XPS study of the band alignment at ITO/oxide (n-type MoO3 or p-type NiO) interface

While they have different electronic properties n-type MoO3 and p-type NiO are very efficient as buffer layers between the ITO anode and the organic electron donor in organic photovoltaic cells. While it is admitted that MoO3 is n-type, its band structure is still under study. Here, the band alignment at the interface of an ITO/MoO3 heterojunction is studied by X-ray photoelectron spectroscopy (XPS). The same study is realized on the structure ITO/NiO, NiO being a p-type semiconductor. The measurements have been performed on samples obtained under the same experimental conditions as those used to achieve organic photovoltaic cells. The MoO3 (NiO) upper layer was 3?nm thick. The semidirect XPS technique used to measure the band offsets allows us to estimate the band discontinuities at the interface ITO/MoO3: ?Ev?=?0.50?eV and ?Ec?=?0.90?eV, while at the interface ITO/NiO we have ?Ev?=?-2.10?eV and ?Ec?=?-1.90?eV. Therefore, n-type MoO3 and p-type NiO, which are both very efficient anode buffer layers (ABLs), exhibit different band structure at the contact with ITO. However, the measurement, by means of a Kelvin probe, of the work functions of the structures ITO/NiO and ITO/MoO3, shows that they are close and significantly higher than that of ITO alone.