Chemical potential pinning due to equilibrium electron transfer at metal/C60-doped polymer interfaces

We report electroabsorption measurements of the built-in electrostatic potential in metal/${\mathrm{C}}_{60}$ -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 ${\mathrm{C}}_{60}$ molecules in the polymer film. The built-in potentials of a series of structures employing thin films of both undoped and ${\mathrm{C}}_{60}$ -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 ${\mathrm{C}}_{60}$ -doped MEH-PPV the maximum built-in potential decreases to 1.5 eV. Electron transfer to the ${\mathrm{C}}_{60}$ molecules close to the metal interface pins the chemical potential of the metal contact near the electron acceptor energy level of ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$ in MEH-PPV is about 1.7 eV above the hole polaron energy level of MEH-PPV.