Electrochemical determination of the density of states of nanostructured NiO films.

Mesoporous p-type NiO films were prepared by aerosol-assisted chemical vapor deposition (AACVD) and characterized by X-ray diffraction (XRD). The nanostructure of the films was investigated by field emission gun scanning electron microscopy (FEG-SEM). The density of states (DOS) in these nanostructured films has been determined by means of electrochemical impedance spectroscopy and cyclic voltammetry. The analysis reveals an exponential distribution of band gap states above the valence band that extends around 1.5 eV. In addition, monoenergetic states were also identified which overlap with the exponential distribution. This distribution of states has an enormous influence in the electronic processes of the devices in which NiO electrodes are employed (electrochromism, water splitting or energy storage). Especially, in p-type dye-sensitized solar cells (p-DSCs), it is thought that intra-band-gap states are responsible for the fast observed recombination processes, whose existence and distribution has not been clearly determined yet and are now confirmed and quantified by our analysis. This provides a better comprehension of the recombination events which represent one of the main losses in p-DSCs.