Barrier heights and surface states of metal-polymer (PET) contacts

The properties of a metal-polymer contact have been investigated using photoinjection of electrons from the metal into the PET. Contact barrier heights were determined from the Fowler's plot of photo-injection currents in PET. The barrier heights for Cu-PET and Al-PET were 2.90 and 2.83 eV respectively; however, the workfunctions of Cu and Al were 4.54 and 3.44 eV respectively. The difference in the barrier height between Cu and Al (0.07 eV) is much smaller than that in the workfunction (1.1 eV). This indicates the existence of surface states on PET. From a simplified contact model with a uniform density of surface states N_s, the value of the N_s was estimated as 1.7*10¹⁴ cm^-3 eV^-1. The barrier height was strongly affected by atmospheric conditions. O₂ molecules adsorbed on PET can act as electron traps, i.e. surface states, because of their large electron affinity.