Spectroscopic investigation of the interfaces in new poly(9,9 - dihexyl – 9H - fluorene - 2,7 - diyl) based electroluminescent devices

Abstract

The highest occupied and lowest unoccupied states of the new electroluminescent material poly(9,9-dihexyl–9Hfluorene-2,7-diyl) (PPV-D) and polyvinylcarbazole (PVK) are investigated using ultraviolet photoelectron and inverse photoemission spectroscopies. Hole injection barriers are determined for interfaces between indium-tin oxide covered substrates with work function ranging from 4.4 to 4.7 eV and these two polymers. 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 0.6 eV. This is due to the accumulation charges at the interface with the polymer when the injection barrier is small. The resulting field bends the polymer levels to limit charge incoming in the bulk of the film. The efficiency of the electroluminescent structures is strongly influenced by the different energy levels alignment at the layer interfaces.