with the sun's energy, to the part of the cell that harvests the energy and converts it into electricity that can ultimately be fed into a power grid. During this migration across the solar cell, the energised electrons quickly dissipate their excess energy. But those calculations assume a specific rate of energy loss.
The Columbia team discovered that the rate of energy loss is slowed down by over three-orders of magnitude in HOIPs "“ making it possible for the harvesting of excess electronic energy to increase the efficiency of solar cells.
"We're talking about potentially doubling the efficiency of solar cells," says Prakriti P. Joshi, a Ph.D. student in Zhu's lab who is a coauthor on the paper. "That's really exciting because it opens up a big, big field in engineering."
Adds Zhu: "This shows we can push the efficiencies of solar cells much higher than many people thought possible."
After demonstrating this, the team then turned to the next question: what is it about the molecular structure of HOIPs that gives them their unique properties? How do electrons avoid defects? They discovered that the same mechanism that slows down the cooling of electron energy also protects the electrons from bumping into defects. This "protection" makes the HOIPs turn a blind eye to the ubiquitous defects in a material developed from room-temperature and solution processing, thus allowing an imperfect material to behave like a perfect semiconductor.
HOIPs contain lead, and are also water soluble, meaning the solar cells could begin to dissolve and leach lead into the environment around them if not carefully protected from the elements. "Now we can go back and design materials which are environmentally benign and really solve this problem everybody is worried about," Zhu says. "This principle will allow people to start to design new materials for solar energy."
The research team was spearheaded by Haiming Zhu and Kiyoshi Miyata, two postdoctoral fellows at Columbia. Other members include graduate students Jue Wang, Prakriti P. Joshi, Kristopher W. Williams and postdoc Daniel Niesner, all of Columbia; Yongping Fu and Song Jin, collaborators from the University of Wisconsin"“Madison; and the team was led by Xiaoyang Zhu. This research received funding from the US Department of Energy and the National Science Foundation.
'Screening in crystalline liquids protects energetic carriers in hybrid perovskites' by Haiming Zhu et al; Science 23 Sep 2016: Vol. 353, Issue 6306
