Electronic Structure Investigation of Doping C$_{60}$ with Metal Oxide

Fullerene (C$_{60})$ has been used extensively as an acceptor material in organic photovoltaic (OPV) cells. Other applications including n-channel organic thin film transistors (OTFT) and C$_{60}$ based organic superconductors have been reported more than a decade ago. We have investigated p-doping of C$_{60}$ with molybdenum oxide (MoO$_{\mathrm{x}})$ with ultra-violet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and atomic force microscopy (AFM). Both surface doping and bulk doping by MoO$_{\mathrm{x}}$ are studied. It was found that the thermally evaporated MoO$_{\mathrm{x}}$ inter-layer substantially increased the surface workfunction. This increased surface workfunction strongly attract electrons towards the MoO$_{\mathrm{x}}$ layer at the C$_{60}$/MoO$_{\mathrm{x}}$ interface, resulting in strong inversion of C$_{60}$. Energy levels of C$_{60}$ relax gradually as the thickness of C$_{60}$ increases. An exceptionally long (greater than 400 Angstrom) band bending is observed during this relaxation in C$_{60}$. Such a long band bending has not been observed for other organic/MoO$_{\mathrm{x}}$ interface. For the bulk doping, MoO$_{\mathrm{x}}$ doping ratios from 1{\%} to over 100{\%} were investigated. The saturation occurs at approximately 20 {\%}, when the highest occupied molecular level (HOMO) of C$_{60}$ starts to be pinned at the Fermi level. These studies demonstrate effective ways to manipulate the electronic structures of the fullerene.