Jul 27 2018
Solar cells are devices that absorb photons from sunlight and turn their energy to move electrons - enabling the generation of clean energy and providing a reliable route to help fight climate change. But a majority of solar cells used commonly today are fragile, thick, and stiff, which restricts their application to flat surfaces and increases the cost to manufacture the solar cell.
An image of a back-reflector surface used by the researchers to test perovskite performance. Each quadrant is a different surface material—gold, titanium, palladium or a silica compound—upon which the perovskite material would be deposited for experiments. (Image credit: University of Washington)
“Thin-film solar cells” could be 1/100th the thickness of a piece of paper and flexible enough to cover surfaces varying from an aerodynamically smooth car to clothing. To design thin-film solar cells, researchers are searching beyond the “classic” semiconductor compounds, such as silicon or gallium arsenide, and utilizing instead other light-harvesting compounds that have the potential to be easier and cheaper to mass produce. The compounds could be extensively accepted if they could work as well as today’s technology.
In a paper published online in the journal Nature Photonics this spring, scientists at the University of Washington explain that a prototype semiconductor thin-film has performed a lot better than current best solar cell materials at producing light.
“It may sound odd since solar cells absorb light and turn it into electricity, but the best solar cell materials are also great at emitting light,” said co-author and UW chemical engineering professor Hugh Hillhouse, who is also a faculty member with both the UW’s Clean Energy Institute and Molecular Engineering & Sciences Institute. “In fact, typically the more efficiently they emit light, the more voltage they generate.”
The UW team realized a record performance in this material, termed as a lead-halide perovskite, by chemically treating in a process called “surface passivation,” which treats flaws and diminishes the likelihood that the absorbed photons will end up wasted instead of converted to valuable energy.
One large problem with perovskite solar cells is that too much absorbed sunlight was ending up as wasted heat, not useful electricity. We are hopeful that surface passivation strategies like this will help improve the performance and stability of perovskite solar cells.
David Ginger, Co-Author & UW Professor of Chemistry
Ginger’s and Hillhouse’s teams partnered together to show that surface passivation of perovskites sharply improved performance to levels that would raise this material to be among the best for thin-film solar cells. They tested with a range of chemicals for surface passivation before spotting one, an organic compound known by its acronym TOPO that increased perovskite performance to levels nearing the best gallium arsenide semiconductors.
“Our team at the UW was one of the first to identify performance-limiting defects at the surfaces of perovskite materials, and now we are excited to have discovered an effective way to chemically engineer these surfaces with TOPO molecules,” said co-lead author Dane deQuilettes, a postdoctoral researcher at the Massachusetts Institute of Technology who conducted this study as a UW chemistry doctoral student. “At first, we were really surprised to find that the passivated materials seemed to be just as good as gallium arsenide, which holds the solar cell efficiency record. So to double-check our results, we devised a few different approaches to confirm the improvements in perovskite material quality.”
DeQuilettes and co-lead author Ian Braly, who conducted this research as a doctoral student in chemical engineering, demonstrated that TOPO-treating a perovskite semiconductor considerably influenced both its external and internal photoluminescence quantum efficiencies—metrics used to establish how good a semiconducting material is at using an absorbed photon’s energy instead of losing it as heat. TOPO-treating the perovskite boosted the internal photoluminescence quantum efficiencies by tenfold—from 9.4% to almost 92%.
Our measurements observing the efficiency with which passivated hybrid perovskites absorb and emit light show that there are no inherent material flaws preventing further solar cell improvements. Further, by fitting the emission spectra to a theoretical model, we showed that these materials could generate voltages 97 percent of the theoretical maximum, equal to the world record gallium arsenide solar cell and much higher than record silicon cells that only reach 84 percent.
Ian Braly, Co-Lead Author
These enhancements in material quality are theoretically predicted to enable the light-to-electricity power conversion efficiency to touch 27.9% under typical sunlight levels, which would push the perovskite-based photovoltaic record past the leading silicon devices.
The subsequent step for perovskites, the scientists said, is to prove a similar chemical passivation that is well-matched with easily manufactured electrodes - as well as to test other types of surface passivation.
“Perovskites have already demonstrated unprecedented success in photovoltaic devices, but there is so much room for further improvement,” said deQuilettes. “Here we think we have provided a path forward for the community to better harness the sun’s energy.”
Other co-authors are Luis Pazos-Outón, a postdoctoral researcher at the University of California, Berkeley; Sven Burke, who recently completed his UW undergraduate degree in materials science and engineering; and Mark Ziffer, who just completed his doctoral degree with the UW Department of Chemistry and the CEI. The research was funded by the U.S. Department of Energy, the National Science Foundation, the University of Washington, the UW Clean Energy Institute, the UW Molecular Engineering & Sciences Institute and the University of California, Berkeley.