Characterization of copper selenide thin film hole-injection layers deposited at room temperature for use with p-type organic semiconductors

Copper selenide, CuxSe(x similar to 2), was examined as a hole-injection layer for low-temperature organic devices. Crystalline CuxSe films grown at room temperature with atomically flat surfaces exhibited metallic conduction with a high electrical conductivity of 4.5 x 10(3) S/cm, a hole concentration of 1.4 x 10(22) cm(-3), and a mobility of 2.0 cm(2)/(V s). Analysis of the free carrier absorption using the Drude model estimated the effective mass of a hole as 1.0m(e). Photoemission spectroscopy measurements of the interfaces between CuxSe and organic hole transport layers, N, N'-bis(naphthalen-1-y1)-N,N']-bis(phenyl)benzidine (NPB) and copper phthalocyanine (CuPc), verified that the hole-injection barriers of these interfaces (0.4 eV for NPB and 0.3 eV for CuPc) are smaller than that of a conventional indium tin oxide (ITO) hole-injection electrode/NPB interface (0.6 eV) but are comparable to that of an ITO electrode/CuPc interface (0.3 eV). Hole-only devices using the CuxSe layer as a hole-injection anode exhibited very low threshold voltages (0.4-0.5 V) and nearly Ohmic characteristics. The NPB layer on the CuxSe layer was found to be highly doped at 10(17)-10(19) cm(-3), probably due to copper diffusion, while the CuPc layer is nearly intrinsic with a doping concentration lower than 10(15) cm(-3). These results indicated that a CuxSe film combined with CuPc is a promising candidate for a low-voltage hole-injection anode or a buffer layer in low-temperature devices such as organic light-emitting diodes and thin film transistors. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3039167]