Energy level alignment and band bending at TPD/metal interfaces studied by Kelvin probe method

In order to examine the validity of Mott-Schottky model at organic/metal interfaces, the position of the vacuum level (VL) of triphenyldiamine (TPD) film formed on five metal substrates was measured as a function of the film-thickness by Kelvin probe method in ultrahigh vacuum (UHV). At all the interfaces, sharp shifts of the VL were observed within 1 nm thickness. Further deposition of TPD up to 100 nm did not change the position of the VL indicating no band bending at these interfaces. These findings clearly demonstrate the Fermi level alignment between metal and bulk TPD solid in UHV is not established within a typical thickness of real devices.