There is a great interest to study the electronic structure of nickel oxide (NiO) due to its potential to act as a hole transport layer in organic photovoltaic cells and dye-sensitized solar fuel cells. Despite several years of research focused on understanding the electronic structure of NiO, the surface electronic structure in this material is not well understood. Using ultrafast extreme ultraviolet (XUV) spectroscopy combined with X-ray photoelectron spectroscopy (XPS), we study the surface electron dynamics in nickel thin film as a function of thermal oxidation of the lattice. Both the XUV spectroscopy and XPS suggest that the sub-surface grain boundary between Ni metal and NiO behaves as an electron trap. The trap state density is higher for the nickel thin film oxidized at lower annealing temperature and reduces with increasing annealing temperature. We find that the photoexcited electron in NiO thin film diffuses to the trapped state through the formation of a small polaron within ~600 fs. Additionally, the charge-carrier recombination dynamics (10-100 ps) exhibit a strong dependence on the trap state density where the higher trap state density leads to faster recombination, indicating a trap-mediated recombination mechanism.<br/>

Effect of lattice oxidation on the surface electronic structure of NiO probed by ultrafast XUV reflection-absorption spectroscopy

256th National Meeting (2018)

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