Photoelectron spectroscopic study of the electronic band structure of polyfluorene and fluorene-arylamine copolymers at interfaces

The occupied and unoccupied states of poly(9,9'-dioctylfluorene) (F8) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)diphenylamine) (TFB) are investigated using ultraviolet photoelectron and inverse photoemission spectroscopies, cyclic voltammetry, and density functional theory calculations. Hole injection barriers are determined for interfaces between substrates with work function ranging from 4.3 to 5.1 eV and these two polymers as well as poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-NM-phenyl-1,4-phenylenediamine) (PFB). 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 away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is due to the accumulation of excess interface charges on the polymer when the barrier is small. The resulting field shifts the polymer levels to limit charge penetration in the bulk of the film.