Formation of Ohmic hole injection by inserting an ultrathin layer of molybdenum trioxide between indium tin oxide and organic hole-transporting layers

Current density-voltage (J-V) characteristics of hole-only devices using indium tin oxide (ITO) anode and N, N′ -diphenyl- N, N′ -bis(1-naphthyl)- 1, 1′ -biphenyl- 4, 4′ -diamine (α-NPD) layers were measured with various thicknesses of a molybdenum trioxide (Mo O3) buffer layer inserted between ITO and α-NPD. The device with a 0.75-nm -thick Mo O3 layer forms Ohmic hole injection at the ITOMo O3 α-NPD interfaces and J-V characteristics of this device are controlled by a space-charge-limited current. Results of X-ray photoelectron and ultraviolet/visible/near-infrared absorption studies revealed that this Ohmic hole injection is attributable to an electron transfer from ITO and α-NPD to Mo O3.