Electronic structures of organic molecular materials for organic electroluminescent devices studied by ultraviolet photoemission spectroscopy

Electronic structures of evaporated films of five organic light-emitting and carrier-injecting materials for organic electroluminescent devices were studied by ultraviolet photoemission spectroscopy. The compounds examined were (i) light-emitting materials tris(8-hydroxyquinolino) aluminum (Alq₃) and 1,2,3,4,5-pentaphenylcyclopentadiene, (ii) a hole-injecting material N,N′ -bis(3-methylphenyl)-N ,N′ -diphenyl-[1,1′ -biphenyl]-4,4′ -diamine, and (iii) electron-injecting materials N,N′ -diphenyl-1,4,5,8-naphthyletracarboxyldiimide and 1,3,5-tris(5-phenyl-1,3,4-oxadiazol-2-yl)benzene. The spectral features corresponding to the top parts of the valence states, which dominate the electric properties of the materials, were assigned by the comparison with the simulated density of states obtained from PM3 molecular orbital calculations. Using these calculations, the evolution of the electronic structure of each molecule from those of constituent parts was discussed. The characters of the unoccupied states obtained by these calculations were also presented. Using these data, the correlation between the ionization threshold energies determined by ultraviolet photoemission spectroscopy and the carrier-injecting and light-emitting properties were discussed.