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 $(MoO3)$ buffer layer inserted between ITO and $α-NPD$⁠. The device with a $0.75-nm$-thick $MoO3$ layer forms Ohmic hole injection at the $ITO∕MoO3∕α-NPD$ 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 $MoO3$⁠.