Electronic structure of MoO3 insertion layer at the interface between organic semiconductor and indium tin oxide (ITO).

We have investigated the electronic structure of the interfaces formed by inserting thin layer of MoO$_{3}$ in between indium tin oxide (ITO) and different organic semiconductors, such as aluminium phthalocyanine chloride (AlPcCl) and copper phthalocyanine (CuPc), with photoemission and inverse photoemission spectroscopy (PES and IPES). The presents of MoO$_{3}$ layer at the interface increases the workfunction dramatically. As a result, the organic HOMO is almost aligned with the Fermi level (E$_{f})$ at the AlPc-Cl/MoO$_{3}$ interface. For thicker AlPc-Cl layers, gradual band bending is observed. However, the recovery of the HOMO is incomplete for AlPc-Cl thickness of 200 {\AA}, leading to a great reduction of the hole injection barrier compare to the case without MoO$_{3}$. Similar situation is found in case of CuPc/MoO$_{3}$, although the energy levels are almost fully recovered for CuPc film thicker than 200 {\AA}. The energy level alignment of these interfaces will be discussed to explain the improvement induced by MoO$_{3}$ layer in device performance.