Chemical potential pinning due to equilibrium electron transfer at metal/C-60-doped polymer interfaces

We report electroabsorption measurements of the built-in electrostatic potential in metal/ C-60-doped polymer/metal structures to investigate chemical potential pinning due to equilibrium electron transfer from a metal contact to the electron acceptor energy level of C-60 molecules in the polymer film. The built-in potentials of a series of structures employing thin films of both undoped and C-60-doped poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were measured. For undoped MEH-PPV, which has an energy gap of about 2.4 eV, the maximum built-in potential is about 2.1 eV, whereas for C-60-doped MEH-PPV the maximum built-in potential decreases to 1.5 eV. Electron transfer to the C-60 molecules close to the metal interface pins the chemical potential of the metal contact near the electron acceptor energy level of C-60 and decreases the built-in potential of the structure, From the systematic dependence of the built-in potential on the metal work function we find that the electron acceptor energy level of C-60 in MEH-PPV is about 1.7 eV above the hole polaron energy level of MEH-PPV. (C) 1997 American Institute of Physics.