Abstract
The research effort in mesoporous p-type semiconductors is increasing due to their potential application in photoelectrochemical energy conversion devices. In this paper an electron-hole pair is created by band-gap excitation of NiO nanoparticles and the dynamics of the electron and the hole is followed until their recombination. By spectroscopic characterization it was found that surface Ni3+ states work as traps for both electrons and holes. The trapped electron was assigned to a Ni2+ state and the trapped hole to a "Ni4+" state positioned close to the valence band edge. The recombination kinetics of these traps was studied and related with the concept of hole relaxation suggested before. The time scale of the hole relaxation was found to be in the order of tens of ns. Finally the spectroscopic evidence of this relaxation is presented in a sensitized film.
Highlights
The excitation wavelengths used in this study with bare nickel oxide (NiO) are all equal to or lower than 355 nm, which leads to band gap excitation (BG ex.)
In this work transient absorption spectroscopy was used to analyse the trapping of electrons and holes generated by band gap excitation of NiO nanoparticle lms
The analysis revealed that a er a fs pulse the hole is trapped on a sub-picosecond time scale in a “Ni4+” state
Summary
In the last y years the electronic properties of nickel oxide (NiO) have been the focus of theoretical and experimental studies and are still under debate.[1,2,3,4,5,6] NiO, in its pure form, is a 3.5 eV indirect band gap semiconductor showing chargetransfer/Mott–Hubbard insulating behaviour.[7,8] It has been used as semiconductor material in a range of potential solar energy conversion applications such as hole transport layer in organic photovoltaic cells, water splitting cells, dye-sensitized solar fuel cells (DSSFCs) and p-type dye-sensitized solar cells (p-DSCs).[9,10,11,12,13,14,15] O en in these devices NiO is in the form of nanostructured crystals which are sintered together to obtain a mesoporous lm with a large active area.[16,17] The surface of the crystals plays a central role in the activity of the semiconductor. Ni3+ surface states can oxidise and trap a hole to “Ni4+”, where “Ni4+” indicates a mixed valence state§ of Ni higher than 3, possibly hybridized with the valence band, which has not been well characterised yet.[20,48,49,50,51]
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