NREL Research Pinpoints Promise of Polycrystalline Perovskites

A team of scientists from the Energy Department’s National Renewable Energy Laboratory (NREL) determined that surface recombination limits the performance of polycrystalline perovskite solar cells.
 
Research into perovskites at NREL and elsewhere has proved the material’s effectiveness at converting sunlight into electricity, routinely topping 20% efficiency. The sunlight creates mobile electrons whose movement generates the power but upon encountering defects can slip into a non-productive process. Known as a recombination, this process reduces the efficiency of a solar cell. For the cell to be the most efficient, the recombination must occur slowly.
 
With prior studies into perovskites focusing on bulk recombination, one area left unexamined until now concerned the surface recombination in lead iodide perovskites. NREL’s scientists determined recombination in other parts of a methylammonium perovskite film isn’t as important as what’s happening on the surface, both the top and bottom.
 
Matthew Beard and his colleagues within NREL’s Chemistry and Nanoscience Center studied surface recombination in single-crystal and polycrystalline films using transient reflection spectroscopy. Their findings, Top and bottom surfaces limit carrier lifetime in lead iodide perovskite films, appear in Nature Energy.
 
“What’s important is to know where the recombination is coming from,” said Beard, lead author of the research paper. “There are multiple sources of possible recombination. In order to improve your device, you’re asked to get rid of all non-radiative recombination. Typically people forget about surfaces. They think about grain boundaries. They think about bulk defects and so forth.”
 
Beard’s co-authors are all from NREL: Ye Yang, Mengjin Yang, David T. Moore, Yong Yan, Elisa M. Miller and Kai Zhu.
 
A second study that concurrently appeared in the journal Physical Chemistry Chemical Physics was authored by Mengjin Yang, Yining Zeng, Zhen Li, DongHoe Kim, Chun-Sheng Jiang, Jao van de Lagemaat, and Kai Zhu further strengthened the conclusions of the Nature Energy paper. This study, using high-resolution fluorescence-lifetime imaging, also showed that surface recombination is the determining factor instead of grain boundary recombination.
 
The research suggested a light coating of a protective material on the surface of the polycrystalline thin films could further improve the performance of perovskite solar cells.