Journal
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
Volume 9, Issue 17, Pages 5047-5054Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpclett.8b01865
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- Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under DOE Grant [DE-SC0014051]
- U.S. Department of Energy (DOE) [DE-SC0014051] Funding Source: U.S. Department of Energy (DOE)
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Understanding the chemical nature of defect sites as well as the mechanism of defect-mediated recombination is critical for the rational design of energy conversion materials with improved efficiency. Using femtosecond extreme ultraviolet (XUV) spectroscopy in conjunction with X-ray photoelectron spectroscopy (XPS), we present results on the ultrafast electron dynamics in NiO prepared with varying concentrations of defect states. We find that oxygen vacancy defects do not serve as the primary recombination center, but rather the recombination rate scales linearly with the density of Ni metal defects. This suggests that grain boundaries between Ni metal and NiO are responsible for fast carrier recombination in partially reduced NiO. Our kinetic model shows that the photoexcited electrons self-trap via small polaron formation on the subpicosecond time scale. Additionally, we estimate an absolute measurement of small polaron formation rates, direct versus defect-mediated recombination rates, and the small polaron diffusion coefficient in NiO. This study provides important parameters for engineering NiO based materials for solar energy harvesting applications.
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