Strong interface p-doping and band bending in C$_{60}$*

C$_{60}$ is a strongly n-type material with its lowest unoccupied molecular orbital very close to the Fermi level, and p-doping C$_{60}$ has been a challenging issue. We measured the electronic energy level evolution of C$_{60}$ on molybdenum oxide (MoO$_{x})$/ conducting indium tin oxide (ITO) interfaces with ultra-violet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and atomic force microscopy (AFM). We found that MoO$_{x}$ strongly p-dopedC$_{60}$at the interface, resulting in an inversion layer in C$_{60}$. The energy levels of C$_{60}$relax gradually as the thickness of C$_{60}$ increases, and the band bending region is observed to be greater than 400 {\AA} in C$_{60}$. The root mean square (RMS) roughness measured with AFM of 581 {\AA} thick C$_{60}$ film was 68 {\AA}, slightly increased from that of the ITO substrate of 55 {\AA}. We have also investigated the effect of exposing the MoO$_{x}$ air, and found that it eliminated the doping effect.