Photoemission and vibrational studies of metal/organic interfaces modified by plasma-polymerized fluorocarbon films

While gold and silver have high work functions, they are known to form relatively poor hole injecting contacts with organic semiconductors. We have found that inserting an ultrathin layer of plasma-polymerized fluorocarbon (CF_x) between the metal and organic materials can substantially improve the hole injection. The working mechanism of the CF_x interlayer was investigated with ultraviolet and x-ray photoemission spectroscopy and high resolution electron energy loss spectroscopy. The hole injection enhancement was caused by the reduced hole injection barrier, attributed to an interfacial electric field induced by the partial charge transfer from the metal to the CF_x layer. Stability of the dipolar interface upon exposure to air was also demonstrated.