N-doping of pentacene by decamethylcobaltocene

We demonstrate n-type doping of pentacene with the powerful reducing molecule decamethylcobaltocene (CoCp ₂ ^* ). Characterization of pentacene films deposited in a background pressure of CoCp ₂ ^* by X-ray photoemission spectroscopy and Rutherford backscattering confirm that the concentration of incorporated donor molecules can be controlled to a level as high as 1%. Ultraviolet photoemission spectroscopy show Fermi level (E _F) shifts toward unoccupied pentacene states, indicative of an increase in the electron concentration. A 1% donor incorporation level brings E _F to 0.6 eV below the pentacene lowest unoccupied molecular orbital. The corresponding electron density of ∼ 10¹⁸ cm^-3 is confirmed by capacitance-voltage measurements on a metal-pentacene-oxide-silicon structure. The demonstration of n-doping suggests applications of CoCp ₂ ^* to pentacene contacts or channel regions of pentacene OTFTs.