Electronic structure of molybdenum-oxide films and associated charge injection mechanisms in organic devices

We report on the electronic structure of freshly evaporated and air-exposed Molybdenum tri-oxide (MoO₃) and the energy-level alignment between this compound and a holetransport material [e.g., N,N'-diphenyl-N,N'-bis (1-naphthyl)-1,1'-biphenyl-4,4'-diamine (a-NPD)]. Ultraviolet and inverse photoelectron spectroscopy show that freshly evaporated MoO₃ exhibits deep-lying electronic states with an electron affinity (EA) of 6.7 eV and ionization energy (IE) of 9.7 eV. Air exposure reduces EA and IE by ∼1 eV, to 5.5 and 8.6 eV, respectively, but does not affect the hole-injection efficiency, which is confirmed by device studies. Thus, MoO₃ can be applied in low-vacuum environment, which is particularly important for low-cost manufacturing processes. Our findings of the energy-level alignment between MoO₃ and α-NPD also leads to a revised interpretation of the charge-injectionmechanism, whereby the hole-injection corresponds to an electron extraction from the organic highest-occupied molecular orbital (HOMO) level via the MoO₃ conduction band.