Interface energetics of polyfluorene and fluorene-arylamine copolymers

The energy level alignment at interfaces between poly(9,9′- dioctylfluorene) (F8), poly(9,9′-dioctylfluorene-co-bis-N,N′-(4- butylphenyl)-diphenylamine) (TFB) and poly(9,9′-dioctylfluorene-co-bis-N, N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) and substrates with work function ranging from 4.3 eV to 5.1 eV is investigated via ultra-violet photoemission spectroscopy. Vacuum level alignment with flat bands away from the interface is found when the interface hole barrier is 0.6 eV or larger. Band bending that moves the filled states away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is presumably due to the accumulation of excess interface charges on the polymer side when the interfacial barrier is small. The resulting field shifts the polymer levels in a way that limits charge penetration in the bulk of the film. We also study metal-on-polymer interfaces. Different metals exhibit different growth modes. While Pt shows complete layer-by-layer type of growth, Al shows island type of growth. Current-voltage measurement shows the presence of hole traps in the Au-on top-contact device, suggesting diffusion of small Au clusters into the polymer film. Furthermore, metal-on-polymer interfaces frequently present different interface energetics than their polymer-on-metal counterpart, e.g. a 0.3-0.4 eV higher hole injection barrier for Pt-on-TFB than TFB on Pt.